CryoNIRSP Project Documentation Document CN-0004 Revision F
Cryogenic-Near-Infra-Red Spectro- Polarimeter (CryoNIRSP)
Critical Design Document (CDD)
Saved: 2/1/2014 5:00:00 PM
Prepared for Critical Design Review
A. Fehlmann, J. Kuhn, T. Bond, D. Mickey, I. Scholl, C. Giebink, J. Messersmith; IfA Maui K. Hnat, D. James, G. Schickling, R. Schickling; Universal Cryogenics, Tucson
Name Signature Date Prepared by: Fehlmann Andre 01/07/2014 Prepared by: Prepared by: Approved by: J. Kuhn Approved by: T. Bond
Critical Design Document (CDD)
REVISION SUMMARY:
1. Date:2/28/11 4:05 PM Revision: Preliminary version (for ID4) Changes: Initial version 2. Date: 6/8/11 6:15 PM Revision: PDR Changes: 3. Date: 7/6/13 Revision: B Changes: - Flux balance paragraphs misplaced - Compliance Matrix updated - Error Budget added - various minor corrections 4. Date: 8/5/2012 Revision: C for Readiness Review Changes: - Levels of sections moved up - requirement referencing changed to section numbers - sections added - sections rearranged - data handling & control system treated separately in CN-Spec-0005 - test results added - optical and hardware design updated 5. Date: 10/10/2013 Revision: D for Readiness Review Changes: - most of the RIX items addressed that were sent to us by the readiness review panel - changed SPEC number to 0004 because CN-SPEC-0003 is the PDD document 6. Date: 12/21/2013 Revision: E for Critical Design Review Changes: - Readiness review recommendations applied - Moved risk analysis to separate document CN-0012 - Changed document number from CN-SPEC-0004 to CN-0004 - Quality assurance and test and verification plans moved to separate document CN- 0011 - Management/Budget and Electronics sections moved to separate documents CN- 0010 and CN-0022 7. Date: 01/30/2014 Revision: F RIX items addressed Changes:
CN-0004 i Critical Design Document (CDD)
- Figure 100 updated and new Figure 101 added - Component efficiencies revised - Earthquake compliance section added - Flux level sections for spectrograph and context imager revised
CN-0004 ii Critical Design Document (CDD)
Table of Contents
1 Requirements Overview 13 1.1 Scope of the Document 13 1.2 PDR Recommendations 13 1.2.1 Supplemental PI/Expert CryoNIRSP Review 14 1.3 CDR Deliverables 15 1.4 Related Documents 16 1.4.1 Related ATST Project Documents 16 1.4.2 Interface Control Documents and Drawings 17 1.5 Specific Definitions and Terminology 17 1.6 Compliance Matrix 17 2 Introduction to the CryoNIRSP Design 18 3 Optical Design 19 3.1 CryoNIRSP Optical Design Requirements 19 3.2 Introduction and nomenclature 19 3.3 Interface to ATST 21 3.3.1 Polarization Modulator 21 3.3.2 Calibration Retarder 21 3.3.3 Mechanical Interface to Coudé Station 21 3.3.4 Optical Interface to Coudé Station 24 3.4 Cryogenic Temperature Optics 24 3.4.1 Common characteristics 25 3.4.1.1 Image Scales and Focal Ratios 25 3.4.1.2 Mirror substrates 26 3.4.1.3 Mirror coatings 26 3.4.1.4 RMS Surface Roughness 27 3.4.1.5 Surface Quality 27 3.4.1.6 Pellicle 27 3.4.1.7 Camera 30 3.4.2 Spectrograph 31 3.4.2.1 Description 31 3.4.2.2 Focal Lengths 31 3.4.2.3 Grating Specifications 33 3.4.2.4 Mirror Specifications 36 3.4.2.5 Alignment Sensitivity 37 3.4.2.6 Cold Pupil 37 3.4.2.7 Slit mask Specifications 37 3.4.2.8 Filter Specifications 38 3.4.2.9 Spectrograph Flux Estimates 39 3.4.2.10 Analyzing Beam-splitter 40 3.4.3 Context Imager 44 3.4.3.1 Description 44 3.4.3.2 Mirror Specifications 45 3.4.3.3 Alignment Sensitivity 46 3.4.3.4 Cold Pupil 46 3.4.3.5 Filter Specifications 46 3.4.3.6 Imager Light Flux Levels 47 3.5 Room Temperature Optics: Feed Optics 49 3.5.1 Description 49
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3.5.2 Modulator 51 3.5.3 Mirrors/ substrate 51 3.5.4 Mirror coatings 51 3.5.5 RMS Surface Roughness 52 3.5.6 Surface Irregularity 52 3.5.7 Surface Quality 52 3.5.8 Mirror Specifications 52 3.5.9 Scan Mirror 53 3.5.10 Focusing Mirror 53 3.5.11 Filter Specifications 53 3.5.12 Calibration Lamps 54 3.5.13 Entrance Window 54 3.6 Optical Performance Analysis 56 3.6.1 Spectral Resolution 56 3.6.1.1 Disk Observation Mode 56 3.6.1.2 Coronal Mode 58 3.6.2 Feed Relay 59 3.6.2.1 Image Scale, Exit Pupil 59 3.6.2.2 Image Quality 59 3.6.2.3 Image Scanning 61 3.6.3 Context Imager 63 3.6.4 Spectrograph Spatial Resolution 64 3.6.5 Polarization Analysis 65 3.6.6 Thermal Background Control 67 3.6.7 Instrument Efficiency 67 3.7 Optical Tolerance Analysis and Wavefront Error Budget 69 3.7.1 Feed Optics 70 3.7.1.1 Figure and Position Errors 70 3.7.1.2 Surface Irregularity 70 3.7.2 Spectrograph 71 3.7.2.1 Figure and Position Errors 71 3.7.2.2 Surface Irregularity 72 3.7.3 Context Imager 72 3.7.3.1 Figure Errors 72 3.7.3.2 Surface Irregularity 73 3.7.4 Metrology Capabilities of Vendor 73 3.7.5 Analyzer Assembly 73 3.8 Optics Surface Quality and Coating Micro-roughness 74 3.9 Scattered-light and Ghost Image Analysis 75 3.9.1 Grating Scattered Light 75 3.9.2 Analyzing Beam Splitter Scattered Light and Ghosts 78 3.9.3 Grating Recombination Ghosts 79 3.9.3.1 Critical Orders 80 3.9.3.2 Littrow Ghost Model Results 80 3.9.3.3 Blaze function 81 3.9.3.4 Effect of rotating the grating 81 3.9.3.5 Conclusion 81 3.9.4 Entrance Window Ghosts 81 3.9.5 Blocking Filter Scattered Light and Ghosts 82 3.10 Optical Alignment 84 3.10.1 Overall System Alignment 84
CN-0004 iv Critical Design Document (CDD)
3.10.2 Alignment: Co-registration of Context Imager and Spectrograph Slit 84 3.10.3 Beam-splitter image registration 85 4 Hardware Design 86 4.1 Cryogenics Temperature Instrument Components 86 4.1.1 Mirror Mounts 86 4.1.2 Adjustable Mounts 90 4.1.3 Analyzing Beam-splitter Mount 90 4.1.4 Camera sub-assembly 93 4.1.5 Pick-off Mirror Assembly 94 4.1.6 Focus Mirror Stages 95 4.1.7 Context Imager Filter Wheel Assembly 96 4.1.8 Spectrograph Filter and Slit Wheel Assembly 98 4.1.8.1 Filter Wheel Test 99 4.1.9 Grating Turret 99 4.1.10 Deployable Cold Stop Mount 101 4.1.11 Baffling 102 4.1.12 Motors 103 4.1.12.1 Location 103 4.1.12.2 Description 104 4.2 Room Temperature Instrument Components: Feed Optics 105 4.2.1 Adjustable Mirror Mounts 105 4.2.2 Scanning Mirror Motorized Stage 105 4.2.3 Focusing Mirror Motorized Stage 107 4.2.4 Warm Filter Wheel Assembly 107 4.2.5 Optical Safety Mechanism 107 4.2.6 Window Mount 108 4.2.7 Baffling 109 4.2.8 Motors 109 4.3 Vacuum System 110 4.3.1 Context Imager Vacuum System 110 4.3.2 Spectrograph Vacuum System 110 4.4 Cryogenic System 111 4.4.1 Handling System 112 4.4.2 Context Imager 112 4.4.2.1 Context Imager Cold Head 113 4.4.2.2 Context Imager Thermal Components 114 4.4.2.3 Context Imager Thermal Analysis 114 4.4.2.4 Context Imager Vibration Isolation 115 4.4.2.5 Context Imager Thermal Link 116 4.4.2.6 Context Imager Electrical Isolation 117 4.4.2.7 Context Imager Vacuum Case 117 4.4.2.8 Context imager Cold Stages 120 4.4.2.9 Context imager Thermal Shields 121 4.4.2.10 Test Dewar Cool Down Time 122 4.4.3 Spectrograph 123 4.4.3.1 Spectrograph Cold Heads 124 4.4.3.2 Spectrograph Thermal Components 125 4.4.3.3 Spectrograph Thermal Analysis 125 4.4.3.4 Spectrograph Vibration Isolation 126 4.4.3.5 Spectrograph Thermal Link 127 4.4.3.6 Spectrograph Electrical Isolation 128
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4.4.3.7 Spectrograph Vacuum Case 128 4.4.3.8 Spectrograph Cold Stages 132 4.4.3.9 Spectrograph Thermal Shields 133 4.4.4 Cryo-cooler Compressors 133 4.4.5 Temperature control 134 4.4.6 Vibration control 134 4.4.6.1 Vibration Tests 134 4.4.7 Operational constraints 137 4.5 Dust Control 138 4.6 Earthquake Compliance 138 5 Calibration Procedures 139 5.1 CryoNIRSP Polarimetric Calibration (PolCal) 140 5.2 Photometric calibrations 142 5.2.1 Dark 142 5.2.2 Gain 143 5.2.3 PhotoCal 143 5.3 Geometry Calibration 144 5.4 Focus Calibration 144 5.5 Wavelength Calibration 145 6 Data Handling 146 7 Control System and motion controll 147 7.1 Instrument Health 147 7.2 Motion Controller 158 7.3 Motor Drivers 158 7.4 Motors 158 7.5 Limit Switches, Home Markers and Encoders 159 7.6 Temperature Sensors 159 7.7 Vacuum equipment 159 7.8 Cryogenic Equipment 160 7.9 Power Supplies 160 7.10 Calibration Lamps 160 7.11 Power Feed 160 7.12 Electrical Layout and Wiring 160 7.13 Safety Interlock 160 7.14 Motion Control Analysis 165 7.14.1 Motor Speed 165 7.14.2 Positional Repeatability 165 7.14.3 Backlash 165 8 Hazard Analysis 168 9 Design Trade Studies Summary 169 10 Construction Phase Planning 171 10.1 Fabrication Plan 171 10.2 Manufacturability 171 10.3 Long Lead Items 172 10.4 Packaging and Transportation Plan 172 10.4.1 Tucson to ATRC 172 10.4.2 ATRC to ATST site 172
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10.5 Quality Control and Quality Assurance Plan 173 10.6 Verification Test Plan 173 11 Schedule, RiskS and Cost Estimates 174 11.1 Project Schedule, WBS, Cost Estimates 174 11.2 Risk Assessment and Mitigation Strategies 174
CN-0004 vii Critical Design Document (CDD)
Table of Figures
FIGURE 1: CRYONIRSP OPTICAL LAYOUT SHOWING THE CONTEXT IMAGER IN THE FRONT AND THE SPECTROGRAPH ABOVE...... 20 FIGURE 2: DRAWING OF THE CLAMPS USED TO MOUNT THE INSTRUMENTS TO THE SUPPORTING BEAMS IN THE COUDÉ ROOM (ATST-DWG-00049)...... 22 FIGURE 3: LAYOUT OF THE COUDÉ ROOM AS PROVIDED BY THE PROJECT...... 23 FIGURE 4: ELECTRONICS BAY NUMBERING AND ASSIGNMENT PROVIDED BY THE PROJECT. OUTLINE OF THE PROVIDED ATST RACK WITH 40 USABLE UNITS...... 23 FIGURE 5: SPRING LOADED ANTI-VIBRATION MOUNTS FOR FLOOR (LEFT) AND CEILING (RIGHT) MOUNTING...... 24 FIGURE 6: A SIMULATED POLARIZED SPECTRAL IMAGE FROM THE CRYONIRSP ILLUSTRATING A CORONAL EMISSION LINE AND BACKGROUND ATMOSPHERIC ABSORPTION LINES WITH THE SPATIAL AND SPECTRAL RESOLUTION OF THE INSTRUMENT. ... 25 FIGURE 7: MIRROR COATING SAMPLES WHERE THE SET ON THE LEFT STAYED AT ATRC AND THE SET ON THE RIGHT WAS KEPT AT THE SUMMIT FOR 18 MONTHS. THE SAMPLES IN THE FRONT WERE COATED WITH PURE, UNPROTECTED SILVER. COMPARED TO THE ALUMINUM COATED SAMPLES IN THE BACK THE DEGRADATION IS CLEARLY VISIBLE...... 27 FIGURE 8: NITROCELLULOSE PELLICLE TRANSMISSION MEASUREMENTS...... 28 FIGURE 9: SURFACE IRREGULARITY MEASUREMENT OF A WARM TEST PELLICLE...... 29 FIGURE 10: SURFACE IRREGULARITY MEASUREMENT OF A 190 K COLD TEST PELLICLE (RIPPLES CAUSE BY VACUUM SYSTEM VIBRATIONS)...... 29 FIGURE 11: SPECTROGRAPH ZEMAX LAYOUT...... 32 FIGURE 12: SPECTROGRAPH SOLID MODEL WITH OPTICS AND MOUNTS (TOP VIEW)...... 32 FIGURE 13: SPECTROGRAPH MODEL WITH OPTICS AND MOUNTS (SIDE VIEW)...... 33 FIGURE 14: TEST GRATING SUBSECTION SURFACE IRREGULARITY MEASURED IN LITTROW CONDITION AFTER THERMAL CYCLING...... 34 FIGURE 15: COLLIMATING AND CAMERA LENSES USED TO MEASURE THE GRATING EFFICIENCY (LEFT). A GOOD PORTION OF THE GRATING WAS ILLUMINATED WITH THIS SETUP (RIGHT)...... 34 FIGURE 16: EFFICIENCY CURVE PUBLISHED BY GRATINGLAB FOR THE B055 MASTER GRATING (RULING FREQUENCY 31.6 LINES/MM, BLAZE ANGLE 63.9°)...... 36 FIGURE 17: DETAILED DRAWING OF THE TRIPLE SLIT...... 38 FIGURE 18: THE HALEAKALA SKY BRIGHTNESS AND K-CORONA...... 38 FIGURE 19: FLUX LEVELS FOR THE SPECTROGRAPH...... 40 FIGURE 20: FTS-MEASURED POLARIZED LIGHT TRANSMISSION OF UV-GRADE FUSED SILICA WIRE GRID POLARIZER FOR INCIDENCE ANGLES UP TO 30 DEGREES. HERE THE TILT AXIS OF THE SUBSTRATE IS PERPENDICULAR TO THE GRID LINES. . 42 FIGURE 21: SAME AS PREVIOUS FIGURE BUT THE TILT AXIS IS PARALLEL TO THE WIRE GRID ELEMENTS...... 42 FIGURE 22: REFLECTED FLUX FROM POLARIZER, INCLINATION 30 DEGREES (PERPENDICULAR TO WIRE GRID)...... 43 FIGURE 23: MEASURED CROSS- AND PARALLEL-POLARIZATION TRANSMISSION THROUGH THE FUSED-SILICA WIRE GRID POLARIZER ...... 43 FIGURE 24: CONTEXT IMAGER OPTICAL LAYOUT...... 44 FIGURE 25: CONTEXT IMAGER SOLID MODEL WITH OPTICS AND MOUNTS...... 44 FIGURE 26: A ZEMAX NON-SEQUENTIAL RAY TRACE MODEL OF THE SLOTTED SINGLE SLIT PICK OFF MIRROR...... 45 FIGURE 27: CONTEXT IMAGER FLUX LEVELS...... 48 FIGURE 28: FEED OPTICS OPTICAL LAYOUT...... 49 FIGURE 29: DETAILED LAYOUT FOR THE CALIBRATION LAMPS, WARM FILTER WHEEL, MODULATOR AND SAFETY SHUTTER...... 50 FIGURE 30: OVERVIEW SPECTRUM OF A TH-AR LAMP. WAVELENGTH RANGE IS 715-5000 NM (14,000-2000 CM–1). THE LINE INTENSITY IS GIVEN IN ARBITRARY UNITS. LONGWARD OF 2500 NM (<4000 CM–1) THERMAL EMISSION FROM THE HOT CATHODE INTRODUCES A CONTINUUM IN THE OTHERWISE PURE EMISSION-LINE SPECTRUM (FIGURE FROM KERBER ET AL. 2008)...... 54 FIGURE 31: SPECTROGRAPH LINE PROFILE IN DISK OBSERVATION MODE...... 57 FIGURE 32: DISK MODE SPOT DIAGRAM...... 58 FIGURE 33: CORONAL MODE SPOT DIAGRAM...... 59 FIGURE 34: FEED RELAY SPOT DIAGRAM FOR DISK MODE FIELD...... 60 FIGURE 35: FEED RELAY SPOT DIAGRAM FOR CORONAL MODE FIELD...... 61 FIGURE 36: FEED RELAY SPOT DIAGRAM...... 62 FIGURE 37: FEED RELAY SPOT DIAGRAM ...... 63 FIGURE 38: CONTEXT IMAGER SPOT DIAGRAM...... 64
CN-0004 viii Critical Design Document (CDD)
FIGURE 39: SPECTROGRAPH SPATIAL RESOLUTION...... 65 FIGURE 40: GRATING SCATTERED LIGHT ANALYSIS LAYOUT ...... 76 FIGURE 41: ILLUMINATION OF THE PSEUDO DETECTOR WITH THE PRINCIPAL ORDER REMOVED – ANODIZED BAFFLES...... 76 FIGURE 42: ILLUMINATION OF THE PSEUDO DETECTOR WITH THE PRINCIPAL ORDER REMOVED – ANODIZED BAFFLES...... 77 FIGURE 43: ILLUMINATION OF THE PSEUDO-DETECTOR PLACED 10 CM BEHIND THE ENTRANCE SLIT...... 77 FIGURE 44: ANALYZING BEAM SPLITTER LAYOUT...... 78 FIGURE 45: ANALYZING BEAM SPLITTER IMAGE FOR A TRIPLE SLIT SET UP...... 79 FIGURE 46: SPECTRUM IN ORDER 42 AT 1.347 µM, WITH RECOMBINATION GHOST IN ORDER 44. THE GHOST POWER IS 0.0023 WATTS...... 80 FIGURE 47: AT THE IMAGE PLANE THE GHOST OF THE INDICATED POINT SOURCE IS A 1.2 MM DIAMETER SPOT CENTERED ABOUT 2 ARCSEC (IN THE IMAGE PLANE) AWAY FROM THE PRIMARY IMAGE WITH A 1.67 DEGREE WINDOW TILT. THE GHOST HAS 0.6% OF THE POINT-SOURCE POWER. OBSERVATIONS BELOW THE SOLAR LIMB WITH THIS LIMB ORIENTATION WILL EFFECTIVELY HAVE NO DISK-SCATTERED LIGHT CONTAMINATION...... 82 FIGURE 48: GHOST IMAGE (GREEN) SEEN ON THE SPECTROGRAPH DETECTOR DUE TO BLOCKING FILTER...... 82 FIGURE 49: DISTORTION MAP FOR CONTEXT IMAGER...... 85 FIGURE 50: DETAILS OF THE KINEMATIC MIRROR MOUNTS...... 86 FIGURE 51: KINEMATIC OPTICAL MOUNT DETAIL...... 87 FIGURE 52: MODEL RESULTS FOR MIRROR DEFLECTION CAUSED BY THE MOUNTS...... 88 FIGURE 53: THERMAL AND MOUNT TESTS WITH A PROTOTYPE MOUNT USING THE FINAL MOUNT DESIGN...... 88 FIGURE 54: COOL DOWN TIMES OF THE TEST CONFIGURATION...... 89 FIGURE 55: OPTICS MOUNTING SCHEME (LEFT, MIDDLE) AND SUBSTRATE SHAPE (RIGHT) ...... 89 FIGURE 56: BEAM-SPLITTER MECHANICAL DESIGN...... 91 FIGURE 57: COMPACT AND LOCKABLE X, Y STAGE TO MOUNT THE PRISM...... 91 FIGURE 58: SIGMA KOKI KSPS 406M ROTATION STAGE...... 92 FIGURE 59: THE LIGHT TRAP THAT IS ATTACHED TO THE POLARIZATION ANALYZER ASSEMBLY WILL CAPTURE ALL THE LIGHT THAT WOULD OTHERWISE BE REFLECTED BACK INTO THE DEWAR...... 92 FIGURE 60: FPA AND ASIC MOUNT (FRONT VIEW)...... 93 FIGURE 61: FPA AND ASIC MOUNT (REAR VIEW)...... 94 FIGURE 62: PICKOFF MIRROR ASSEMBLY...... 95 FIGURE 63: FOCUS MIRROR STAGE DESIGN. HERE THE MOUNT FOR SM5 IS SHOWN...... 96 FIGURE 64: CONTEXT IMAGER FILTER WHEEL ASSEMBLY...... 97 FIGURE 65: FILTER AND SLIT WHEELS ASSEMBLY...... 98 FIGURE 66: GRATING TRAVEL RANGE...... 99 FIGURE 67: GRATING TURRET DETAILS...... 100 FIGURE 68: DEPLOYABLE COLD STOP (APERTURE COLORED IN MAGENTA) IN THE SPECTROGRAPH CONTEXT...... 101 FIGURE 69: DEPLOYABLE COLD MASK MECHANISM...... 101 FIGURE 70: CROSS SECTION OF THE BAFFLING ON THE SPECTROGRAPH SLIT AND FILTER WHEEL ASSEMBLY AND THE PICK-OFF ASSEMBLY...... 102 FIGURE 71: CYLINDRICAL HOUSING AROUND THE SPECTROGRAPH GRATING...... 103 FIGURE 72: STEPPERS, FEED-THROUGH, AND MOUNTS...... 104 FIGURE 73: SCANNING MIRROR MOUNT...... 106 FIGURE 74: SAFETY MECHANISM DESIGN...... 108 FIGURE 75: DUAL DEWAR DESIGN OF CRYONIRSP...... 111 FIGURE 76: CONTEXT IMAGER DEWAR (LEFT), TOP SECTION AND SHIELDS REMOVED (RIGHT)...... 112 FIGURE 77: LEFT, TEST DEWAR CRYO-ENGINE. RIGHT, CROSS SECTION THROUGH CRYO-COOLER COLD FINGERS...... 113 FIGURE 78: SUMITOMO RDK-408 LOAD MAP...... 114 FIGURE 79: FLOATING ANTI-VIBRATION MOUNT DESIGN...... 115 FIGURE 80: THERMAL LINKS FOR THE CONTEXT IMAGER CRYO-COOLER (FLEXIBLE COPPER ROPES ARE DRAWN STRAIGHT IN THE CAD MODEL BUT ARE ACTUALLY BENT)...... 116 FIGURE 81: THERMAL LINK BETWEEN THE 1STSTAGE OF TEST DEWAR AND THE 1ST STAGE OF CONTEXT IMAGER...... 116 FIGURE 82: CONTEXT IMAGER ELECTRICAL ISOLATION...... 117 FIGURE 83: DISPLACEMENT OF TOP CASE COVER UNDER VACUUM IS 2.3 MM...... 118 FIGURE 84: CONTEXT IMAGER CASE PANELS AND FRAMES: ...... 118 FIGURE 85: WELDED CONTEXT IMAGER TOP CASE ASSEMBLY...... 119 FIGURE 86: BOTTOM VIEW OF THE CONTEXT IMAGER DEWAR...... 119
CN-0004 ix Critical Design Document (CDD)
FIGURE 87: CONTEXT IMAGER COLD STAGES ARE ISOLATED BY G-10 STAND-OFFS SHOWN IN GREEN...... 120 FIGURE 88: THE SUPPORT ASSEMBLY CONSISTS OF TWO ALUMINUM BLOCKS AND G-10 STRAP MATERIAL. THE ALUMINUM BLOCKS HAVE THREADED HOLES FOR MOUNTING OF THE STAGE PLATES...... 120 FIGURE 89: MODULAR THERMAL HEAT SHIELD DESIGN...... 121 FIGURE 90: CROSS SECTION OF THE CONTEXT IMAGER. THE THERMAL HEAT SHIELD IS SHOWN IN BLUE...... 122 FIGURE 91: COOL DOWN TIMES MEASURED WITH THE TEST DEWAR. THE TEMPERATURE BUMP AT 18 HOURS IS DUE TO THE COOLER BEING SWITCHED OFF FOR VIBRATION MEASUREMENTS. THE COOLER WAS TURNED OFF AFTER 21 HRS...... 122 FIGURE 92: COOL DOWN TIME FOR A DEWAR THAT HAS A SIMILAR SIZE AS THE CONTEXT IMAGER, WHICH WAS BUILT BY UCRYO FOR THE BIG BEAR OBSERVATORY...... 123 FIGURE 93: SPECTROGRAPH DEWAR...... 124 FIGURE 94: SUMITOMO RDK-400 LOAD MAP...... 125 FIGURE 95: THERMAL LINKS FOR THE SPECTROGRAPH RDK-400 CRYO-COOLER (FLEXIBLE COPPER ROPES ARE DRAWN STRAIGHT IN THE CAD MODEL BUT ARE ACTUALLY BENT)...... 127 FIGURE 96: SPECTROGRAPH ELECTRICAL ISOLATION...... 128 FIGURE 97: SPECTROGRAPH TOP CASE COVER MASS IS 248.4 KG...... 129 FIGURE 98: SPECTROGRAPH UPPER CASE...... 129 FIGURE 99: SPECTROGRAPH PERMANENT LOWER CASE (96 KG)...... 130 FIGURE 100: SPECTROGRAPH DEWAR BOTTOM VIEW SHOWING THE ELECTRICAL FEED THROUGHS AND VACUUM PORTS...... 130 FIGURE 101: ELECTRICAL FEED THROUGHS ON THE TEST DEWAR...... 131 FIGURE 102: SPECTROGRAPH DEWAR WITH THE TOP CASE AND RADIATION SHIELDS REMOVED...... 131 FIGURE 103: SPECTROGRAPH DESIGN SHOWING THE G-10 STAND-OFFS IN GREEN...... 132 FIGURE 104: THE SUPPORT ASSEMBLY CONSISTS OF TWO ALUMINUM BLOCKS AND G-10 STRAP MATERIAL. THE ALUMINUM BLOCKS HAVE THREADED HOLES FOR MOUNTING OF THE STAGE PLATES...... 132 FIGURE 105: VIBRATION CONTROL SYSTEM ON COLD HEADS OF CRYO-COOLERS...... 134 FIGURE 106: TEST DEWAR SET UP TO MEASURE VIBRATIONS...... 136 FIGURE 107: TIME SERIES OF THE MOVEMENTS MEASURED IN THE Z-DIRECTION ON THE 2ND STAGE...... 136 FIGURE 108: POWER SPECTRUM GENERATED FROM THE 2ND STAGE Z-DIRECTION MOVEMENT TIME SERIES. THE DOMINANT EXCITATION NEAR 30HZ IS PRESENT WHEN THERE IS NO MECHANICAL EXCITATION OF THE DEWAR. MECHANICAL RESONANCES AT HIGHER FREQUENCIES HAVE BEEN ELIMINATED BY MODIFYING THE COLD BAFFLE...... 137 FIGURE 109: CRYONIRSP CALIBRATION HIERARCHY...... 139 FIGURE 110: GEOMETRY CALIBRATION ILLUSTRATION...... 144 FIGURE 111: CRYONIRSP HEALTH REPORTING FLOW DIAGRAM...... 147 FIGURE 112: CRYONIRSP SYSTEM LAYOUT DIAGRAM...... 148 FIGURE 113: MOTOR SPEED AND TORQUE PERFORMANCE CURVES (BLUE DASHED 24 VDC, BLUE SOLID 48 VDC, AND RED 75 VDC)...... 159 FIGURE 114: POWER FEED DIAGRAM...... 162 FIGURE 115: CRYONIRSP RACK LAYOUT...... 163 FIGURE 116: CRYONIRSP CABLE CONNECTIONS...... 164 FIGURE 117: CM5 SOLID MODEL DRAWING...... 172
CN-0004 x Critical Design Document (CDD)
Table of Tables
TABLE 1: GRATING SPECIFICATIONS. (GRATING 2 MIGHT BE ADDED LATER TO UPGRADE CRYONIRSP)...... 33 TABLE 2: SUMMARY OF THE GRATING EFFICIENCY MEASUREMENTS FOR 637 NM...... 35 TABLE 3: SUMMARY OF THE GRATING EFFICIENCY MEASUREMENTS FOR 1000 NM...... 35 TABLE 4:OPTICAL SPECIFICATIONS FOR THE SPECTROGRAPH MIRRORS...... 36 TABLE 5: SPECTROGRAPH COLD PUPIL SPECIFICATIONS...... 37 TABLE 6: SLIT SPECIFICATIONS...... 37 TABLE 7: SPECTROGRAPH FILTERS COMMON REQUIREMENTS...... 39 TABLE 8: SPECTROGRAPH FILTERS SPECIFICATIONS ...... 39 TABLE 9: OPTICAL SPECIFICATIONS FOR THE ANALYZING BEAM SPLITTER MIRRORS...... 41 TABLE 10: OPTICAL SPECIFICATIONS FOR THE CONTEXT IMAGER MIRRORS...... 45 TABLE 11: OPTICAL SPECIFICATIONS FOR THE PICK OFF MIRRORS...... 45 TABLE 12: CONTEXT IMAGER COLD PUPIL SPECIFICATIONS...... 46 TABLE 13: CONTEXT IMAGER FILTERS COMMON REQUIREMENTS...... 46 TABLE 14 POSSIBLE CONTEXT IMAGER FILTER CONFIGURATION...... 47 TABLE 15: OPTICAL SPECIFICATIONS FOR THE FEED OPTICS MIRRORS...... 52 TABLE 16: WARM OPTICS FILTERS COMMON REQUIREMENTS...... 53 TABLE 17: INSERTS FOR THE WARM OPTICS FILTER WHEEL...... 53 TABLE 18: OPTICAL SPECIFICATIONS FOR THE ENTRANCE WINDOW...... 55 TABLE 19: CRYONIRSP SPECTROGRAPH EFFICIENCY. THE CALCULATION IS DONE FOR THE MORE DEMANDING CASE WHERE THE BEAM HAS TO PASS TWO WIRE GRID POLARIZERS IN THE POLARIZATION ANALYZER...... 67 TABLE 20: CRYONIRSP CONTEXT IMAGER EFFICIENCY...... 68 TABLE 21: IMAGE RESOLUTION ERROR BUDGET...... 69 TABLE 22: FEED OPTICS SURFACE FIGURE ERROR BUDGET...... 70 TABLE 23: FEED OPTICS SURFACE IRREGULARITY ERROR BUDGET...... 71 TABLE 24: SPECTROGRAPH OPTICS SURFACE FIGURE ERROR BUDGET...... 71 TABLE 25: SPECTROGRAPH OPTICS SURFACE IRREGULARITY ERROR BUDGET...... 72 TABLE 26: CONTEXT IMAGER OPTICS SURFACE FIGURE ERROR BUDGET...... 73 TABLE 27: SPECTROGRAPH OPTICS SURFACE IRREGULARITY ERROR BUDGET...... 73 TABLE 28: FIXED MOUNT REQUIREMENTS...... 90 TABLE 29: BEAM-SPLITTERFIXED MIRROR MOUNTS REQUIREMENTS ...... 90 TABLE 30: BEAM SPLITTER PRISM MOUNT REQUIREMENTS...... 90 TABLE 31: CAMERA MOUNT REQUIREMENTS ...... 93 TABLE 32: PICKOFF MIRROR ASSEMBLY REQUIREMENTS...... 94 TABLE 33: FOCUS MIRRORS REQUIREMENTS ...... 95 TABLE 34: CONTEXT IMAGER FILTER WHEELS ASSEMBLY REQUIREMENTS ...... 96 TABLE 35: SPECTROGRAPH FILTERS AND SLITS WHEEL ASSEMBLY REQUIREMENTS...... 98 TABLE 36: GRATING TURRET REQUIREMENTS ...... 99 TABLE 37: SPECTROGRAPH COLD PUPIL MOUNT SPECIFICATIONS...... 101 TABLE 38: FIXED MOUNTS REQUIREMENTS ...... 105 TABLE 39: SCANNING MIRROR REQUIREMENTS...... 105 TABLE 40: AEROTECH GIMBAL MOUNT SPECIFICATIONS...... 106 TABLE 41: WARM FOCUSING MIRROR REQUIREMENTS...... 107 TABLE 42: CONTEXT IMAGER FILTER WHEELS ASSEMBLY REQUIREMENTS ...... 107 TABLE 43: SAFETY MECHANISM REQUIREMENTS...... 108 TABLE 44: WINDOW MOUNT REQUIREMENTS...... 108 TABLE 45: VACUUM SYSTEM SPECIFICATIONS...... 110 TABLE 46: DEWAR PHYSICAL CHARACTERISTICS...... 111 TABLE 47: SUMITOMO RDK-408S INFO...... 113 TABLE 48: CONTEXT IMAGER THERMAL COMPONENTS...... 114 TABLE 49: SUMITOMO RDK-400S INFORMATION...... 124 TABLE 50: SPECTROGRAPH THERMAL COMPONENTS...... 125 TABLE 51: CRYOCOOLER COMPRESSOR SPECIFICATIONS ...... 133
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TABLE 52: VIBRATION TESTS SUMMARY...... 135 TABLE 53: SYSTEM LEVEL CALIBRATION PROCEDURES...... 140 TABLE 54: CRYONIRSP SPECTROPOLARIMETRY CALIBRATION MODES...... 142 TABLE 55: EQUIPMENT LIST...... 157 TABLE 56: MOTOR SPECIFICATIONS...... 159 TABLE 57: CRYONIRSP MOTION CONTROL SUMMARY...... 167
CN-0004 xii Critical Design Document (CDD)
1 REQUIREMENTS OVERVIEW
1.1 SCOPE OF THE DOCUMENT
This document describes the design of the Cryogenic Near InfraRed SpectroPolarimeter (CryoNIRSP). The CryoNIRSP is a unique cryogenic spectropolarimeter that samples the broadest wavelength range and highest photometric sensitivity of all first generation ATST instruments. The document describes the instrument and control hardware planned for achieving the ATST science requirements called out in: the CryoNIRSP Instrument and ATST Science Requirement Documents, its Operations Concept Document, and the CryoNIRSP Design Review Document. These documents and this Critical Design Document are prepared for review during the project sponsored critical design review. The information contained in this document is proprietary and not intended for distribution outside of the immediate ATST review and instrument project teams.
1.2 PDR RECOMMENDATIONS
The CryoNIRSP Preliminary Design Review (PDR) was conducted in June 2011. The review committee made recommendations to the CryoNIRSP team which can be found in the Report from Cryo-NIRSP Preliminary Design Review. A summary and our responses are: Instrument scientist should be hired early. CryoNIRSP has hired a full time instrument scientist. Cost contingency of 8.2% seems too low. Estimates are being revised and will be updated for CDR. Schedule between PDR and CDR is short. CryoNIRSP applied for a two month no cost extension. Project management is shared with the ATST DL-NIRSP instrument. CryoNIRSP has its own project manager now. Tip-tilt correction is assumed by CryoNIRSP to achieve diffraction limited image quality at 4 microns. This has not been considered a requirement by ATST. No tip/tilt correction is to be provided by the CryoNIRSP instrument as directed by the ATST. Polarization modulator design and software implementation should be elaborated with ATST. This has been done but no modulator specifications were given to the CryoNIRSP team. Grating Littrow recombination ghosts and coatings should be investigated Grating ghosts have been investigated. Grating performance should be investigated after thermal cycling. Substrate and coating resilience should be verified. Done. Pellicle optical and thermal properties should be investigated. Pellicle beam splitter tests have been done. A large rectangular cryostat design is considered a challenging task. CryoNIRSP has now a dual cryostat design where two smaller dewars – one containing the spectrograph and one for the context imager – are connected.
CN-0004 13 Critical Design Document (CDD)
CryoNIRSP does not meet the maximal “total non-scheduled down time” requirement of two days. CryoNIRSP will not be able to meet the unscheduled downtime requirement of two days. With the minimal cool- down and warm-up rates required of the IR arrays as well as the practical implications on cooling equipment, an actual unscheduled downtime of 6 days is more likely (48hrs warm-up, 48hrs service, 48hrs cool-down). A formal waiver of this requirement has been accepted by the ATST project. Heat rejection before the instrument might be achieved using a dichroic as pick up between M9 and M10. The ATST project will use a mirror between M9 and M10 to direct all the light towards CryoNIRSP. CryoNIRSP uses a heat rejection filter in front of the safety shutter to limit the energy entering the dewar. CryoNIRSP needs to control limb occulting disk and must provide tip tilt signals to M2. The CryoNIRSP team assumes that no interaction shall occur between CryoNIRSP and the M2 tip/tilt system. The software design is expected to be described in full detail, i.e. addressing all operation modes. Done in Software Design Document (CN-0005). All calibration options should be developed concurrently. CryoNIRSP shall collaborate with the ATST project and other instruments. Described in section 5 of this document and the CryoNIRSP OCD. CryoNIRSP shall collaborate with the ATST project do define night time use requirements. In progress. Feasibility of dome screen flats should be explored. Dome flats are no longer required. CryoNIRSP shall work closely with the project to make sure an IR camera is selected soon enough to keep the instrument development on schedule. In progress but remains a main concern of the CryoNIRSP team as well. We understand that the project is considering detectors other than H2RG and defining detector timing and synchronization specifications. We have completed a phase I study of an IR array H2RG detector option that will meet CryoNIRSP requirements. Overlap with DL-NIRSP: difference and unique capabilities of each instrument should be clearly spelled out. Done. The scope of the context imager shall be made clear by showing the flow down of the requirements from the ATST SRD (Spec-0001) to the CryoNIRSP ISRD (Spec-0056). Done. CryoNIRSP should examine modes of multi-instrument operation. The ATST project has chosen a mirror to direct the light towards CryoNIRSP, thus no other instrument can run at the same time. Test measurements of the Si IX 3.935 um line should be taken. Could be done if needed.
1.2.1 Supplemental PI/Expert CryoNIRSP Review
A preliminary version of the CryoNIRSP design and this document were presented to an expert internal (informal) IfA 7-member review panel during the second week of May 2010. The panel was led by the IfA Instrumentation director (Alan Tokunaga) and each member of the panel has had experience as the PI of a major astronomical instrument for Subaru, Gemini, the NASA IR detector program (Don Hall), the NASA IRTF, or satellite x-ray
CN-0004 14 Critical Design Document (CDD) instruments. This panel had several cryogenic systems experts (John Rayner, Alan Tokunaga, Klaus Hodapp, and Don Hall). The panel’s ½ day review concluded by endorsing the CryoNIRSP design. It also made several suggestions the project could consider to improve CryoNIRSP. All of these suggestions have been studied and/or implemented in the current CryoNIRSP design: That we should consider enhancing the thermal baffling, cold stops (implemented) That the G10 thermal standoffs should be reexamined for mechanical rigidity (studied) That our thermal design might be “too conservative” even with only two cryo-coolers (reviewed and current thermal design verified) That we should verify light-tightness of the mechanical feed-throughs (implemented) That we should verify the detector operating temperature (implemented) That our mirror mounts might be simplified (this was reviewed in more detail prior to the PDR and all concluded that the mirror mounts were sound kinematic designs) That the beam splitter should be verified early in the project (implemented) That we may have difficulty getting narrow-band IR filters (quotes obtained, some filter specifications relaxed) That LN2 precool might not be needed and that precool safety issues should be studied (LN2 precool has been eliminated from the CryoNIRSP design) The committee felt that a full time instrument scientist would be preferred, perhaps one shared between CryoNIRSP and the ATST? (noted)
1.3 CDR DELIVERABLES
The list of recommended deliverables for CDR is detailed in the CryoNIRSP Critical Design Phase Statement of Work and in PMCS-0017 – Instrument Management Plan. The deliverables for the CryoNIRSP CDR are primarily contained in the three documents: Design Requirements Document (DRD SPEC-0158), the Software Design Document (SDD CN-0005) and the Critical Design Definition (CDD CN-0004) which is the document you are reading. Some deliverables will be presented in a separate document. The DRD contains the flow-down of requirements from the Instrument Science Requirements Document (ISRD Spec- 0056). The DRD contains the development and explanation of all requirements for the CryoNIRSP. Requirements are then captured and summarized in the compliance matrix (CN-0017), along with all external project requirements (see 1.6 Compliance Matrix). The CDD contains the details of the CryoNIRSP design and the following SOW deliverables: CryoNIRSP optical design . optical tolerance analysis . optical error budget . optical alignment plan CryoNIRSP hardware design . Motion stages . Optical mounts . Error budget Thermal system design Cryogenic system design
CN-0004 15 Critical Design Document (CDD)
Final block diagram Final motion control design Safety interlock design Specific hardware solutions Hardware layout Control system design report Control design & analysis Equipment list Spares list Summary specs for commercial parts Long lead items Manufacturability report Construction phase . Fabrication plan . Transportation plan
1.4 RELATED DOCUMENTS
SPEC-0001 - ATST Science Requirements Document SPEC-0056 – CryoNIRSP Instrument Science Requirements Document SPEC-0157 – CryoNIRSP Operations Concept Document SPEC-0158 – CryoNIRSP Design Requirement Document CryoNIRSP Statement of Work for Critical Design Phase CN-0005 – CryoNIRSP Software Design Document CN-0017 – CryoNIRSP Compliance Matrix CN-0020 – FM2 Mirror Drawing CN-0021 – CM5 Mirror Drawing CN-0022 – Electrical and Wiring Diagrams
1.4.1 Related ATST Project Documents
SPEC-0005 – Software and Control Requirements SPEC-0012 – ATST Acronym List and Glossary SPEC-0013 – Software Concepts Definitions SPEC-0014 – Software Design SPEC-0022 – Software Users’ Manual SPEC-0023 – ICS Specification SPEC-0036 – Operational Concepts Definition SPEC-0037 – ATST Risk Management Plan SPEC-0041 – Spares SPEC-0063 – Interconnects and Services Specification Document SPEC-0070 – ATST Standard Environmental Conditions SPEC-0080 – Polarimetry Analysis and Calibration Design Requirements Document SPEC-0088 – Camera System Software Specifications SPEC-0101 – Infrared Camera Requirements SPEC-0134 – Polarimetry Analysis and Calibration Specifications
CN-0004 16 Critical Design Document (CDD)
SPEC-0144 – Facility Instrument to Polarization Modulator Controller Specification TN-0127 – ATST Control Systems Standards TN-0147 – Polarization Issues TN-0155 – Measured and Modeled Mirror Reflectivity TN-0180 – CryoNIRSP Modulators TN-0181 – CryoNIRSP Calibration Retarder
1.4.2 Interface Control Documents and Drawings
ICD-1.1/3.1.3 Telescope Mount Assembly to Coudé Station ICD-1.3/3.4 TEOA to NIRSP ICD-3.1.3/3.4.2 Coudé Station to CRYO-NIRSP
1.5 SPECIFIC DEFINITIONS AND TERMINOLOGY
ATRC Advanced Technology Research Center CDR Critical Design Review CryoNIRSP Cryogenic-Near-Infra-Red Spectro-Polarimeter CS Control Software DLPC Dual Linear Polarization Calibration DRD Design Requirement Document FPA Focal Plane Array GCS Global Coordinate System GOS Gregorian Optics System IC Instrument Controller ICD Instrument Control Document ICS Instrument Control System IS Instrument Sequencer ISRD Instrument Science Requirement Document LSC Line Symmetry Calibration OCD Operations Concept Document PA&C Polarization, Analysis and Calibration System PDD Preliminary Design Document SOW Statement of Work SWG Science Working Group TCS Telescope Control System TEOA Top-End Optical Assembly TMA Telescope Mount Assembly
1.6 COMPLIANCE MATRIX
All the requirements in this document track back to the CryoNIRSP Design Requirements Document (DRD SPEC- 0158) through the compliance matrix in the document CryoNIRSP Compliance Matrix (CN-0017). The matrix serves two purposes: 1) It lists all DRD requirement numbers and traces to their original source document and source requirement numbers. 2) It traces all design requirements from the DRD to the sections of the design documents that provide information on compliance.
CN-0004 17 Critical Design Document (CDD)
2 INTRODUCTION TO THE CRYONIRSP DESIGN
The primary purpose of the Cryogenic Near-InfraRed Spectro-Polarimeter (CryoNIRSP) is the study of solar coronal magnetic fields over a large field-of-view at near- and thermal-infrared wavelengths. CryoNIRSP will measure the full polarization state (Stokes I, Q, U and V) of spectral lines originating on the Sun at wavelengths from 1000 nm (500 nm goal) to 5000 nm. It is the only ATST instrument with the capability of sensitively imaging the relatively faint infrared corona and the thermal infrared solar spectrum. CryoNIRSP depends on the full coronagraphic capabilities of ATST to observe both the near-limb (using ATST’s prime focus and secondary occulting) and the more distant corona and heliosphere. Its thermal infrared capabilities allow sensitive study of the solar disk in the CO lines. Near-limb capabilities allow unique observations of spicules, prominences, flares, and eruptive events in the low corona.
Grating Spectrograph In order to provide very high spectral resolving power over a very wide wavelength range, a fully reflecting grating spectrograph using a large echelle grating is required. The grating spectrograph also provides simultaneous measurement of the entire profile of the line being observed, an advantage for interpreting the data in terms of, say, the magnetic field in the solar atmosphere. This configuration does, however, require scanning the image across the spectrograph entrance slit, since the slit samples the field of view in only one direction. A multi-slit operation mode is available that efficiently uses CCD pixels for large area single-line spectropolarimetry.
Operation at Cryogenic Temperatures Observations of faint or low-contrast solar coronal sources at wavelengths longer than 1.8 µm require that the surfaces visible to the detector is cooled to cryogenic temperatures; this means that the system must be enclosed in a vacuum chamber. At thermal infrared wavelengths this design attenuates the background radiation at the detector by more than a factor of 1000. The largest remaining photon background comes from the residual emissivity of the ATST mirrors. Our design favors a relatively fast optical system, with fold mirrors in the collimator arm of the spectrograph. Out-of- field thermal radiation is eliminated in our optical design by providing a cold pupil stop within both the spectrograph and context imager optical paths. For achieving the full spectral resolution required for disk observations the articulated spectrograph pupil stop is removed from the beam. For solar disk observations the out-of-field thermal radiation is much fainter than the image, so the spectrograph cold stop is not required.
Context Imager A separate optical system is required to image the spectrograph field of view at wavelengths of coronal emission lines of interest. The context imager will have the same detector array as the spectrograph, and will observe the entire 4 x 3 arcminute field of view. Image quality will be adequate for diffraction-limited imaging at 4.65 µm. A selection of broad- and narrow-band filters will be provided. The context imager will be able to be operated concurrently with the spectrograph.
CN-0004 18 Critical Design Document (CDD)
3 OPTICAL DESIGN
3.1 CRYONIRSP OPTICAL DESIGN REQUIREMENTS
The following are the ISRD requirements for the CryoNIRSP optical design: Wavelength range: 1000 – 5000 nm Spectral resolution: 30,000 coronal observations 100,000 on-disk observations Total transmission: 10% Thermal emission: 10 millionths of disk brightness at 3934 nm Polarimetric accuracy: better than 5x10-4 Temporal modulation: dual beam polarimeter with frame rate ≥ 10 Hz Spectral modulation: time for wavelength change ≤ 10 s Spatial sampling: 0.5 arcsec/pixel for coronal observations 0.15 arcsec/pixel for on-disk observations (diffraction limited at 4.7 µm) Spatial FOV: 4 arcmin parallel to limb, 3 arcmin perpendicular to limb for coronal observations 1.5 arcmin square for on-disk observations Context imaging: sample coronal FOV with ≤ 0.5 arcsec/pixel
3.2 INTRODUCTION AND NOMENCLATURE
The CryoNIRSP instrument is composed of several subsystems which include the 11 motion controlled components indicated with (M) below: the optical system (CN-OPT) the feed optics relay system (CN-FR) the thermal system (CN-THR) the control system (CN-CSYS) the vacuum system (CN-VAC) the handling system (CN-HAN)
Acronym Instrument Assembly Sub-assembly CN-SP Spectrograph CN-SW wheel assembly CN-SFW filter wheel (M) CN-SSW slit mask wheel (M) CN-SGT grating turret (M) CN-SM2 fold-mirror CN-SM3 collimator CN-SM4 fold-mirror CN-SAM spectrograph articulated cold mask (M) CN-SM5 camera mirror (M) CN-SBS analyzing beam-splitter CN-SBC compensator
CN-0004 19 Critical Design Document (CDD)
CN-SB1 beam 1 mirror 1 CN-SB2 beam 2 mirror 1 CN-SB3 beam 2 mirror2 CN-SBP prism mirror CN-SCAM camera system CN-CI Context imager CN-PO pick-off mirror assembly (M) CN-M1 pick-off mirror CN-M1A empty mount CN-M1B slotted mirror (single) CN-M1C slotted mirror (triple) CN-CW wheel assembly CN-CWF1 filter wheel 1 (M) CN-CWF2 filter wheel 2 (M) CN-CM2 fold-mirror CN-CM3 re-imaging mirror CN-CM4 re-imaging mirror CN-CM5 camera mirror (M) CN-CCAM camera system CN-F Feed optics relay system CN-FBM beam-splitter (project provided) CN-FM1 scanning mirror (M) CN-FM2 re-imaging & focusing mirror (M) CN-CAL calibration lamps (M) CN-FW wheel assembly (M) CN-F1 ND filters, diffusor CN-SM Safety shutter CN-W1 Entrance window
Figure 1: CryoNIRSP optical layout showing the context imager in the front and the spectrograph above. Optical elements visible are [1] polarization modulator [2] dewar entrance window (W1), [3] context pickoff mirror (M1), [4] context fold mirror (CM2), [5] context relay a (CM3), [6] context relay b (CM4), [7] context camera mirror (CM5), [8] location of context filter wheel (CW), [9] context image (CCAM). Spectrograph components visible in this view are [10] entrance slit (SSW), [11] spectrograph filter wheel (SFW), [12] first fold mirror (SM2), [13] collimator (SM3), [14] second fold mirror (SM4), [15] echelle grating (SGT), and [16] spectrograph camera mirror (SM5).
CN-0004 20 Critical Design Document (CDD)
3.3 INTERFACE TO ATST
3.3.1 Polarization Modulator
The modulator hardware and software system provided by ATST and defined in the draft of SPEC-0144 will be used by the CryoNIRSP instrument to enable polarization measurements. Using TRADS and the reference time, rate and state parameters provided by the modulator interface, the CryoNIRSP instrument will synchronize data acquisition with the modulator. This will satisfy the science goals required for all polarimetric observation tasks. The acquisition of the appropriate modulators is the responsibility of the project. Technical note (TN-180) from the project originally described a set of three modulators. Informal ATST documents (30 Oct 2013 email) now imply that a single modulator spanning the full CryoNIRSP wavelength range will be provided. We understand that the modulator will meet the requirements given in the Polarimetry Analysis and Calibration Design Requirements Document (SPEC- 0080): Beam deflection < 10 arcsec
AR coating < bulk MgF2 at all wavelengths Modulation efficiency: Average >0.9 of theoretical maximum (0.52) with no wavelength <0.8 of theoretical maximum (0.46) Thermal drift for 1C <7.0·10-3 for on-diagonal elements and 3.5·10-3 for off diagonal elements of polarimeter response matrix. This is for a 1° C change in temperature in the Coudé Lab. CryoNIRSP has provisions in its warm front-end optics for the polarization modulator but its physical and electrical mounting design await further project information on its detailed specifications.
3.3.2 Calibration Retarder
Provision for GOS calibration retarders is the responsibility of the project. The calibration retarders must span the same range as the polarization modulator. The ViSP/DL-NIRSP calibration retarder includes the range of 0.5 to 2.5 μm. Thus only the range from 2.5 to 4.0 μm must be spanned by the CryoNIRSP calibration retarder. The details of the CryoNIRSP retarder can be found in TN-0181. They meet the requirements given in the Polarimetry Analysis and Calibration Design Requirements Document (SPEC-0080): Beam deflection <90 arcsec
AR coating < bulk MgF2 at all wavelengths Retardation accuracy: 90° ± 30°. Thermal drift for 1C < 1.33·10-2 for diagonal elements of the retarder Mueller matrix, <0.67·10-2for elements mapping crosstalk among Q, U, and V.
3.3.3 Mechanical Interface to Coudé Station
The CryoNIRSP design does meet the requirements given in a draft version of the Interface Control Document – Coudé Station to CryoNIRSP (ICD-3.1.3/3.4.2):
Beam height is 1250 mm above Coudé floor Use liquid coolant provided by the Coudé Station Use compressed air provided by the Coudé Station Mass is < 5000 kg
CN-0004 21 Critical Design Document (CDD)
Components do not dissipate more than 20 W of heat during operational state (calibration lamps for wavelength calibration will only be switched on when the telescope is not observing) Components do not have temperatures more than +1.5 C / -3 C different from the ambient air
CryoNIRSP will be mounted on the instrument support beams provided by the project. These beams can be moved to any position on the Coudé floor to accommodate our instrument (specified in SPEC-0011). We will use the clamps provided by the TMA contractor to mount our instrument (see Figure 2). Machinable shims with 1" starting thickness, 0.75" design thickness (0.25" nominally removed) and 0.5" minimal thickness will be placed in between the clamps and the dewar feet to allow a ±0.25 inch height adjustment.
Figure 2: Drawing of the clamps used to mount the instruments to the supporting beams in the Coudé room (ATST-DWG-00049).
We have designed our instrument to fit into the layout provided by the project (Figure 3). The compressor units for the cryo-coolers, the vacuum roughing pumps and all the electronics will be mounted in the four assigned CryoNIRSP racks (Figure 4). The project is responsible for the anti-vibration mounting of the compressors and roughing pumps. The complete rack layout including all pumps and compressors is described in the Electrical and Wiring Diagrams document (CN-0022). The helium lines between the cryo-compressors and the cold heads will be mounted on spring loaded anti-vibration mounts every three feet. Figure 5 shows floor and ceiling mounts onto which the lines can be mounted.
CDD-3.3.3 Source: ISRD-5.4 Verification: Design Review
CN-0004 22 Critical Design Document (CDD)
Figure 3: Layout of the Coudé room as provided by the project.
+x-axis
16 01
15 C 02 r y o N I R
14 S 03 P
V
T F V I
13 S 04 D P L N I R
2222..55 S P
12 05
W F
C VBI
11 06 WFC
10 07 09 08
Figure 4: Electronics bay numbering and assignment provided by the project. Outline of the provided ATST rack with 40 usable units.
CN-0004 23 Critical Design Document (CDD)
Figure 5: Spring loaded anti-vibration mounts for floor (left) and ceiling (right) mounting.
3.3.4 Optical Interface to Coudé Station
The Coudé Station to CryoNIRSP optical interface is a mirror with mounts and supports, supplied by ATST, to reflect or transmit the required wavelength range of (0.5 goal) 1-5 microns to the CryoNIRSP instrument. The beam-splitter or mirror interface location is currently not determined but it will provide a pupil image to the CryoNIRSP scanning mirror FM1. The optics following the mirror interface – except the polarization modulator – are the responsibility of the instrument builder (ICD-3.1.3/3.4.2). The ATST delivered image jitter (project email 10/3/2012) to CryoNIRSP is 0.162 arcsec. Thus all CryoNIRSP science requirements are satisfied without tip/tilt control.
3.4 CRYOGENIC TEMPERATURE OPTICS
The echelle spectrograph and the context imaging system are contained within separate dewars which are connected by a vacuum bellows. The dewars may be sealed individually by a gate valve. All optics are maintained at a temperature near 80 K. The spectrograph design uses off-axis conic sections as both collimator and camera, with the off-axis direction perpendicular to the plane of diffraction. The focal length of the camera is shorter than that of the collimator, providing the f:8 focal ratio needed to image the field onto the nominal focal plane array. A selection of entrance slits is provided in a slit wheel, and grating order selection is accomplished by a set of interference filters in a wheel located behind the slit plane. A polarizing beam-splitter is placed just ahead of the detector so that both beams of the two-beam polarimeter have a common path except for the final few centimeters. The context imager is fed by a pickoff mirror or beam-splitter that can be inserted under program control ahead of the spectrograph entrance slit. A three-mirror relay reimages the entire field onto an IR Focal Plane Array (FPA). The context imager also has a selection of filters, located in a pair of wheels placed just ahead of the FPA.
CN-0004 24 Critical Design Document (CDD)
3.4.1 Common characteristics
3.4.1.1 Image Scales and Focal Ratios The slit width is chosen to provide the required spatial sampling, then the collimator and camera focal lengths are chosen to properly illuminate the grating, and to map the slit width to the detector pixel size. An important consideration is that diffraction by the slit widens the beam. This effect can be utilized to fill the grating for high- resolution observations, but since we want to minimize the number of powered optics in the system it is prudent to control aberrations by limiting the f-ratio of the collimator to 10 or greater. For example, if the focal ratio of the beam arriving at the spectrograph is set to 20, the image scale at the slit is 388 µm/arcsec, so the slit width equivalent to 0.15 arcsec is 58 µm. Now consider the diffraction by the slit. The width of the diffraction pattern is 휆⁄푤, so at 휆 = 4.65 µm the diffraction width is 0.08 radians or f:12.5. If we set the feed to f:16, the image scale is 310 µm/arcsec, the slit with required is 푤 = 46.5 µm and the width of the diffracted beam toward the collimator is 0.1 radians or f:10. The collimator focal length needs to be chosen so that the beam width matches the projected width of the grating, so a narrower diffraction pattern means the collimator focal length must be greater, increasing the size of the vacuum dewar. On the other hand, a wider diffraction pattern increases the optical aberrations in the collimator optics. The warm feed optics must provide an image at the selected focal ratio, so this decision defines the spectrograph collimator, the feed optics and the input to the context imager. The chosen spectrograph collimator focal ratio is 18.0. A simulated spectrograph detector image in single-slit mode is illustrated in Figure 6. The image shows the coronal mode spectrum near 3.92 µm and the image geometry due to the polarizing beam splitter that provides simultaneous orthogonal polarizations.
Figure 6: A simulated polarized spectral image from the CryoNIRSP illustrating a coronal emission line and background atmospheric absorption lines with the spatial and spectral resolution of the instrument.
CN-0004 25 Critical Design Document (CDD)
CDD-3.4.1.1 Source: ISRD-5.2, 5.9, 5.10 Verification: Inspection
3.4.1.2 Mirror substrates The substrates in the dewar will be made of low expansion glass. Homogeneous differential contraction of the mirror optics, with respect to the aluminum structure, has no effect on the optical performance because of the a thermal kinematic mirror mounts, and the inhomogeneous contraction due to non-isothermal temperature gradients in the glass substrates yields errors smaller than our optical figure tolerance.
CDD-3.4.1.2 Source: ISRD-3, 5.2, 5.9, 5.10 Verification: Analysis
3.4.1.3 Mirror coatings The mirror coatings will be silver or protected silver. The many optical surfaces from the primary mirror through the instrument to the detector imply that the overall optical throughput can be significantly improved with high reflectivity mirror optics. This is especially true inside the dewar where the effects of dust and chemical degradation on the mirror throughput will be small and a silver coating directly improves the system throughput. The low sulfur environment and normal vacuum conditions suggest that non-overcoated silver will avoid some of the polarization issues of protected silver while preserving long mirror lifetimes. Unprotected silver also has a slight advantage in that the wavelength dependence of most terms in the instrument Mueller matrix is smaller – making polarization calibration more accurate. We would be interested to use the Quantum FSS99-500 coating which is the preferred choice of the project for all mirrors past M1 (TN-0155).
CDD-3.4.1.3 Source: ISRD-5.1, 5.6, 5.7 Verification: Test
3.4.1.3.1 Mirror coatings Test We made two sets of coating samples. Each set has a test substrate coated with pure unprotected silver and one sample coated with pure aluminum. One sample set stayed at ATRC for the test period of 18 months. The other set was brought to Mees observatory on the summit of Haleakala shortly after coating. Figure 7 clearly shows how both silver coated samples degraded. The unprotected silver coating is not an option for the CryoNIRSP mirrors kept in ambient atmosphere even under Haleakala summit conditions. The dewar optics may be silver-coated.
CN-0004 26 Critical Design Document (CDD)
Figure 7: Mirror coating samples where the set on the left stayed at ATRC and the set on the right was kept at the summit for 18 months. The samples in the front were coated with pure, unprotected silver. Compared to the aluminum coated samples in the back the degradation is clearly visible.
3.4.1.4 RMS Surface Roughness The surface roughness of all mirrors shall be 1.5 nm. Large angular surface brightness gradients (e.g. for observations near the limb) may be sensitive to mirror-scattered light. This degree of micro-roughness is sufficient to minimize mirror light scatter in these large optical dynamic range observations and is easily available from vendors. In this case the total root-mean-square core Strehl reduction from all CryoNIRSP optics over most of the wavelength range will be less than a few percent.
CDD-3.4.1.4 Source: ISRD-5.10 Verification: Inspection
3.4.1.5 Surface Quality All optical elements shall have a surface quality of 40-20 scratch-dig.
CDD-3.4.1.5 Source: ISRD-5.10 Verification: Inspection
3.4.1.6 Pellicle The pickoff mirror for the context imager is mounted on a four-position turret as described in section 4.1.5 below. One of these positions could be used for a beam-splitter, which will transmit most of the light and reflect a small fraction into the context imager. A series of tests were performed to determine if a pellicle could be used in the CryoNIRSP beam-splitter. Measurements indicate that a nitrocellulose membrane a few microns in thickness will
CN-0004 27 Critical Design Document (CDD) have adequate transmission throughout the CryoNIRSP wavelength range. There are two absorption bands, at 2.8 and 3.4 µm, which would be significant if the membrane thickness were greater than about 4 µm. According to our wavefront error budget, the pellicle should have an RMS surface irregularity of less than 63 nm.
Figure 8: Nitrocellulose pellicle transmission measurements.
CDD-3.4.1.6 Source: ISRD 5.2, 5.3, 5.6, 5.9 Verification: Design Review, Test, Inspection
3.4.1.6.1 Wavefront Distortion at Room Temperature Two 1 inch sample pellicles were obtained from National Photocolor. The nitrocellulose membranes are glued to the aluminum frame using two different adhesives (M3 - #4224-NF clear Pressure Sensitive adhesive, Devcon Epoxy).The wavefront distortion at room temperature has been measured in the reflected and the transmitted beam using the Zygo XPI PS interferometer at ATRC. The surfaces show a saddle structure which is probably due to the uneven machining of the frame or an uneven spread of the adhesive. The RMS surface irregularity over the whole aperture is 0.007 waves in transmission and 0.12 waves in reflection (@633 nm) and thus already very close to the budgeted 0.1 waves. More careful fabrication and using only the central part of an oversized pellicle will yield a surface irregularity that will meet the requirement.
CN-0004 28 Critical Design Document (CDD)
Figure 9: Surface irregularity measurement of a warm test pellicle.
3.4.1.6.2 Wavefront Distortion at Cryogenic Temperatures The Devcon Epoxy pellicle mounted inside the test dewar could be cooled to below 190 K for initial tests. To measure the wavefront distortion introduced by the pellicle we had to switch off the cryo-cooler. This was necessary because vibrations made measurements impossible otherwise. The vibrations caused by the vacuum pump can still be seen as ripples in the results. However, the measured cold wavefront distortion – 0.17 waves RMS - is only slightly worse than the warm results. Thus a carefully manufactured and oversized pellicle could be used as a cold beam splitter (except for the following considerations).
Figure 10: Surface irregularity measurement of a 190 K cold test pellicle (Ripples cause by vacuum system vibrations).
CN-0004 29 Critical Design Document (CDD)
3.4.1.6.3 Pellicle Performance when Cryo-cooler is operating The pellicle performance measurements in the test dewar showed that vibrations are a problem when using a pellicle. When the cryo-cooler was running, the pellicle membrane was vibrating as well and made interferometric measurements impossible. The wavefront distortions introduced by these vibrations are larger than the budgeted values. Consequently our CDR analysis leads us to abandon the use of a pellicle beam splitter in the CryoNIRSP. A solid ‘slotted’ mirror will be used as described below.
3.4.1.7 Camera The IR array detectors, electronics, and associated software will be provided by the ATST project, but the CN team is actively supporting this effort by laboratory testing of camera options. The camera and a (possible) single ASIC assembly will be directly mounted on the analyzing beam splitter assembly and its counterpart in the context imager. Vibrations of the camera assembly, induced by the cryo-cooler, have the potential to blur the images. The camera array assembly must be insensitive to vibrations: 1) to less than 2 µm RMS on timescales longer than 40 ms for the spectrograph. 2) To less than 10 µm RMS on timescales longer than 500 ms for the context imager. An IR detector has not been selected by the project. One possibility which may satisfy all requirements is the Teledyne Hawaii-2 RG 2K x 2K 18µm pixel HgCdTe array doped for operation between 0.8 – 5 µm. It is desirable to use the same type of array and readout electronics for both the spectrograph and context imager. Detailed specifications for the detector and camera are described in project documents SPEC-0101 and SPEC-0088. The CryoNIRSP will satisfy all science requirements using IR cameras which achieve these requirements. The CryoNIRSP team is anxious to support the project in the camera selection process where possible, in order to keep the instrument construction and delivery on schedule. Based on an independent study of the H2RG and its ASIC design we now understand that the H2RG frame acquisition can be externally triggered (e.g. by TRADS) with an indeterminacy of less than 0.1 millisecond. The summary of a report from an independent SIDECAR ASIC engineer that the CryoNIRSP team commissioned is included below that describes how this can be done: Summary Recommendations for H2RG Camera Option for CryoNIRSP from UCryo-ASIC Engineer Summary requirements: Timing jitter between external start-of-frame –integration must be less than 0.1 ms (this is required to achieve 10-4 polarimetric accuracy) 10 Hz frame rate must be sustainable with at least 50 ms frame integration time with external trigger Rolling shutter, with 0.1 ms exposure time must be possible with at least 10 Hz frame rate (this is required to allow disk image observations without neutral density filters) A mode allowing images with at least a 10 s integration time and 90% duty cycle must be possible (this is required to achieve read noise limited performance during coronal spectroscopy) Summary Recommendations: These requirements may be achieved with minor modifications of the JADE/SAM card and ASIC reprogramming. The optimal solution will require additional information about the Teledyne JADE card schematics showing pin access to the ASIC SIDECAR. Option 1: Use the SyncP/SyncN input pads to synchronize the external timing to the ASIC. Exposure begins by writing to register 6900 a value 8001. The fastest mechanism for writing this 16bit control word using the existing hardware must be examined Option 2: Use the FSyncP/FSyncN pads to synchronize frame signal. Synchronize word trigger with ASIC serial communication, and dynamically adjust ASIC master clock input delay and frequency. The worst-case level-of-effort for creating the optimal solution was less than 1000 hours of effort from a qualified ASIC engineer (ROM quotation).
CN-0004 30 Critical Design Document (CDD)
CDD-3.4.1.7 Source: ISRD-5.9 Verification: Test
3.4.2 Spectrograph
3.4.2.1 Description The CryoNIRSP spectrograph is based on a concept described by Gil and Simon [Appl. Opt. 22, 152 (1983)] and Schieffer et al. 2007 [Appl. Opt. 46, 3095 (2007)]. The instrument design is fully reflective for high throughput and broad spectral coverage. The collimator and camera optics are off-axis conic section mirrors whose tangential plane is perpendicular the dispersion direction of the grating. This layout distributes the field curvature and distortion in two axes, thereby flattening the focal plane. The design permits a long entrance slit and a flat focal plane. The focal lengths of the two mirrors are quite different, 2096 mm and 932 mm, in order to image the slit onto the expected detector. Because the collimator focus is long, flat fold mirrors have been added between the slit and the collimator and between the collimator and grating. The second fold mirror is tilted so its deflection is in the dispersion direction; this permits separation of the collimator and camera optics. The primary grating is an R2 echelle, with 408 mm ruled width, the largest available. This width is sufficient to support the required resolving power at 4.5 microns. The grating – and a possible second grating with different ruling frequency – are mounted on a single turret that can be rotated 360°, to change gratings or position the grating for direct (zero-order) reflection. A beam splitting polarizer located just ahead of the detector acts as the analyzer component required to do polarimetry. The corresponding modulator element is assumed to be placed in the CryoNIRSP feed optics prior to the entrance window as close to the slit as possible. The beam-splitter consists of a pair of wire-grid polarizers on IR-transmitting substrates, together with reflective optics to place the two images side by side at a common focus. Half the detector width, in the spectral direction, is allocated to each beam.
3.4.2.2 Focal Lengths We have chosen to feed the spectrograph with an f:18 focal ratio, i.e. 푓 = 72 m. Then the slit width w must be 52 µm and the diffracted beam width is .09 radians. The convolution of the diffraction pattern with the geometrical beam width is about 0.11 radians. An appropriate grating for at least the longer-wavelength part of the required spectral range has a ruling frequency of 31.6 mm-1. We use 훼 and 훽 for the angles of incidence and diffraction, and the Littrow angle 휃 = ½(훼 − 훽). For this grating, with 휆 = 4.65 µm and 휃 = −5.5°, 훼 = 56.9°. The projected grating width is 408 cos 훼 = 223 mm, so to fill the grating the collimator focal length is 2096 mm. The camera focal length must be short enough to fit the desired spatial field (length of slit image) onto the detector, and long enough to provide adequate linear dispersion of the spectrum. The first requirement, imaging a 4 arcminute field onto a detector array with 2048 18 µm pixels, leads to an effective focal length for the telescope-spectrograph system of 32 m or a focal ratio of 8.This requires that the camera focal length be 푓2 = (8⁄18) × 2096 = 932 mm. It remains to verify that the detector pixel size adequately samples the required spectral resolution, in the case of disk- mode observations. The angular dispersion of the diffracted beam is given by
sin 훽+sin 훼 퐴 = . 휆 cos 훽
The linear dispersion is then 퐴 푓2 , where 푓2 is the focal length of the spectrograph camera optics. The grating dispersion must be sufficient that the needed spectral sampling is not degraded by the detector pixel size. To get Nyquist-sampled resolution of 100000 at 휆 = 4.65 µm, we need 훿휆 = 23.3 pm. For the same grating, 퐴 = 18 1.006 at this wavelength. The camera focal length must be 푓 ≥ = 0.768 m, so the choice 푓 = 932 mm 2 1.006 × 23.3 2 will provide enough dispersion. The dispersion depends somewhat on where a given line falls within the order for a given grating, i.e. the actual value of 훽, but this is a typical value for a grating blazed at 63°, near the center of the order.
CN-0004 31 Critical Design Document (CDD)
CDD-3.4.2.2 Source: ISRD-5.2, 5.9, 5.10 Verification: Design Review, Inspection
Figure 11: Spectrograph Zemax layout.
Figure 12: Spectrograph solid model with optics and mounts (top view). The grating is located in a separate housing to reduce stray light.
CN-0004 32 Critical Design Document (CDD)
Figure 13: Spectrograph model with optics and mounts (side view).
3.4.2.3 Grating Specifications The grating is a standard aluminum-coated replica echelle grating. The possibility for a second grating is provided for future instrument upgrades to sample some lines with higher throughput or to achieve different spectral resolution. The single-grating solution was selected during early cost savings. The science impact was deemed to be acceptable (longer integration times for some lines) by the SWG. Large grating polarizations may decrease the spectropolarimetric efficiency of our measurements. Vendor grating efficiency data suggest that the grating polarization will be small when the wavelength is large compared to the ruling spacing. Grating measurements verify that our echelle grating does not induce a significant imbalance between the two beam-splitter polarized spectra. The grating substrate will be made of low expansion glass, to minimize temperature-dependent changes in grating behavior. The grating coating is aluminum. The designated grating has been obtained for testing in the ATRC labs.
Requirement Thickness Length Width Ruling Ruling Ruling Blaze Surface [mm] [mm] [mm] length width frequency angle irregularity Name [mm] [mm] [µm-1] [deg] [nm] Grating 1 70 160 420 153 408 0.0316 63.9 < 31 RMS Grating 2 70 160 420 153 408 0.079 63.4 < 31 RMS Table 1: Grating specifications. (Grating 2 might be added later to upgrade CryoNIRSP).
CDD-3.4.2.3 Source: ISRD-5.1, 5.2, 5.9 Verification: Design Review, Inspection, Analysis, Test
3.4.2.3.1 Grating Surface Irregularity Measurements The wavefront distortion introduced by the test grating was measured with our ZYGO interferometer before and after thermal cycling. Since the aperture of the interferometer is smaller than the grating, we had to do the measurements on subsections of the grating. The peak to valley variations were all smaller than 0.25 waves: the RMS errors were on the order of 0.03 waves and thus much smaller than the budgeted error.
CN-0004 33 Critical Design Document (CDD)
Figure 14: Test grating subsection surface irregularity measured in Littrow condition after thermal cycling.
3.4.2.3.2 Grating Thermal Cycling After thermal cycling, the Aluminum coating of our test grating showed no damage on visual inspection. The performed surface irregularity measurements also showed no spalling of the coating. Since such gratings were already used in cryogenic instruments and no problems were reported, we expect no issues with the grating coating.
3.4.2.3.3 Grating Scattered Light and Ghosts Sections 3.9.1 and 3.9.3 describe models of grating scattered light and recombination ghosts.
3.4.2.3.4 Grating Efficiency We measured the efficiency of our echelle grating at the ATRC. We had two 6 inch, uncoated lenses available to illuminate a large portion of our grating with a collimated beam and to focus the diffracted light onto our camera (Figure 15). The Littrow angle for this spectrograph set up was 10.75°. A monochromator served as source and delivered the light through a fiber bundle to illuminate the grating with an unpolarized beam.
Figure 15: Collimating and camera lenses used to measure the grating efficiency (left). A good portion of the grating was illuminated with this setup (right).
CN-0004 34 Critical Design Document (CDD)
The grating was mounted to a rotation stage together with a flat mirror. We used the mirror to determine the total amount of light that is illuminating the grating. Then a wire grid polarizer (Edmund Optics Ultra Broadband wire grid, fused silica, sandwiched) was placed as an analyzer in front of the detector, to measure the intensity in the Q and U polarization states. We assume a polarizer transmission of 84% - 2 fused silica to air surfaces from the substrate and the cover plate. After the mirror was replaced by the grating, we determined the Q and U polarized intensities diffracted into the most intense order. The efficiency of the grating is then calculated as the ratio of the grating and the mirror intensities for the different polarization states. Our measurements were limited by the monochromator to wavelengths of 637 and 1000 nm. 637 nm: For this setup the angle of incidence was α=53° and the most intense diffraction order was m=88. The chosen wavelength is 1/3 order from the blaze wavelength (634.5 nm) for this order. Table 2 summarizes the measurements. From the mirror measurements we can calculate a total mirror efficiency of 95%. This could indicate the polarizer transmission is about 4% lower than estimated. However in the relative grating efficiency calculation the polarizer transmission cancels when we calculate the ratio. The grating efficiency results show that the p-polarized light is about 3% more efficiently diffracted by the grating than s-polarized light. The total efficiency is about 28% and would be even higher for wavelengths closer to the blaze wavelength.
Polarization Mirror efficiency Grating efficiency state (no analyzer / with analyzer) (grating with analyzer / mirror with analyzer) Q+ 49.2% 29.9% Q- 46.2% 26.2% U+ 47.5% 27.8% U- 47.4% 28.2% Table 2: Summary of the grating efficiency measurements for 637 nm.
1000 nm: For this setup the angle of incidence was α=53.25° and the most intense diffraction order was m=56. The chosen wavelength is 1/6 higher than the blaze wavelength (997 nm) for this order. Table 3 summarizes the measurements. From the mirror measurements we can calculate a total mirror efficiency of 96%. This could indicate the polarizer transmission is about 3% lower than estimated. However in the relative grating efficiency calculation the polarizer transmission cancels when we calculate the ratio. The grating efficiency results show no difference between the p- and s-polarized light. The total efficiency is about 32% and would be even higher for wavelengths closer to the blaze wavelength.
Polarization Mirror efficiency Grating efficiency state (no analyzer / with analyzer) (grating with analyzer / mirror with analyzer) Q+ 47.1% 31.7% Q- 48.4% 31.7% U+ 46.2% 31.0% U- 50.0% 31.9% Table 3: Summary of the grating efficiency measurements for 1000 nm.
Conclusion: Our grating measurements set a lower efficiency boundary of about 56 %. Closer to the blaze wavelength the efficiency will increase as the data published by Gratinglab in Figure 16 shows. For longer wavelengths, as expected, the absolute efficiency will increase whereas the difference between p- and s-polarized light decreases.
CN-0004 35 Critical Design Document (CDD)
Figure 16: Efficiency curve published by Gratinglab for the B055 master grating (ruling frequency 31.6 lines/mm, blaze angle 63.9°). 35 nm away from the blaze wavelength, the efficiency drops below 5%.
3.4.2.4 Mirror Specifications
Name SM2 (fold 1) SM3 (collimator) SM4 (fold 2) SM5 (camera) Requirement Clear aperture [mm] 140x110 200x206 160x200 160x152 Size [mm] 156x122 200x228 176x220 176x168 Center Thickness [mm] 25 30 30 25 Radius of curvature[mm] 4192.00 ± 13.83 1861.465 ± 18.615 Plano/concave/convex Plano Concave Plano Concave Conic constant -1.2650 ± 0.02 -0.8930 ± 0.02 Off-axis distance [mm] 403.79 125.00 (to element center) De-center tolerance ±2 mm Surface irregularity <31 nm RMS Fixed/Adjustable/Motorized A A A M Table 4:Optical specifications for the spectrograph mirrors.
CDD-3.4.2.4 Source: ISRD-5.2, 5.9 Verification: Design Review, Analysis, Test, Inspection
CN-0004 36 Critical Design Document (CDD)
3.4.2.5 Alignment Sensitivity See also section 3.7 and 3.10. Spectrograph mirrors shall be alignable to within ±0.5 mm of their nominal position in the z-direction. Decenters of up to 2 mm are acceptable. Provision for angular alignment to ±1 mrad (±0.06°) about the X and Y axes shall be made. The tolerance analysis shows that positional uncertainties of this size cause acceptable bore sight error and minimal degradation of the image quality. The mirror surfaces will have conical fiducials that allow the optical coordinate system of each optic to be readily mapped in the global instrument coordinate system with coordinate measuring instruments.
CDD-3.4.2.5 Source: ISRD-5.2, 5.9 Verification: Analysis
3.4.2.6 Cold Pupil A non-reflective (diffusive black) cold pupil made of aluminum is located at SM4 to eliminate out-of-field thermal radiation. It is mounted on a motorized stage and can be removed from the beam path during full spectral resolution disk observations. It has a rectangular size of 160 x 120 mm with round edges.
Requirement Angular accuracy [°] 1 Decenter [mm] 1 Axial accuracy [mm] 1 Table 5: Spectrograph cold pupil specifications.
CDD-3.4.2.6 Source: ISRD-5.4 Verification: Design Review, Analysis
3.4.2.7 Slit mask Specifications Slits will be cut into a standard 25 × 101 mm invar substrate. Slits for high-resolution disk observations will have lengths equivalent to 120 arcsec on the sun; coronal slits will have 233 arcsec length. The slit widths are matched to the required spectral resolving power and spatial sampling. The chosen substrate might change, depending on the manufacturing process.
Requirement Use Number of slits Separation Length Width (multi slits) [mm] [mm] [] Name Slit 1 Disk observations 1 42 52 Slit 2 Corona 1 1 81 175 Slit 3 Triple slit 3 8.8 81 175 Slit 4 Double slit 2 13.8 81 175 Slit 5 Empty slot Slit 6 Dark mask Co-registration Beam-splitter image Non-redundant Various N/A N/A Slit Mask co-registration pinholes Table 6: Slit Specifications.
CN-0004 37 Critical Design Document (CDD)
CDD-3.4.2.7 Source: ISRD-5.2, 5.9 Verification: Design Review, Test, Inspection
Figure 17: Detailed drawing of the triple slit.
3.4.2.8 Filter Specifications The CryoNIRSP operates over nearly one decade in wavelength from the visible to the mid-IR. Part of this spectral region is partially extinct by the atmosphere as illustrated in Figure 18. Most of this spectral “real-estate” has yet to be fully explored and the CryoNIRSP includes a range of IR filters to sample this broad spectral region.
Figure 18: The Haleakala sky brightness and K-corona.
CN-0004 38 Critical Design Document (CDD)
Sky brightness (thick line) and K-corona (thin lines at two limb distances) over the wavelength range of the CryoNIRSP. Major atmospheric extinction band are apparent near 1.4, 1.9, 2.7 and 4.3 μm. Some of the brighter observed and predicted coronal emission lines are indicated by “diamonds”.
Blocking filter wavelengths described below are nominal. Additional or a restricted set of blocking filters may be negotiated later during instrument operation.
Requirement Transmission ripple tolerance over pass band (standard deviation) <10% Peak transmission >80% Blocking from 400 to 6000 nm >5 OD Surface irregularity <633 nm RMS Band pass tolerance 20% of nominal band pass Thickness <5 mm Dimensions 91 × 25 mm Clear aperture 90% of physical Table 7: Spectrograph filters common requirements.
Center CW tolerance FW # Filter Name Wavelength FWHM Min Trans Shape (nm) tolerance (nm) 1 Fe XIV 530 1 5.5 1.0 80 4 cavity 2 Fe X 637 2 8.0 1.5 80 4 cavity 3 H I 656 2 8.5 2.0 80 4 cavity 4 Fe XI 789 3 12.0 2.5 80 4 cavity 5 He I, Fe XIII 1080 5 22.5 5.0 80 4 cavity 6 S IX 1252 7 30.6 6.0 80 4 cavity 7 Si X 1430 9 35.4 7.0 80 4 cavity 8 Fe IX 2218 25 97.7 20.0 80 4 cavity 9 CO 2326 10 42.7 9.0 80 4 cavity 10 Si X 2580 12 51.5 10.0 80 4 cavity 11 Mg VIII 3028 17 70.0 15.0 80 4 cavity 12 Si IX 3935 30 123.6 25.0 80 4 cavity 13 CO 4651 40 170.7 35.0 80 4 cavity 14 TBD 15 Dark Non-reflective aluminum stop 16 Empty slot Table 8: Spectrograph filters specifications
CDD-3.4.2.8 Source: ISRD-5.1 Verification: Design Review, Test, Inspection
3.4.2.9 Spectrograph Flux Estimates The spectrograph design attenuates the environmental thermal flux by more than a factor of 1000, such that the dominant background contribution in some observing modes may be the warm ATST mirror optics. We assume a per mirror thermal emissivity of 2%. The mirror reflectivity was also assumed to be 98% (average) at all wavelengths for
CN-0004 39 Critical Design Document (CDD) all mirrors (including ATST telescope mirrors). Allen (1973) IR solar flux values were assumed. CryoNIRSP optical component transmission efficiencies are described in Table 17. Wavelength independent average values were used for the calculations in this section and an average detector QE of 80% was assumed here. With a pixel scale of about 165 mA and spatial scale of about 0.12” the expected continuum coronal flux should be between 500-2000 photo- electrons/s/pixel at the ISRD required flux sensitivity. The thermal background approaches and exceeds this level at wavelengths beyond about 4.5 micron (dependent on the warm ATST mirror emissivity achieved). Disk observations will yield a detected flux of about 107 photoelectrons/s/pixel. These flux levels are accessible to the expected exposure times (minimum of about 1 ms) and well capacity (105). To achieve these background levels a cold pupil aperture stop is oriented at SM4 during coronal observations.
Figure 19: Flux Levels for the spectrograph. Solid lines show the expected per pixel continuum detected signal for disk and corona. The broken lines show the expected background signal from the ATST and Coudé mirrors assuming 2% and 0.5% emissivity.
3.4.2.10 Analyzing Beam-splitter To achieve the required polarimetric accuracy of 5 × 10−4, a dual-beam polarimeter is necessary, that is, the analyzer transmits two orthogonal polarizations that are recorded simultaneously. The CryoNIRSP layout assumes rotating wave-plate modulators as required to cover the wavelength range. The beam splitting analyzer is placed at the end of the system, just before the detector (see Figure 44). The strong advantage of this position is that the optical system is common to both polarizations except for the last few reflections before the detector, facilitating co-registration of the two images and ensuring that aberrations are the same in both. Its disadvantage is that there are many optical surfaces that can affect the Mueller matrix of the system.
CN-0004 40 Critical Design Document (CDD)
The polarizing beam-splitter is based on micro-wire polarizers (Moxtek, Inc.), with both the transmitted and reflected beams utilized. A second polarizer is added in the reflected beam from the primary polarizer, both to enhance the polarization contrast and to equalize the optical paths. The polarizer substrate will be chosen for high transmission from 0.5 to 5 µm. The reflected beam from the second wire grid polarizer is absorbed in a light trap. The final fold mirrors are implemented from a single polished aluminum 90-degree prism that is coated with electroless nickel and gold. All mirrors are held by fixed mounts and the final beam-splitter alignment is accomplished by tilting and displacing the final dual-mirror fold prism. Two spatially registered, orthogonally polarized, spectra are formed on a single 2048 × 2048-pixel detector (the current baseline model) using this geometry.
Optics Name SB1 SB2 SB3 Prism Requirement Clear aperture [mm] 54 x 31 54 x 47 54 x 31 20 x 54 Size [mm] 60 x 34 60 x 52 60 x 34 22 mm reflecting surfaces 60 mm height Center Thickness [mm] 6 7 6 Plano/concave/convex Plano Plano Plano Plano 90° prism De-center tolerance ±0.5 mm Surface irregularity <79 nm RMS Fixed/Adjustable/Motorized F F F A Table 9: Optical specifications for the analyzing beam splitter mirrors. CDD-3.4.2.10 Source: ISRD-3, 5.1, 5.2, 5.3, 5.6, 5.9 Verification: Analysis, Design Review, Test
3.4.2.10.1 Polarizer Performance Measurements The novel polarizing beam-splitter of the CryoNIRSP depends on realizing the advertised properties of broadband wire grid polarizers. The CryoNIRSP beam-splitter design is based on the performance of the “Ultrabroadband” Thorlabs (e.g. WP12L-UB). Polarizer (which is manufactured by Moxtex Inc.). We obtained polarizer samples for laboratory optical testing. These use a UV-grade fused silica substrate and were obtained without a protective window over the metallic wire-grid. The transmission and reflection profiles for s- and p-polarization light was measured using our Varian FTS and a Cary spectrophotometer. Wire grid polarizers work far in a “vector diffraction” regime and their transmission properties depend not just on the linear polarization state of the incident light, but on the azimuth of the incidence angle with respect to the wire grid orientation. Figure 20 and Figure 21 show the polarized transmission through a single polarizer versus tilt angle for substrate tilt axis oriented parallel and perpendicular to the wire grid. These data are obtained from single-beam FTS measurements.
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Figure 20: FTS-measured polarized light transmission of UV-grade fused silica wire grid polarizer for incidence angles up to 30 degrees. Here the tilt axis of the substrate is perpendicular to the grid lines.
Figure 21: Same as previous figure but the tilt axis is parallel to the wire grid elements.
The wire grid polarizers have diminished, and wavelength dependent, transmission properties for incident light directed more than 20 degrees from the normal direction in the plane perpendicular to the wire grids. The reflected light from the wire grids does not have a strong wavelength dependence. Figure 22 illustrates the unpolarized reflection coefficient.
CN-0004 42 Critical Design Document (CDD)
Figure 22: Reflected flux from polarizer, inclination 30 degrees (perpendicular to wire grid).
At shorter wavelengths the transmitted intensities are described in Figure 23 with data from a laboratory grating spectrophotometer. Here the parallel and crossed polarized transmitted beam intensities have been measured out to about 2.5μm. The extinction ratio over this range is no larger than about 0.001 – limited by the alignment accuracy in these measurements. At short wavelengths, down to 500 nm, the wire grids perform well. For coronal measurements at longer wavelengths, including the 3.9 μm SiX line, these fused silica substrates will meet CryoNIRSP and ATST efficiency requirements. We note that the transmission dip at 2.6-2.7 μm does not correspond to important solar spectral features and lies within a deep atmospheric absorption band. Disk observations in the CO fundamental band near 4.6 μm will suffer some transmission loss but this will have minimal science impact because maximum non- saturated CryoNIRSP integration times will be smaller than the expected IR array read time, thus larger integration times (by about a factor of 2) will be inconsequential.
Figure 23: Measured cross- and parallel-polarization transmission through the fused-silica wire grid polarizer
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3.4.3 Context Imager
3.4.3.1 Description The context imager system is intended to provide band pass-filtered images of the field of view at frequent intervals during spectropolarimetric observations. The context imager optics are all reflective except for filters and polarizers. The nominal detector is identical to the detector used in the spectrograph. Light is delivered to the context imager by means of a mirror or slotted mirrors that can be inserted into the beam, within the spectrograph dewar and ahead of the spectrograph entrance slit. The slotted mirrors allow the spectrograph slit to be fully illuminated, but simultaneously direct the outer region of the field to the imager. Thus light into CryoNIRSP can either be directly to the imager, the spectrograph, or simultaneously to both optical arms of the instrument. A flat mirror redirects the light beam parallel to the spectrograph axis. The f:18 image delivered by the feed optics is located between the two flat mirrors; it is re-imaged at f:8 by a three-mirror relay. The relay includes a pupil image located on CM4 that allows the use of a cold stop for the context imager.
Figure 24: Context Imager Optical Layout. Mirror CM1 is mounted on a turret that permits selecting a plane mirror, either of the two slotted plane mirrors or an open position for context-only, shared or spectrograph-only observations. A cold stop is located on CM4.
Figure 25: Context imager solid model with optics and mounts. Filter wheel and detector array are surrounded by a cold radiation shield.
CN-0004 44 Critical Design Document (CDD)
3.4.3.2 Mirror Specifications
Optics Name CM2 CM3 CM4 CM5 Requirement Clear aperture [mm] 108x125 152x152 58x58 260x256 Size [mm] 120x140 170x170 64x64 290x290 Center Thickness [mm] 25 35 12 60 Radius of curvature 2000.00 ± 20.00 829.92 ± 8.30 1426.59 ± 14.27 Plano/concave/convex Plano Concave Convex Concave Conic constant 0.1391 ± 0.05 1.5631 ± 0.3 0.1094 ± 0.005 Off-axis distance [mm] 150.00 0.00 150.00 De-center tolerance ±1 mm Surface irregularity <63 nm RMS Fixed/Adjustable/Motorized A A A M Table 10: Optical specifications for the context imager mirrors.
The pick off mirror CM1 is either a non-slotted or a slotted plane mirror. Two different slot sizes are available to allow single or triple slit measurements. The slot size is the same all the way through the mirror. It is cut at a 45 degree angle to follow the spectrograph beam path. Figure 26 shows a Zemax model of the slotted single slit pick off mirror.
Optics Name CM1 CM1 slotted (single slit) CM1 slotted (triple slit) Requirement Clear aperture [mm] 90x117 90x117 90x117 Size [mm] 100x130 110x130 110x130 Center Thickness [mm] 20 20 20 Slot size [mm] - 78x18 78x44 Slot end Semi circular Semi circular Plano/concave/convex Plano Plano Plano De-center tolerance ±1 mm Surface irregularity <63 nm RMS Fixed/Adjustable/Motorized M Table 11: Optical specifications for the pick off mirrors.
Figure 26: A Zemax non-sequential ray trace model of the slotted single slit pick off mirror. The right-panel color scale indicates that 18.7 arcsec from the slit the image is 50% vignetted.
CN-0004 45 Critical Design Document (CDD)
CDD-3.4.3.2 Source: ISRD-5.9, 5.14 Verification: Design Review, Analysis, Inspection
3.4.3.3 Alignment Sensitivity See also section 3.7 and 3.10. Context imager mirrors shall be aligned to within ±0.5 mm of their nominal position in the z-direction. Decenters of up to 1 mm are acceptable. Provision for angular alignment to ±0.5 mrad (±0.03°) about the X and Y axes shall be made. Tolerance analysis shows that positional uncertainties of this size cause acceptable bore sight error and minimal degradation of the image quality. The mirror surfaces will have conical fiducials that allow the optical coordinate system of each optic to be readily mapped in the global instrument coordinate system with coordinate measuring instruments. CDD-3.4.3.3 Source: ISRD-5.14 Verification: Analysis
3.4.3.4 Cold Pupil A non-reflective (diffusive black) cold pupil made of Aluminum is located at CM4 to eliminate out-of-field thermal radiation. It is permanently attached to the mirror.
Requirement Angular accuracy [°] 1 Decenter [mm] 1 Axial accuracy [mm] 1 Table 12: Context imager cold pupil specifications. CDD-3.4.3.4 Source: ISRD-5.4 Verification: Analysis
3.4.3.5 Filter Specifications Band pass filters will be provided to permit context images at the wavelengths of the spectrograph observations. Two filter wheels in tandem arrangement are provided, each with nine filter positions. One possible configuration is described in the following table. Filters will be designed to function properly in the f:8 beam just ahead of the context imager detector array and will be antireflection coated and may be tilted to move secondary ghost images to one side of the core PSF.
Requirement Transmission ripple tolerance over pass band (standard deviation) <10% Peak transmission >80% Blocking from 400 nm to 6000 nm >5 OD Surface irregularity <31 nm RMS Band pass tolerance 20% of nominal band pass Thickness <5 mm Dimensions ø 56 mm, +0, -0.5 Clear aperture 90% of physical Antireflection Coating <0.1% over pass band Table 13: Context Imager filters common requirements.
CN-0004 46 Critical Design Document (CDD)
Center Filter CW tolerance Effective Band #/wheel Wavelength Shape Comment Name (nm) pass (nm) (nm) 1-a Green Line 530.27 0.1 0.2 3 cavity 2-a Halpha 656.28 0.1 0.2 3 cavity 3-a R 700 10 220 2 cavity Calibration/PSF 4-a FeXIII(1) 1074.7 0.2 1 3 cavity 5-a HeI 1083.0 0.2 1 3 cavity 6-a Open 7-a J 1250 5 20 2 cavity Calibration/PSF 8-a K 2200 20 480 2 cavity Calibration/PSF 9-a SiIX 3923 5 20 2 cavity 1-b M’ 3950 5 20 3 cavity SiIX/CO cont. ref. 2-b CO 4651 5 20 3 cavity 3-b ND 500-5000nm Density TBD 4-b Wire grid 500-5000nm 5-b Open 6-b Open 7-b Open 8-b Open 9-b Dark (non-reflective aluminum stop) Table 14 Possible context imager filter configuration. The two 9-position context imager filter wheels allow 81 possible combinations. The configuration above allows QU polarimetry from visible to thermal IR wavelengths, selected coronal and disk line observations with continuum image reference observations, and neutral density observations of lines and continuum from visible to thermal wavelengths.
CDD-3.4.3.5 Source: ISRD-5.1, 5.6, 5.7, 5.16 Verification: Design Review, Analysis, Test, Inspection
3.4.3.6 Imager Light Flux Levels The cryogenic imager design attenuates the environmental thermal flux by more than a factor of 1000, such that the dominant background contribution in some observing modes may be the warm ATST mirror optics. Thus, the light flux and thermal background calculations are somewhat dependent on the, as yet undetermined, emissivity and reflectivity of the ATST optical mirror train. In our calculations we will assume a net emissivity per mirror of 2% and a reflectivity (per mirror) of 98%. Under these conditions the expected flux in coronal observing mode (10-5 of disk intensity; source SRD) is about 106 photons per second per pixel with a 10 nm bandwidth filter. Calculations here assume the same telescope and instrument efficiencies and pixel illumination geometry described in section 3.4.2.9. Due to the ATST mirror thermal emissivity the background reaches the coronal signal level at wavelengths beyond about 4.5 microns. Disk observations through the same filter would yield between 1012 and 1010 photons/s/pix from near- to thermal-IR wavelengths. In disk observing mode we require a smaller band pass, and/or neutral density filter with short exposure time to limit the flux to less than 105 photo-electrons/pix/exposure. The background from out- of-field thermal emission from the telescope structure is minimized with a cold pupil stop fixed on CM4. Light flux levels are monitored using a broad-band diode that continuously observes scattered light from the face of the detector. Unsafe light flux levels detected by this diode trigger the safety shutter.
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Figure 27: Context imager flux levels. Solid lines show the per pixel detected signal level (photo electrons) for continuum disk and corona observations assuming a 10 nm band pass filter. The dotted line shows the expected thermal background assuming 2% ATST mirror emissivities.
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3.5 ROOM TEMPERATURE OPTICS: FEED OPTICS
3.5.1 Description
The 4 × 3 arcminute field delivered by the telescope must be presented to the CryoNIRSP entrance slit at a focal ratio of f:18. This focal ratio is determined by engineering considerations in the spectrograph collimator optics; see section 3.4.1.1 for the discussion. In addition, it must be possible to scan the field across the slit in the 3 arcminute direction. These functions are achieved by means of a set of three mirrors in the Coudé room, external to the CryoNIRSP dewar. The first of these, provided by the facility, is a flat mirror that intercepts the beam between M9 and M10. The pupil image at M10 is thereby redirected to a location convenient for placement of CryoNIRSP on the Coudé floor. The scanning mirror FM1 is located at this pupil image; it is tilted under motor control about a vertical axis to precisely control the angle of the beam reflected from FM1. Finally, mirror FM2 images the field onto the spectrograph slit plane, and converts the angular deviation at FM1 to a linear translation. FM1 is slightly convex in order to permit the image to be telecentric, while keeping FM1 outside the space of the dewar. Figure 3 shows how the CryoNIRSP and its feed optics might be placed on the Coudé platform. The feed optics layout is shown in Figure 28. All warm optics, except FM2, are mounted on a platform in front of the two dewars. FM2 is mounted on a pedestal that is similar in design as the pedestal of M9. A motorized linear stage allows the wavelength calibration lamps (a halogen flat-field lamp and Th-Ar line source) to be moved in and out of the instrument optical path. A filter wheel containing pellicle neutral density filters for disk observations, diffusors and wire grid calibration polarizers, is placed prior to the project-provided polarization modulator. A hardware actuated safety shutter is used to protect internal CryoNIRSP optics from unexpected high light-flux levels in case of telescope pointing failure. Figure 29 shows the detailed layout for the calibration lamps, warm filter wheel, modulator and safety shutter.
CDD-3.5.1 Source: ISRD-5.2, 5.9, 5.10 Verification: Design Review
Figure 28: Feed Optics Optical Layout. M9 is a facility fold mirror; FBM is a facility mirror that is inserted to direct light to CryoNIRSP. The CryoNIRSP scan mirror is FM1 and the reimaging and focusing mirror is FM2.
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Figure 29: Detailed layout for the calibration lamps, warm filter wheel, modulator and safety shutter. Top: The calibration lamps are mounted on a linear stage a can be moved into the instrument beam path. Bottom: The project provided polarization modulator will be placed between the warm filter wheel and the safety shutter.
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3.5.2 Modulator
The modulator is a critical component of the CryoNIRSP spectropolarimeter, that will be provided by the ATST project. This will be located in front of the CryoNIRSP dewar in the warm optics system. The software interface to the Polarization Modulation Controller is described in SPEC-0144 (draft). The CN software systems interact with the PMC and timing (TRADS) generator according to SPEC-0144 in order to control the synchronization of the IR camera frame acquisition. Typical control sequences are described in the CryoNIRSP SDD. The hardware design of the PM is described in TN-0180 (draft). The general performance requirements for the modulator design are contained in that document, SPEC-0080 and SPEC-0134. The specified thermal sensitivity, polarization efficiency, reflective ghost properties, beam deviation and overall wavelength coverage will satisfy CryoNIRSP and ATST requirements. We note that the proposed TN-0180 design will achieve a 105 mm clear aperture. This will produce some vignetting at the edge of the CN field (13% at the ends of the slit at 4’, and none with a 3.5’ diameter FOV). The CryoNIRSP warm optics optical design allocates 120 mm of optical path length for placement of the modulator in the beam ahead of the dewar. Accurate polarimetric calibration of the modulator is necessary to satisfy the ATST polarimetry requirements. The modulator must be precisely calibrated (polarimetrically) and stable to temperature and operational timing jitter. For CryoNIRSP the most stringent timing jitter conditions occur for disk observations at the maximum cadence. Assuming 16-state demodulation with a CryoNIRSP 10 Hz camera sample rate, and following TN-0137 we require angular uncertainty of the modulator during IR array frame acquisition of less than about 0.14 degrees. A timing “jitter” specification of 0.1 ms synchronized to the modulator achieves this requirement with some margin. For most IR array detector options this level of frame acquisition synchronization is readily achievable.
CDD-3.5.2 Source: ISRD-5.2, 5.9 Verification: Analysis
3.5.3 Mirrors/ substrate
The mirror substrate shall be BK7 glass.
CDD-3.5.3 Source: ISRD-5.2, 5.9 Verification: Analysis
3.5.4 Mirror coatings
The mirror coating shall be protected silver. Throughput should be maximized for visible to 5 micron performance. See section 3.4.1.3.1 for test results. We would be interested to use the Quantum FSS99-500 coating which is the preferred choice of the project for all mirrors past M1 (TN-0155).
CDD-3.5.4 Source: ISRD-5.2, 5.9 Verification: Test
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3.5.5 RMS Surface Roughness
The large spatial optical surface brightness gradients near the solar limb may be measurably contaminated by mirror- scattered light. To minimize large-angle scattered light from the Sun’s disk, our strategy is to minimize the mirror micro-roughness subject to common vendor practices, and to carefully calibrate the large-angle system PSF. Thus, the surface roughness of all mirrors shall be less than 1.5 nm. This degree of micro-roughness should be sufficient to minimize mirror light scatter in these large optical dynamic range observations and is easily available from vendors.
CDD-3.5.5 Source: ISRD-5.2, 5.9 Verification: Inspection
3.5.6 Surface Irregularity
Our tolerance analysis, based on the image resolution requirements, implies that a 63 nm RMS surface irregularity will not significantly degrade the imaging performance of the CryoNIRSP.
CDD-3.5.6 Source: ISRD-5.2, 5.9 Verification: Inspection
3.5.7 Surface Quality
All optical elements shall have a surface quality of 40-20 scratch-dig.
CDD-3.5.7 Source: ISRD-5.2, 5.9 Verification: Inspection
3.5.8 Mirror Specifications
Optics Name FBM FM1 FM2 Requirement Clear aperture [mm] 320 205 353 Size [mm] 350 220 400 Center Thickness [mm] 50 30 60 Radius of curvature[mm] 20000 ± 100 7067.5 ± 35.3 Plano/concave/convex Plano Convex Concave Conic constant 0.0 -1.000 ± 0.005 Off-axis distance [mm] 0.0 0.0 110.0 De-center tolerance ±2 mm Substrate BK7 Surface irregularity < 63 nm RMS Fixed/Adjustable/Motorized A M M Table 15: Optical specifications for the feed optics mirrors.
CDD-3.5.8 Source: ISRD-5.2, 5.9 Verification: Design Review, Analysis, Inspection
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3.5.9 Scan Mirror
The scan mirror is an off the shelf motorized dual gimbal mount, driven by standard NEMA23 stepper motors (see section 4.2.2 for mount specifications). The precision required is driven by the resolution specified for disk mode observations. The slit width for this mode is 52 µm; a step of this size requires an angular mirror step of 7.38 µrad. The total range of travel required is set by the size of the coronal field, 60 mm at the slit plane. To scan the entire field requires a mirror rotation of 8.5 mrad or 0.5 degree. CDD-3.5.9 Source: ISRD-5.2, 5.9 Verification: Analysis
3.5.10 Focusing Mirror
In order to accommodate thermal changes in the post-M2 ATST optical path, CryoNIRSP requires a focus mechanism to maintain proper transfer focus of the Gregorian image onto the slit. Thus our feed optics mirror FM2 will be mounted on a linear stage with a travel range of 25 mm. CDD-3.5.10 Source: ISRD-5.2, 5.9 Verification: Design Review
3.5.11 Filter Specifications
For CryoNIRSP-only polarimetric calibrations and to provide longer integration (anti-aliased) full-disk observations a filter wheel outside of the dewar is provided. For disk observing modes a pellicle neutral density filter can be rotated into the beam after FM2. This filter transmits about 1 W of the 65 W that would be incident on CryoNIRSP in disk observing modes. The RMS surface irregularity shall be less than 315 nm. Polarimetric calibration is achieved using two wire-grid polarizers that may be rotated into the beam. Flat-field and emission-line wavelength calibration use a ground-glass diffuser in the filter wheel. During normal observations the filter wheel is set to an open position. Requirement Surface irregularity <315 nm RMS (for pellicles and wire grids) Thickness <10 mm Dimensions ø 6 inches Clear aperture 90% of physical Table 16: Warm optics filters common requirements.
#/wheel Filter Name Description 1 ND1 Reflective pellicle neutral density filter 2 OD (National Photocolors) 2 ND2 Reflective pellicle neutral density filter 3 OD (National Photocolors) 3 diffusor Ryotek IR grad (low OH fused silica) 4 wire grid Q+ 5 wire grid Q- 6 open 7 open Table 17: Inserts for the warm optics filter wheel. CDD-3.5.11 Source: ISRD-3 Verification: Design Review, Analysis, Inspection
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3.5.12 Calibration Lamps
We use a Thorium Argon hollow cathode lamp to perform the CryoNIRSP wavelength calibration. This source offers well documented lines over the entire instrument wavelength range. The lamp will be powered by a commercial power supply from Newport (model nr. 69907).
Figure 30: Overview spectrum of a Th-Ar lamp. Wavelength range is 715-5000 nm (14,000-2000 cm–1). The line intensity is given in arbitrary units. Longward of 2500 nm (<4000 cm–1) thermal emission from the hot cathode introduces a continuum in the otherwise pure emission-line spectrum (Figure from Kerber et al. 2008).
We use a Tungsten halogen lamp to illuminate the diffusor in order to perform flat field calibrations. The continuous lamp spectrum covers the entire instrument wavelength range. The lamp will be powered by a commercial power supply from Newport (model nr. 69907).
3.5.13 Entrance Window
See also section 3.9.4. The material used for the entrance window will be CaF2 since it provides the best mix of availability, mechanical strength, transmission and spurious reflection intensity. Sapphire has been disregarded because of its high refractive index which means that a double-bounce ghost image will be formed about three times the window’s thickness ahead of the slit plane. The ghost intensity is about 0.5%. CaF2 has a refractive index around 1.43, thus the ghost is formed 50 mm ahead of the slit plane, with an intensity of 0.1%. By tilting the window by 1.67°, the ghost image is moved laterally by 0.615 mm. Thus we are able to minimize scattered light by placing the large intensity gradients on the ghosted side when observing near the limb.
CDD-3.5.13 Source: ISRD-5.1, 5.2, 5.4, 5.9 Verification: Design Review, Inspection, Analysis
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Optics Name W1 Requirement Clear aperture [mm] 140 Size [mm] 155 Center Thickness [mm] 25 mm Plano/concave/convex Plano De-center tolerance ±2 mm Substrate CaF2 Surface irregularity <315 nm RMS Coating Uncoated Fixed/Adjustable/Motorized F Table 18: Optical specifications for the entrance window.
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3.6 OPTICAL PERFORMANCE ANALYSIS
3.6.1 Spectral Resolution
3.6.1.1 Disk Observation Mode
Requirement For disk observations at 4.65 µm wavelength, the resolving power must be at least 100,000, with a slit length of at least 90 arcsec on the sun.
Performance Analysis The requirement driving the spectrograph layout is that it delivers a spectral resolving power of at least 100,000 at a wavelength of approximately 4.65 µm, along with a spatial resolution equal to the telescope diffraction limit at the same wavelength. In addition the assumed detector geometry of 2048 × 2048 pixels with 18 µm pitch affects the scale, and a requirement that for coronal observations the effective slit length is 4 arcmin on the sky, affect focal length choices. The spectral resolving power is limited by the width and incidence angle of the grating: the angular width of the line profile can be no better than the single-slit diffraction pattern defined by the projected width of the grating. One expression is W(sin 훽 + 푠푖푛훼) 푅 = 0 휆 where 훼 and β are the angles of incidence and diffraction, respectively, and W is the grating width. This relationship is independent of the groove width and the diffraction order, as they are proportional to each other. The CryoNIRSP grating has 푊 = 408 mm and a 64° blaze angle. This is the widest standard grating available, and increasing the blaze angle increases the angular dispersion but doesn’t affect the limiting resolution very much. The difference between 훼 and 훽, called 2휃, is defined by the layout requirements of the spectrograph. With 휃 = −5.5°, 훼 = 55.5° and 훽 = 66.5°, 푅0 = 155,000 at 휆 = 4.651 휇m (the profile width is 30 pm in wavelength). This should be adequate, but it must be convolved with the detector pixel size, the entrance slit width, and the aberration function of the spectrograph. The angular dispersion is given by 푑훽 sin 훽 + sin 훼 퐴 = = 푑휆 휆 cos 훽 which, with the wavelength in µm, is equal to 1.01 µrad/pm (to pick a convenient set of units). Then with the camera focal length 푓2 = 0.932 m, the linear dispersion is 퐴푓2 = 0.94 µm/pm. The detector pixel of 18 µm then samples 19 pm of spectrum. The dispersion measured at the entrance slit is less than that on the camera side: 푑훼 cos 훽 퐵 = = 퐴 = 0.69 푑휆 cos 훼 and with 푓1 = 2.096 m the linear dispersion is 퐵푓1 = 1.45 µm/pm. The slit width of 52 µm corresponds to a wavelength interval of 36 pm. The convolution of the rectangular slit and pixel profiles with the grating diffraction profile (and the geometrical aberrations of the spectrograph optics) gives the resultant spectrograph line profile, as shown in the Figure 31 below. The full width at 50% is 40 pm, equal to 휆⁄116,000.
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Figure 31: Spectrograph line profile in disk observation mode. The blue curve is the diffraction profile resulting from the grating dimensions; the magenta curve shows the convolution of detector pixel width and entrance slit, and the red curve is the final profile from the convolution of slit and detector sampling with the grating profile.
The predicted performance also depends on the geometric aberrations of the optical system. The following Figure 32 shows a spot diagram for the spectrograph. The top row represents field positions at the center of the slit, and 60 arcsec on either side - the slit length longer than that required for the high-resolution disk mode. The second and third row are the same, but for two additional slits that could be used in a multi-slit option. Two wavelengths separated by one part in 105 are shown easily resolved, an indication that aberrations won’t significantly degrade the line profiles shown above.
CN-0004 57 Critical Design Document (CDD)
Figure 32: Disk mode spot diagram. The colors represent two wavelengths separated by one part in 100,000. The top row shows three points at the center and ends of a 120 arcsec slit; the second and third rows represent potential additional slits in a multi-slit configuration. These geometric patterns must be convolved in the vertical axis (the wavelength axis) with the profiles in the previous figure.
3.6.1.2 Coronal Mode
Requirement Resolving power 30,000 over 4 arcmin slit length.
Performance Analysis The differentiating features of “coronal mode” observations are that the required field of view (i.e. slit length) is 4 arcmin, the spatial sampling is 0.5 arcsec, and the required spectral resolving power is 30,000. The spectrograph aberrations increase towards the ends of the slit, but do not unduly limit the performance, as the spot diagram below demonstrates (Figure 33). Again the fields are chosen to demonstrate the performance with multiple entrance slits, but in this case the slit length is equivalent to 4 arcmin. At this wavelength, around 1.08 µm, the linear dispersion is 4.3 µm/pm, so an 18 µm pixel samples 4.2 pm and the RMS spot radius is less than a pixel. In coronal mode, the entrance slit width is 175 µm or 0.5 arcsec, and slit diffraction essentially has no effect on the beam width of the grating. The limiting resolving power is less, since the grating is not completely filled, but is still
CN-0004 58 Critical Design Document (CDD) approximately 130,000 at the wavelength of the 3.9 µm Si IX line. Pixel binning will be used to select the desired spatial and spectral sampling.
Figure 33: Coronal mode spot diagram.
3.6.2 Feed Relay
3.6.2.1 Image Scale, Exit Pupil
Requirement Image delivered to spectrograph entrance slit has a focal ratio of 18, i.e. an effective telescope focal length of 72 m. Image is to be telecentric, i.e. the effective exit pupil of the relay is more than 100 m from the image plane.
Performance Analysis The beam from M9 is collimated, and forms a pupil image on the CryoNIRSP scan mirror FM1. An imaging mirror whose focal length is chosen to provide the required focal ratio, located one focal length from FM1, satisfies both requirements. The requirement is satisfied by design.
3.6.2.2 Image Quality
Requirement Spatial resolution better than 0.3 arcsec over a 90 × 90 arcsec field; better than 1.0 arcsec over a 240 × 180 arcsec field.
CN-0004 59 Critical Design Document (CDD)
Performance Analysis The feed relay provides a telecentric f:18 image at the spectrograph slit, and a steerable mirror at a pupil image that can be used to scan the image across the slit. To place the image inside the dewar by 0.7 m as required by the mechanical design, the scan mirror is made slightly convex. This moves the image position, while still allowing the imaging mirror to collimate the pupil. The scan mirror is spherical; the reimaging mirror is an off-axis parabola. The following spot diagram (Figure 34) of the feed relay image over the disk-observation field shows some aberrations, but the image is diffraction limited for all wavelengths longer than 1.6 µm.
Figure 34: Feed relay spot diagram for disk mode field.
The field positions represent the center, edges and corners of a 90 × 90 arcsec field. The spectrograph slit position corresponds to the central row of this diagram. Since all the optics are reflective, the spot size is independent of wavelength. The Airy disk is shown for 1.09 µm; it would be four times larger at 4.6 µm.
Figure 35 shows the modeled spot diagram over the entire 4 × 3 arcmin field. The image is slightly defocused at the ±Y edges (perpendicular to the spectrograph slit, but the spot radius is no worse than 0.16 arcsec anywhere in the field.
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Figure 35: Feed relay spot diagram for coronal mode field. The field positions represent the center, edges and corners of a 240 x 180 arcsec field. The spectrograph slit position corresponds to the central row of this diagram. Since all the optics are reflective, the spot size is independent of wavelength.
3.6.2.3 Image Scanning To verify that the feed relay image quality is maintained as the image is scanned across the slit, we rotate the FM1 mirror in the model. Figure 36 shows the spot pattern for the disk-mode field when it is scanned to put the edge of the 90 × 90 arcsec field on the slit. The spot sizes are slightly larger than at the center of the scan, but well within the tolerance. Scanning to the other edge of the field shows very similar behavior. The results for a scan to the edge of the coronal 4 × 3 arcmin field are shown in Figure 37. Again the image quality is more than adequate for the science requirement.
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Figure 36: Feed relay spot diagram. 90 × 90 arcsec disk field, scanned to place the lower row on the slit. The spot radii are all smaller than 0.1 arcsec.
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Figure 37: Feed relay spot diagram 240 × 180 arcsec coronal field, scanned to place the lower edge of the field on the slit. The top part of the field falls off the context imager at this scan position.
3.6.3 Context Imager
Context images are provided by a mirror to intercept the beam, inside the dewar and before the entrance slit of the spectrograph. The image is reduced, using a three-mirror relay, to f:8 in order to fit the entire 4 arcmin field onto a standard 2k × 2k infrared detector. The relay performance is excellent, providing diffraction limited image quality over the field. The spot diagram shown in Figure 38 represents only the context imager, so the actual image quality will be defined by the telescope and feed system described above.
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Figure 38: Context imager spot diagram. The performance of the context imager relay alone is demonstrated here. The field points represent the corners of a 4 × 4 arcmin field, together with three points near the center.
3.6.4 Spectrograph Spatial Resolution
The spatial scale at the spectrograph detector is the same as that of the context imager, 155 µm per arcsec. The spatial resolution can be seen from the spot diagrams in section 3.6.1 above. The following plot (Figure 39) shows the same information as an ensquared energy plot for the disk configuration, showing 70% of the rays from one point falling within an 0.3 arcsec square area. In the coronal configuration the image quality falls off at the ends of the slit image, but is still well within requirement of 1 arcsec resolution.
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Figure 39: Spectrograph spatial resolution. The curves represent energy on the focal plane within a square, as a function of the half-width of the square. Individual curves are for various field points within a 2 × 2 arcmin field. The image scale is such that 18 µm represents 0.12 arcsec.
3.6.5 Polarization Analysis
The stringent polarization requirements for the ATST instruments place tight constraints on the instrument and telescope stability and the polarization calibration routines. The CryoNIRSP OCD describes our Class I and II calibration algorithms. We expect to achieve the ISRD polarization requirements and will detail the performance and limitations of these during instrument commissioning when we have access to direct measurements of the telescope polarization properties. A critical component of the CryoNIRSP system is the polarizing beam-splitter. To meet the requirements on polarization measurement accuracy, a dual-beam polarimeter is necessary. This means observing two images in orthogonal polarizations simultaneously. The two beams need to have a common path to the extent possible, so the images are geometrically identical. And the analyzer must be usable over the wavelength range 0.5 to 5 µm. We have chosen to place the analyzer as the last element in the optical system, just ahead of the spectrograph detector. The analyzer optics separates the image into two spectra, and places both of them side by side on a common detector FPA. All its optics are plane surfaces, so differential distortion of the two images is minimal. A polarizing beam-splitter is required that can provide adequate polarization contrast over the required wavelength range. The device modeled at this time is a micro-wire polarizer (Moxtek, Inc.) which consists of fine aluminum wires deposited on a thin transparent substrate. The optimum substrate would transmits to 5 µm. Water-free fused silica is available, though its transmission falls off to 20% by 4.7 µm, and sapphire wafers are also likely to be available. Critical performance requirements of the beam-splitter components were tested and are described in section 3.4.2.10.1. This includes the wavelength dependent transmission and reflectivity of the polarizers. Locating the analyzer at the end of the optical train offers the major advantage that the “two” beams are strictly common-path until they reach the analyzer itself. However this layout places the diffraction grating within the polarimeter, as well as a number of other optical components in the Coudé train and the spectrograph.
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Efficiency curves published by Gratinglab (Figure 16) show some differences in efficiency for P- and S-plane polarizations, but for echelle gratings operated in moderately high orders, the differences are small. By itself, a modest efficiency difference only affects the gain calibrations for the two parts of the detector, not the character of the polarization measurement. A more serious issue would be relative phase shift between the two polarizations. We intend to evaluate the retardance shown by candidate gratings, but prior experience with gratings of this type indicates that it will be acceptable. The effects of the remaining spectrograph optics on the polarization-modulated light delivered by the telescope can be described by a Mueller matrix for the spectrograph, relating the Stokes vector delivered to the analyzer to that present at the entrance window. The desired condition is that the two Stokes vectors are identical, i.e. the Mueller matrix is diagonal. The ZEMAX optical design software has the capability to trace polarized rays through a system, accounting for polarization effects at each surface. By running such a ray trace for six canonical states of completely polarized input, and integrating over the pupil, it is possible to compute a Mueller matrix for the system. A difficulty in doing this for the CryoNIRSP spectrograph is that ZEMAX sees the echelle grating as a highly tilted mirror. The result is quite large differences in reflectivity and phase between X and Y polarizations. Since the reflections are actually near-normal reflections from the steps of the grating ruling, the polarization effects are better simulated by a plane mirror oriented to reflect the beam to the detector assembly. We have implemented this in the model by setting the diffraction order to zero and rotating the grating to reflect the beam at the Littrow angle in the system. A sample calculation, for the central field point and assuming bare silver coatings on the mirrors, gives the following Mueller matrix (I(normalized) QUV format). The first element is the overall intensity transmission; the remaining terms are fractional. This is an encouraging result, since the largest crosstalk terms are below 1%, and since the spectrograph is fixed and enclosed in a vacuum chamber we expect its Mueller matrix to be stable. Since our calibration techniques should achieve an accuracy of 1% we expect to meet the overall ISRD polarization requirements. Wavelength 1.09 µm 0.8007 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0018 [ ] −0.0001 −0.0134 0.9999 0.0010 −0.0000 −0.0018 −0.0010 1.0000 Wavelength 1.5 µm 0.7835 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0015 [ ] −0.0001 −0.0134 0.9999 0.0008 −0.0000 −0.0014 −0.0008 1.0000 Wavelength 2.0 µm 0.8025 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0134 [ ] −0.0001 −0.0134 0.9999 0.0006 −0.0000 −0.0000 −0.0006 1.0000
If the mirror coatings are silver with a protective dielectric layer, each reflection has an added phase shift due to the dielectric. So long as the optical thickness of the dielectric layer is small compared to the wavelength, the effects on the crosstalk terms should be small and vary smoothly with wavelength. The following are computed Mueller matrices for silver with a 50 nm layer of SiO for three wavelengths in the NIR. Wavelength 1.09 µm 0.7717 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0046 [ ] −0.0001 −0.0134 0.9999 0.0025 −0.0000 −0.0045 −0.0026 0.9999 Wavelength 1.5 µm 0.7647 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0035 [ ] −0.0001 −0.0134 0.9999 0.0019 −0.0000 −0.0034 −0.0019 1.0000
CN-0004 66 Critical Design Document (CDD)
Wavelength 2.0 µm 0.7943 0.0050 −0.0000 0.0000 0.0050 0.9999 0.0134 0.0025 [ ] −0.0001 −0.0134 0.9999 0.0014 −0.00000 −0.0025 −0.0014 1.0000
The QV and UV crosstalk terms are larger with the dielectric overcoat, but in either case they are quite small. A much more serious issue for polarimeter calibration will be the four large-angle reflections in the Coudé train at M4 – M7. The wavelength dependence of the polarization cross-talk is generally larger for the coated silver and may constrain the polarization calibration wavelength to be close to the data wavelength.
3.6.6 Thermal Background Control
Requirement Thermal radiation from other than the intended field of view must be strictly controlled to limit coronal background flux to less than 10-5 of the disk brightness 100 arcsec from the limb (ISRD) Performance Analysis Cryogenic optics, enclosure, and cold pupil stops achieve a thermal background for both imager and spectrograph that meet this requirement at wavelengths shortward of 4 microns if the ATST per mirror emissivity is less than approximately 2% (see Figure 19 and Figure 27).
3.6.7 Instrument Efficiency
By using the efficiency measurements of the grating and the wire grid polarizers, we can determine a lower boundary for the total instrument efficiency. We assume a value of 98% for the mirror reflectivity which is the lowest value stated for the FSS99-500 coating which is the preferred choice of the project for all mirrors past M1 (TN-0155).
transmissivity cumulative optical component or total reflectivity efficiency FM1 (scanning mirror) 98% 98% FM2 (warm focus mirror) 98% 96% warm filter (open) 100% 96% modulator (AR coated MgF2) 95% 91% entrance window 90% 82% spectrograph filter 80% 66% SM2 (fold mirror) 98% 64% SM3 (collimator) 98% 63% SM4 (fold mirror) 98% 62% grating 60% 37% SM5 (camera mirror) 98% 36% wire grid 1 (analyzing beam splitter reflection) 80% 29% wire grid 2 (analyzing beam splitter transmission) 80% 23% SB1 (fold mirror) 98% 23% prism 98% 22% Total 22%
Table 19: CryoNIRSP spectrograph efficiency. The calculation is done for the more demanding case where the beam has to pass two wire grid polarizers in the polarization analyzer.
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transmissivity cumulative optical component or total reflectivity efficiency FM1 (scanning mirror) 98% 98% FM2 (warm focus mirror) 98% 96% warm filter (open) 100% 96% modulator (AR coated MgF2) 95% 91% entrance window 90% 82% CM1 (pick off mirror) 98% 80% CM2 (fold mirror) 98% 79% CM3 (relay mirror) 98% 77% CM4 (relay mirror) 98% 76% CM5 (relay mirror) 98% 74% context imager filter 1 80% 59% context imager filter 2 80% 48% Total 48%
Table 20: CryoNIRSP context imager efficiency.
The overall efficiency of the spectrograph is mostly dependent on the grating efficiency and the wire grid transmission. The 60% grating efficiency is a lower boundary as described in section 3.4.2.3.4. The wire grid transmission depends on the substrate choice. Our test wire grid polarizers have an UV grade fused silica substrate. Thus the transmission drops to about 50% around 4.5 µm and then to nearly zero around 4.8 µm. This will reduce the total efficiency to below 10% above 4.6 µm. However, the CryoNIRSP team believes the approximately 50% efficiency hit at wavelengths beyond 4.5 μm will not affect CO-disk scientific goals. We will seek project approval to relax the instrument efficiency at the longest wavelengths. Our alternative to perform polarimetry up to 5 µm with 10% efficiency over the entire useful wavelength range, would be to use wire grid polarizers with a sapphire or CaF2 substrate. CaF2 is the preferred solution since it has a transmission over 90% over the entire wavelength range of interest and keeps the wire grid ghost intensities around the 10-3 level (see section 3.9.2). The context imager has a total efficiency above nearly 50% at all wavelengths and we could not identify a critical component.
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3.7 OPTICAL TOLERANCE ANALYSIS AND WAVEFRONT ERROR BUDGET
In general the most stringent optical resolution requirements of the CryoNIRSP follow from the need to observe the solar disk at 4.65 µm. Coronal observations at lower spectral and spatial resolution and shorter wavelengths will be consequently satisfied. Instrument optical tolerances are derived from the ISRD such that the CryoNIRSP does not significantly degrade the delivered telescope resolution. There are several contributions to the overall instrument image resolution error budget which are summarized in the table below. Understanding each term here is important to describing our alignment tolerances and procedures. Currently the largest error contribution, and most uncertain, is the delivered wavefront error to the Coudé transfer optics for the CryoNIRSP. Because the CryoNIRSP does not rely on adaptive optics there is no “front-end” ATST wavefront control. The ISRD states that image stability delivered to the instrument will be “at the level of 0.5 arcsec.” This is equivalent to an RMS wavefront error of about 0.2 waves at 4.65 µm. The actual wavefront aberration will depend on the seeing conditions and telescope jitter. A future ATST interface control document may refine the angular and temporal specifications for this delivered wavefront. While the CryoNIRSP is designed with the error budget described below, where ever possible and when there is no cost increment, the CryoNIRSP will allow improved image resolution if the telescope-delivered wavefront improves. The next most important error budget contribution comes from the cumulative mid-spatial-frequency irregularity of the optical surfaces. This irregularity is a manufacturing error contribution which, for CryoNIRSP, is not particularly demanding. The manufacturing tolerance on the geometry of each optic (each radius, conic constant, off-axis vertex decenter) can also be relatively “loose” because we have excellent metrology available to characterize the as-built optics. The actual optic geometry (within our manufacturing tolerance range) allows a final instrument resolution that is relatively insensitive to the “residual alignment” error contributions. This is because the final optical alignment strategy allows us to compensate for a range of manufactured optic shapes, and because of the dynamic de-space of some of the optics in the CryoNIRSP optical bench, which are effective compensators. An additional error contribution comes from possible indeterminacies in our model (and measurements) of the thermal contraction of the optical system from 300 K to cryogenic temperatures. This optical wavefront error is also small because of the dynamic de-space compensation of the optical bench design. The spectrograph itself, however, has the additional requirement that it delivers resolving power of 100,000 at 4.65 µm. The wavelength resolution element of 46.5 pm covers 46.5 µm on the spectrograph focal plane, just equal to the Airy radius at the f:8 spectrograph focus. This means that the allowable wavefront error within the spectrograph optics is smaller than in the feed optics or imager. Our top level image degradation error budget is summarized in the following table, expressed in terms of RMS waves at 4.65 µm. The spectrograph total wavefront is diffraction limited as regards spectroscopy, but the spatial resolution is still limited by the telescope performance.
(all units in Telescope Feed Surface Surface Residual Total Contingency Budget waves at Wavefront Optics Figure Irregularity Alignment Wavefront 4.65 µm ) Context 0.2 0.06 0.05 0.05 0.03 0.22 0.05 0.23 Imager Spectrograph 0.2 0.06 0.05 0.05 0.03 0.22 0.05 0.23 Imaging Spectrograph NA NA 0.05 0.05 0.03 0.077 0.05 0.09 Spectral Table 21: Image resolution error budget.
For high-resolution disk observations, the spectrograph entrance slit is narrow enough that its diffraction widens the beam so that the aperture – within the spectrograph – is defined by the projected width of the grating. The aperture in the context imager (as well as in the spectrograph for coronal observations with a wide entrance slit) is defined by telescope and the f:18 CryoNIRSP feed optics. Since the rectangular aperture for the spectrograph is easiest to simulate by feeding it with an ideal optical system having an appropriate rectangular aperture, we have chosen to evaluate the three subsystems individually, then RSS-combine the feed optics errors with either the spectrograph or context imager to ensure performance at either focal plane.
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The Zemax tolerancing procedure is used to evaluate positioning errors (primarily tilt and decenter) and overall figure errors (vertex curvature and conic constant). To estimate wavefront aberration due to surface irregularities, we stipulate an RMS surface sag error over the surface clear aperture, then multiply that by 2 for a reflective surface and (n-1) for an air-glass interface. The result is then scaled by the square root of the ratio of the beam footprint from a single field point to the surface clear aperture to take into account the proximity to a pupil location. For each subsystem, a table shows the Zemax tolerance operands for the significant tolerances; other possible operands that have negligible effect on the wavefront are omitted. For each operand the table shows the maximum allowable error (specification) and its effect on the wavefront. A similar table shows the specified surface irregularity on each surface and its contribution to the wavefront error. We have added the errors by assuming each parameter is at its limit and combining them as a root-sum-squares total. This approach is conservative, but for a one-off system is probably appropriate, and it is not greatly different from using the three-sigma limit of a normal distribution or the result of a Monte Carlo test with a parabolic distribution. Each approach assumes that the manufacturing – or alignment – process proceeds until the specification is met, so the remaining errors are likely to tend toward the limits.
3.7.1 Feed Optics
This subsystem contains three mirrors FBS, FM1 and FM2, a filter, a two-component retarder, and the dewar entrance window. In the Zemax tolerance computation, compensation by adjusting the final focus is permitted. The nominal RMS wavefront error for the feed optics is 0.0194 waves at 4.65 µm. This includes a very small contribution from the ATST telescope optics.
3.7.1.1 Figure and Position Errors We assume we can align the optics to decenters of 2 mm and tilts of 1 mrad. Curvature errors are specified so that the maximum deviation from nominal is one fringe at 633nm for FBS, 1% of nominal curvature for FM1, and 0.5% of the nominal curvature for FM2.
Error Limit Wavefront change (waves) Tilt X FBS 0.05 deg 0.0014 Tilt Y FBS 0.05 deg 0.0016 Flatness FBS 1 fringe at 633 3.2E-5 Tilt X FM1 0.05 deg 0.0049 Tilt Y FM1 0.05 deg 0.0022 Curvature FM1 0.01 of nominal 0.0001 Tilt X FM2 0.05 deg 0.0048 Tilt Y FM2 0.05 deg 0.0018 Curvature FM2 0.005 of nominal 0.0005 Conic FM2 0.4 0.0025 Total RSS 0.0061 Table 22: Feed optics surface figure error budget.
3.7.1.2 Surface Irregularity We set the tolerances on mirror surfaces to be 63 nm RMS. The wavefront error in reflection is then 126 nm RMS or about 440 nm P-V. The transmissive elements are set at 315 nm RMS, except that the modulator plates are assumed to be tenth-wave flat.
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Surface RMS surface error Mirror (2) or Lens Sqrt(Footprint to Wavefront error in in nm (n-1) CA) waves at 4.65 µm FBS 63 2 0.8 0.0217 FM1 63 2 1.0 0.0271 FM2 63 2 0.85 0.0232 Filter S1 315 0.403 0.57 0.0153 Filter S2 315 0.403 0.56 0.0152 Modulator S1 63 0.403 0.61 0.0033 Modulator S2 63 0.403 0.61 0.0033 Modulator S3 63 0.644 0.61 0.0053 Modulator S4 63 0.644 0.61 0.0053 Window S1 315 0.403 0.54 0.0148 Window S2 315 0.403 0.53 0.0145 Total RSS 0.0521 Table 23: Feed optics surface irregularity error budget.
The worst-case (all parameters at the specification limit) wavefront error is√0.01942 + 0.00612 + 0.05212 = 0.0559 waves 4.65 µm.
3.7.2 Spectrograph
The spectrograph optics consist of a filter SF, plane fold mirrors SM2 and SM4, off-axis conics SM3 and SM5, the echelle grating and the beam-splitting analyzer. The latter consists, for either beam, of one transmissive element and three mirrors. Zemax tolerancing appears to work better if the RMS spot radius is used as the criterion. The nominal radius is 9.1 µm and the required radius is less than 23 µm.
3.7.2.1 Figure and Position Errors Tolerances similar to those used in the feed optics are adequate. Tilts, decenters, flatness not listed in the table had negligible effect if compensated with a focus adjustment.
Error Limit RMS Radius Change (µm) Tilt X SM2 0.06 deg 0.05 Tilt Y SM2 0.06 deg 0.06 Decenter X SM3 2 mm 0.07 Decenter Y SM3 2 mm 0.04 Tilt X SM3 0.06 deg 0.47 Tilt Y SM3 0.06 deg 0.21 Curvature SM3 0.0033 of nominal 0.02 Conic SM3 0.02 0.25 Tilt X SM4 0.06 deg 0.97 Tilt Y SM4 0.06 deg 0.25 Decenter X SM5 2 mm 0.01 Tilt X SM5 0.06 deg 0.19 Tilt Y SM5 0.06 deg 0.14 Curvature SM5 0.01 of nominal 0.02 Conic SM5 0.02 0.10 Table 24: Spectrograph optics surface figure error budget. With all parameters at either positive or negative limit, the spot size is increased by 8.7 µm to 17.8 µm. This RMS radius, 18 µm, is equivalent to 0.048 waves at 4.65 µm.
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3.7.2.2 Surface Irregularity
Surface RMS surface error Mirror (2) or Lens Sqrt(Footprint to Wavefront error in in nm (n-1) CA) waves (4.65 µm) Filter SF Surface 1 633 0.45 0.15 0.0091 SF Surface 2 633 0.45 0.15 0.0091 SM2 31 2 0.82 0.0112 SM3 31 2 0.97 0.0131 SM4 31 2 0.98 0.0133 Echelle 31 2 1.00 0.0136 SM5 31 2 0.96 0.0130 Analyzer M1 79 2 0.71 0.0240 Polarizer S1 79 0.40 0.62 0.0043 Polarizer S2 79 0.40 0.61 0.0042 Analyzer M4 79 2 0.53 0.0181 Analyzer M5 79 2 0.42 0.0142 Total RSS 0.0461 Table 25: Spectrograph optics surface irregularity error budget.
3.7.3 Context Imager
The context imager optics consists of two plane mirrors CM1 and CM2, three conics CM3, CM4 and CM5, and a filter CF.
3.7.3.1 Figure Errors The off-axis conics CM3 and CM5 are pretty aggressive, so alignment tolerances are stricter than in other parts of the system. Vertex curvatures changes of 1% of nominal are acceptable, and 3-fringe flatness tolerance on CM1 and CM2 is adequate. The 0.05 change in the CM3 conic gives a maximum sag error of 3.9 µm, 6 waves (@633 nm). The 0.3 change to the CM4 conic of 1.6 causes a maximum sag error of 0.10 µm. A change of 0.005 to the CM5 conic, nominally 0.116, causes a maximum sag error of 2.74 µm. At least one (and our preferred) optics manufacturer has a profilometer that can measure 0.1 µm errors so these conic errors are easily within manufacturing tolerances. Compensators: CM5 is allowed to decenter along Y and tilt about the X axis. This permits compensating for errors in axis location, vertex curvature and conic constant. The back focus and focal plane tilt may also be used for compensation. The nominal wavefront error for the CI is 0.0237 waves.
Error Limit Wavefront error in waves (4.65 µm) Decenter X CM3 1 mm 0.0020 Decenter Y CM3 1 mm 0.0016 Decenter X CM4 1 mm 0.0090 Decenter Y CM4 1 mm 0.0010 Decenter X CM5 1 mm 0.0144 Tilt X CM1 0.03 deg 0.0008
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Tilt Y CM1 0.03 deg 0.0006 Tilt X CM2 0.03 deg 0.0010 Tilt Y CM2 0.03 deg 0.0006 Tilt X CM3 0.03 deg 0.0014 Tilt Y CM3 0.03 deg 0.0024 Tilt X CM4 0.03 deg 0.0002 Tilt Y CM4 0.03 deg 0.0053 Tilt Y CM5 0.03 deg 0.0192 Flatness CM2 3 fringes 0.0008 Vertex curvature CM3 0.01 of nominal 0.0071 Vertex curvature CM4 0.01 of nominal 0.0040 Vertex curvature CM5 0.01 of nominal 0.0057 Total RSS 0.0470 Table 26: Context imager optics surface figure error budget.
3.7.3.2 Surface Irregularity
Surface RMS surface error Mirror (2) or Lens Sqrt(Footprint to Wavefront error in in nm (n-1) CA) waves (4.65 µm) CM1 63 2 0.33 0.0044 CM2 63 2 0.47 0.0128 CM3 63 2 0.62 0.0170 CM4 63 2 1.0 0.0271 CM5 63 2 0.77 0.0208 CF surface 1 315 0.45 0.44 0.0133 CF surface 2 315 0.45 0.42 0.0127 Total RSS 0.0444 Table 27: Spectrograph optics surface irregularity error budget.
The total wavefront error is√0.02372 + 0.0472 + 0.04442 = 0.069 waves at 4.65 µm.
3.7.4 Metrology Capabilities of Vendor
The vendor quoting the best price for the mirrors is located on Maui and has excellent metrology capabilities for mirrors up to 16 inches. The vicinity of the manufacturer allows us to closely collaborate with them and regularly check on the progress. As they have much better metrology capabilities than we do, we will use their set ups for the acceptance tests of the mirrors. The conjugates of the mirrors can be determined with an accuracy of 2 µm and their profilometer allows surface irregularity measurements on the 0.1 µm level. Thus the actual shapes of our mirrors will be very accurately known and will certainly meet the specified tolerances. As our optical design allows to compensate for manufacturing imperfections, we will use the accurate shape measurements to determine the final alignment of our optical components
3.7.5 Analyzer Assembly
The analyzer components need to be placed so that the two images are reasonably well registered on the detector array. Errors of small tilts on individual components have the effect of displacing one image with respect to the other; no significant rotation or distortion effects are seen in the model. Common displacements and differential focus are possible but can be compensated by adjusting the final prism-mirror. The most sensitive component is the primary beam-splitter, since it’s farthest from the focal plane: an angular error of 0.04 degree results in one of the images being
CN-0004 73 Critical Design Document (CDD) displaced by ten pixels on the detector. Since the two images will necessarily be co-registered in data processing, the only result of such displacement is to lose a fraction of the image at the edge of the field. These mirrors are small, so machining tolerances in the mirror cells have a strong effect on angular error, but we expect that achieving 0.1 degree accuracy on the mirror angles is possible, with fixed mirror cells.
3.8 OPTICS SURFACE QUALITY AND COATING MICRO-ROUGHNESS
The ATST, as a coronagraph, severely limits the out-of-field light flux on the secondary optics beyond the Lyot pupil stop. While the large intensity contrast for near-limb observations has consequences for our CryoNIRSP transmissive window and filter designs because of spurious “ghost” multiple reflection, the diffuse non-specular reflection from CryoNIRSP optical components has negligible effect on the delivered Strehl while yielding a nearly uniform background light flux. The diffuse scattered intensity, or the fractional Strehl, for an 11 reflection CryoNIRSP 2 configuration follows from the Marechal formula S = Iscat / Iinc = exp(-11*(4*π*sigma/lambda) ). Our vendors are comfortable delivering optics with micro-roughness of 1.5 nm or better. For sigma = 1.5 nm at lambda = 1 micron then S = 0.004. Micro-roughness of twice this yields less than a 2% Strehl reduction. Since this energy will be distributed over wide angles and is smaller than the energy lost in ghost reflections it is within acceptable limits and has no science impact. All CryoNIRSP optics will be specified with a standard 1.5 nm micro-roughness to limit the diffuse scattered energy to be less than ghost scattered light from transmissive elements. Our measurements at the Maui ATRC of the surface quality degradation from Aluminum coatings show that the coated micro-roughness of the optics (with 100 nm thickness) will be less than 2 nm – still negligible for Strehl reduction and the diffuse background light flux. The surface quality requirements for scratch-dig are similarly not extraordinary. Standard MIL-0-13830 offers guidance on this somewhat qualitative requirement. Since none of the CryoNIRSP reflective optics are near an image plane, the optical standard “40-20” scratch-dig requirement will scatter less than 1% of the incident light with no image degradation.
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3.9 SCATTERED-LIGHT AND GHOST IMAGE ANALYSIS
Scattered light and ghost images in CryoNIRSP are minimized by its reflective optical design. Out-of-field thermal emission is minimized by the cryogenic enclosure and cold pupil stops. Spurious light (e.g. from the grating) is minimized with a careful cold optical baffling design within the dewar. Nevertheless the instrument will have some ghosting due to the front entrance window, the interference filters, and the polarizing beam-splitter. Non-sequential ray trace analysis was used to investigate scattered light and ghost images
Requirement According to the ATST SRD stray reflections and scattered light must be less than 1% of the core (peak) surface brightness 10 arcsec from a point source. Coronagraphic observations should achieve a scattered light of <5x10-4 beyond 5 arcsec of the photosphere.
3.9.1 Grating Scattered Light
This section assesses the amount unwanted light coming off the grating in CryoNIRSP, using the non-sequential ray trace capability in Zemax. There are two kinds of stray light: light diffracted by the grating in orders other than the on-blaze order, and light scattered from the grating or surfaces near it. We examined the stray light on the spectrograph detector and in the area around the camera mirror. The spectrograph was modeled as a slit, a grating in a housing, two paraxial lenses, two baffles and an FPA detector. Figure 40 shows the model layout. It contains an entrance slit with dimensions 0.046 x 81 mm, a paraxial-lens collimator, a grating, a paraxial-lens camera, and a FPA detector. The slit emits light of a single wavelength uniformly over its area, but into an elliptical cone that has f:18 in the cross-dispersion direction and f:11 in the dispersion direction, to simulate the effect of diffraction at the slit. The total power from the slit is 1 W. The grating is placed in a cylindrical housing with a rectangular window to admit the incident and diffracted beams. The inside surface of the housing is intended to mimic a black-anodized surface with a sand-blasted texture: absorption of 90% of the light incident on it, the remaining 10% is scattered (half of it is reflected specularly, the other half uniformly). This scattered light mostly hits the grating again, and some of it comes back out the aperture of the housing. Zemax permits diffracting light into up to 12 (sequential) orders, with user input of the power allocated to each order. We used a blaze function formula from Schroeder to calculate the intensities of all the diffracted orders for each of the several lines of interest. The on-blaze order gets 50 to 90 percent of the total energy. 5% Lambertian scattering have been added to the grating, in order to account for dust and rough edges on the rulings.
Detector surfaces may be inserted at any location in the Zemax model. This permits looking at the illumination field not only at the actual detector, but at other locations as well, without interfering with the ray propagation. We have placed one detector surface on the FPA, made of a fully absorbing material to terminate propagation there. A second detector is located on the surface of the camera lens, and a third is a large 30 x 400 cm rectangle, placed near the camera lens but tilted 45° as shown in the first figure so that it intercepts all the rays emerging from the grating housing. A fourth detector, called Post Slit, is 20 x 20 cm wide and placed 10 cm to the right of the entrance slit. This detector observes light from either side, so it sees the beam from the slit, plus any slit images due to a non-principal diffracted order coming back via the collimator. Besides the grating housing, we have placed baffles on the outer side of both the incident and diffracted beam, extending about 50 cm from the grating housing. These baffles may defined to be transparent (made of air), totally absorbing, reflecting, or partially absorbing, reflecting and scattering.
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Figure 40: Grating scattered light analysis layout The grating is shown in orange, the detectors for the analysis are white, and other components gray or black. The top cover of the grating housing has been removed in this drawing.
Case Study: 4651 nm We ran the ray trace for four cases: with the on-blaze order power at nominal and set to zero (to measure the power in the stray light), both with the baffles and with them ignored. This wavelength is nearly on-blaze in order 12, so were able to include orders 1-12.
Figure 41: Illumination of the pseudo detector with the principal order removed – anodized baffles. Result for a black-anodized surface on the baffles, with 90% absorption, 5% specular reflection and 5% uniform scattering. This works well in this configuration (Wavelength 4651 nm in low order), reducing the stray light to about 1 mW.
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Case Study Higher-order Wavelength, 2326 nm The wavelength 2326 nm is in order 24 of the grating. The grating blaze model puts 714 mW of the 1 W total incident power into this order and 95 mW into all the others (the total isn’t 100%, indicating the loss due to groove shading). The Zemax model can trace 12 orders at a time, but by tracing several groups of orders successively, co-adding power onto the detectors.
Figure 42: Illumination of the pseudo detector with the principal order removed – anodized baffles. With 90% absorbing baffles as described above, the stray light is still 35 mW, because order 23 comes back out between the baffles. The diffuse background is low.
Figure 43: Illumination of the pseudo-detector placed 10 cm behind the entrance slit. The detector has been set to record rays from either side. The bright rectangle at the center is the 1 W beam emitted from the slit, and the curved image above it is the slit image in order 23 formed by the collimator.
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Conclusions There is a significant fraction of light that does not belong to the primary diffraction order. A few percent in the presented cases – but up to ten percent at other wavelengths. Most of the extraneous orders come out toward the outside of the incident beam and can be trapped with a suitable baffle. One or more of the extraneous orders are diffracted back toward the collimator and will be imaged near the entrance slit. There will be a housing around the slit assembly which will trap these beams so that they cannot scatter back to the spectrograph. In conclusion, the total amount of randomly scattered light is small with the applied baffling strategy and meets the requirement.
3.9.2 Analyzing Beam Splitter Scattered Light and Ghosts
Using the non-sequential ray trace capability in Zemax, we determined the amount of unwanted light coming out of the CryoNIRSP analyzing beam splitter. A baffling strategy is examined which traps this light and prevents stray light within the spectrograph. In this model we use three line sources close together which are re-imaged at f:8 using paraxial lenses. A non-absorbing detector is placed in front of the light trap to measure the light reflected from the analyzer as well as the light emerging from the trap. We assume here that all the light entering the assembly is in the desired spectrograph image beam, the 1 mm thick Moxtek wire grid polarizers have a CaF2 substrate. A fused-silica analyzer will have similar performance to this model over most of the CryoNIRSP useful wavelength range (see section 3.4.2.10.1). The incidence angles off the wire-grids will be less than 20 degrees (see section 3.4.2.10.1) to maximize transmission and minimize wavelength dependent transmission. In this simulation the total power registered by the spectrograph detector is 333 mW. Secondary optical paths due to reflections at the beam-splitter surfaces, and from scattering at any of the optical surfaces, cause 29 mW of unwanted light to be emitted from the analyzer. Assuming an anodized surface of the light trap with 80% absorptivity, only 29 nW may escape the trap. Increasing the absorptivity of the trap material to 90% increases the overall efficiency of the trap by a factor of 100.
Figure 44: Analyzing beam splitter layout. The Zemax detector (orange line) is used to detect light going into and emerging from the light trap.
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Since we use a triple slit set up in this model, the CryoNIRSP detector image (Figure 45) shows six groups of lines. The three groups above the black line are associated with one polarization. The groups bellow the black line represent the images of the other – orthogonal – polarization state. The main images (red), in the upper half, have ghosts at the 10-3 (green) and 10-6 (blue) level origination from 2 respectively 4 reflections within the beam splitter. In the lower part of the image, there are more ghosts visible due to 2 and 4 reflections within the second beam splitter but also due to a combination of reflections inside the two beam splitters. The intensity level though is also at the10-3 and 10-6 level.
Figure 45: Analyzing beam splitter image for a triple slit set up. The black line separates the two orthogonal polarization states. The images (red) are accompanied by ghosts at the 10-3 (green) and 10-6 (blue) level.
3.9.3 Grating Recombination Ghosts
In spectrographs with array imagers, light reflected from the imager is returned to the dispersing element. The angle of incidence is just equal to the original diffraction angle. This reflected beam may have a flux of ten percent of the flux absorbed by the detector. It is then diffracted again by the grating, into all orders from zero up to the original diffraction order m or in some cases up to m+1 or m+2. Two cases are of interest: if we say the diffraction order of the ghost beam is m’, then if m’ – m = ∆m is zero, the light is returned exactly along its original path back to the collimator and hence to the entrance slit, with the wavelength range recombined. If ∆m is a small positive number, there are values of m and the corresponding wavelength ranges for which the re-diffracted beam is returned not to the collimator but to the camera mirror, and hence back to the detector a second time. For these values of ∆m the spectrum is partially, but not completely recombined so that it appears as a ghost spectrum covering a relatively small part of the detector. Its brightness depends on the grating blaze function for the order m’ [Burgh et al. 2007]. We created a Zemax model to illustrate the behavior of the ghosts, and calculated the wavelength ranges where the ghosts are superimposed on the primary spectrum. We also used a grating blaze function to estimate the ghost intensity.
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3.9.3.1 Critical Orders To investigate which orders can cause ghost spectra, we calculated the direction of the diffracted ghost beams for ∆m = 1, 2, 3, 4, and 5. We approximated the wavelength boundaries of each order as the mean of the center wavelengths of two adjacent orders. Then, we calculated α for the boundaries from the grating equation. Only a limited number of ∆m values have α within the angular range of the chosen order. And we expect only few orders – 21, 42, 63, 84 and 105 – to have ghosts on the detector. These orders, with ∆m = 1, 2, 3, 4, and 5 respectively, have exactly the same geometry and show the same ghost pattern except that the ghost is wider in the higher orders.
3.9.3.2 Littrow Ghost Model Results The Zemax Littrow Ghost Model is a simplified Non Sequential Component model of the CryoNIRSP layout. The source object surface is a rectangle 0.175 x 80 mm in size at the slit plane. The collimator consists of a paraxial lens with focal length 2096 mm, equal to that of the spectrograph collimator mirror. The pupil is placed at the grating and sized to provide an f:18 beam in the collimator section. The grating is an echelle with blaze angle 63.9 degrees and a groove frequency of 31.6 lines per mm. The Littrow angle is 5.5 degrees, i.e. β – α = 11.0 deg. The camera optic is a second paraxial lens of focal length 930.73 mm. The detector is sized at 37 x 18.5 mm to match half the expected FPA. A thin window of sapphire is placed on the detector to provide a reflected beam similar in intensity to that we expect from the actual detector. A range of wavelengths, chosen to fill the detector, is propagated through the system and the predicted ghosts can be observed. Figure 46 shows the result for m = 42 and ∆m = 2. The curved lines are slit images in several wavelengths near 1.347 µm, and the broad straight feature is the ghost caused by the reflection from the detector being re-diffracted by the grating in order 44. The brightness of the ghost depends on the reflectivity at the detector and on the grating efficiency in this order at this angle. With the sapphire window on the detector and a likely pessimistic efficiency of 0.05 in order 44, the power in the ghost is somewhat less than 1% of the power in the principal spectral image.
Figure 46: Spectrum in order 42 at 1.347 µm, with recombination ghost in order 44. The ghost power is 0.0023 watts.
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3.9.3.3 Blaze function We made an attempt to calculate an approximate blaze function, i.e. grating efficiency vs. order number for a number of selected wavelengths. The wavelengths are mostly lines of interest or the central wavelengths of orders 21, 42 and 84. The blaze function for the orders causing the ghost images, i.e. 20, 40 and 80 at their respective wavelengths, is very small in all three cases – below 10-5. If this is correct, the ghost intensity would be negligible.
3.9.3.4 Effect of rotating the grating If one of these ghosts should happen to interfere with a line of interest, it is possible to separate it simply by rotating the grating by a small amount. The position of the main spectrum is sensitive, of course, to the grating angle, but the ghost position moves a small amount in the opposite direction to that of the main spectrum.
3.9.3.5 Conclusion Our model and calculations show that we should have only a few orders that cause ghost images on the detector. The intensity of these ghosts is expected to be very low.
3.9.4 Entrance Window Ghosts
A non-sequential Zemax model was set up to investigate the ghost images due to multiple reflections inside the dewar entrance window. We placed a 15 mm thick CaF2 window 544 mm from the spectrograph slit. The wavelength was 2.3 µm and the total incident power was 1000 mW. 940 mW are transmitted to the principal image of the point source. 59 mW are reflected back into the room. The ghost image produced by two reflections inside the window contains a total power of 0.87 mW and has a diameter of 1.2 mm. Having a wedged window, i.e. tilting one surface by 0.022°, results in the ghost image being moved up by 0.706 mm (equivalent to just over 2 arcsec). The principal image is moved up by 0.083 mm. The separation of the two image centers is just more than the radius of the ghost. Inclining the entire window by 1.67°, the ghost moves up by 0.485 mm and the principal image down by 0.13 mm. Again the two images are just separated. The detected background would be 0.0009 times the average of the sky from the pointing position to 3.5 arcsec farther out for any of the above choices. Further this implies that by orienting the large intensity gradients so they are on the “ghosted” side of the image we can effectively eliminate stray light when observing, for example, the chromospheres near the photosphere. Figure 47 illustrates a non-sequential ray trace analysis of the CryoNIRSP front window and optics that shows the out-of-focus ghost image at the detector created by a point source illumination and window tilt angle of 1.67 degrees. For a non-point-like illumination (e.g. the solar limb) there is a diffuse ghost of about 10-3 of the surface brightness of the incident scene only “above” the true image.
CN-0004 81 Critical Design Document (CDD)
Figure 47: At the image plane the ghost of the indicated point source is a 1.2 mm diameter spot centered about 2 arcsec (in the image plane) away from the primary image with a 1.67 degree window tilt. The ghost has 0.6% of the point-source power. Observations below the solar limb with this limb orientation will effectively have no disk-scattered light contamination
3.9.5 Blocking Filter Scattered Light and Ghosts
We used the non-sequential Zemax model developed for looking at grating stray light, then inserted a filter sheet of MgF2, 30 mm after the entrance slit. The filter thickness is 5 mm. We tilted its rear surface by 0.05°, probably more than we would expect for fabrication tolerance. The reflectance of the surfaces is either uncoated MgF2 or an ideal coating. This results in a ghost about 0.5 mm wide, with intensity depending on the surface reflectance. Tilting the filter by 3° moves the ghost so it almost separates from the main line. Here the surface reflectance was 2%.
Figure 48: Ghost image (green) seen on the spectrograph detector due to blocking filter.
CN-0004 82 Critical Design Document (CDD)
A single-layer AR coat ought to give around 2% reflectance, so the ghost power would be around 0.04% of the main image. Since the spectrum is continuous, this will be the amount of contrast reduction. We see no particular benefit to tilting these filters, unless either the reflection back toward the slit is problematic, or the reflection of a non-principal order back toward the grating is causing trouble. But these orders are much fainter and would then be weakened by the reflectivity of the filter surface (e.g. 2%) and defocused by twice the slit-to-filter distance. We aim for a surface reflectance of 1% or less to further reduce this background. Since the filters are narrow-band anyway, they could have a two-layer V-coat and get the reflectance down to 0.5% or better.
CN-0004 83 Critical Design Document (CDD)
3.10 OPTICAL ALIGNMENT
3.10.1 Overall System Alignment
Placement of CryoNIRSP in the Coudé room: Optical elements within dewars are fiducialized to 3 corner cubes on each of the dewar assemblies that determine the CryoNIRSP global coordinate system (GCS). Optical elements are localized to 0.5 mm (at room temperature) in the GCS. Warm elements are located in the GCS to 0.5 mm accuracy using corner cubes and laser tracker Warm focus assembly on FM1 and tilt adjustment on FM2 provide tilt, focus and displacement alignment of ATST Coudé beam with CryoNIRSP assembly Alignment of CryoNIRSP spectrograph Warm to cold shifts on the optical bench will be measured using corner cube references at room temperature and at operating temperature – the laser tracker will probe points visible through front window of imager and spectrograph dewar assemblies. Corner cube fiducials on all optical elements localize optics to 0.5 mm accuracy in CryoNIRSP GCS defined by exterior dewar corner cubes. Alignment at room temperature with compensation for thermal contraction obtained from shift measurements. Alignment telescope and alignment laser jig at entrance window, with centering alignment masks on each mirror optic, are used to bore-sight mirror optics separately through spectrograph and imager. This aligns each optic tip/tilt and centration. Z position and focus tolerance is satisfied with laser-tracker mirror placement at room temperature and thermal contraction compensation. CryoNIRSP imager and spectrograph angular alignment fixed at room temperature using dewar alignment jacks. CN-I clamped to CN-S and anchored to ATST Coudé mounts.
3.10.2 Alignment: Co-registration of Context Imager and Spectrograph Slit
The context image is not derived from a reflective slit-jaw so that the spectrograph slit sky pointing must be determined within the context imager field-of-view by a co-registration algorithm. The registration may be complicated by atmospheric differential refraction. For example, observations taken with a pointing angle 30 degrees above the horizon experience about 0.1” of apparent image displacement for wavelengths near 1 micron that differ by 100 nm. At longer wavelengths and smaller zenith angle telescope pointing, the angular dispersion decreases. Our co- registration requirement is driven by the 0.5 arcsec IR natural seeing width over the CryoNIRSP field-of-view and will be corrected for mean differential refraction when the context imager wavelength differs by more than 100 nm from the spectrograph observations. The co-registration algorithm will be completed after every warm-up/cool-down CryoNIRSP cycle and uses day or nighttime lunar observations. The similarity of lunar light-flux levels to coronal conditions and the fixed image diversity/contrast of the cratered lunar surface allow spectral lunar observations to be accurately registered against near-simultaneous imagery. Image registration will be determined for each of the three flip mirror options (‘open’, ‘slotted mirror’, and ‘mirror’) and for various filters to map possible differential image registration. The co-registration is complicated by optical distortion in both the context imager and spectrograph, and by the curvature of the spectral lines. The following figure shows, exaggerated by a factor of ten, the context imager distortion over the full 3 × 4 arcmin field. The horizontal (X) axis is parallel to the spectrograph slit. The central row is conjugate to the slit; its maximum distortion is +0.12% at the -X limit and -0.18% at the +X edge, expressed as a percentage of the field half-width. These are equivalent to 0.11 and 0.16 arcsec, approximately one pixel. The spectrograph optics images the slit (in monochromatic light) to a curved image. The ends of the image are offset in the Y direction by about 5%; the distortion along the X axis is similar to that of the context imager but about 2.5 times larger and in the opposite direction. The largest difference between the slit image and the corresponding line on the context imager is equivalent to 116 µm or 0.75 arcsec, at the +X edge of the field. In the 1.5 x 1.5 arcmin disk mode field, the largest
CN-0004 84 Critical Design Document (CDD) discrepancy is 0.25 arcsec. The focal plane mapping procedures will enable mapping the distortion, including curvature of the slit image, in both cameras.
Figure 49: Distortion map for context imager. The image locations are marked for a uniform grid of field points over a 4 × 3 arcmin field. The deviations have been exaggerated by a factor of ten.
3.10.3 Beam-splitter image registration
The two polarized spectra formed on the detector correspond to orthogonal polarization states analyzed from the same long-slit image. Each image will be a maximum of 2K pixels along the spatial direction and 1K pixels in the wavelength direction. Beam-splitter component misalignment is expected to produce a shift of as much as 10 pixels (predominantly in the spatial direction), which is not removed by adjusting the beam-splitter right-angle prism rotation or displacement. Image distortion effects may be as large as 1 or 2 pixels (but smaller than the required spatial resolution). A slit mask will be provided in the slit wheel that places a non-redundant mask in place of the slit in order to register in wavelength and spatial directions the two beam-splitter images.
CN-0004 85 Critical Design Document (CDD)
4 HARDWARE DESIGN
4.1 CRYOGENICS TEMPERATURE INSTRUMENT COMPONENTS
4.1.1 Mirror Mounts
The mount design is the same for all mirrors. It has been driven by the high angular tolerance for some mirrors and the need to achieve repeatable optical alignment between warm up and cool down cycles. The thermal contraction of aluminum from room temperature to 77 K is approximately 3 mm per meter. Since all mirrors are rectangular and some are large pieces, a finite element analysis has been done to verify how much gravitational distortion to expect when mounting them from their edge. The final design is based on a kinematic mount with an edge support and fixed ball supports at the corners of the mirrors. In addition, a spring loaded clip at three corners of each optic, plus one horizontal edge cone-ball support point ensures that alignment can be reliably obtained with good thermal contact to the mirror substrate.
Figure 50: Details of the kinematic mirror mounts. All of the optical mounts have been designed to positively retain the optic without overly constraining it. A kinematic type approach was used in the design of the optical mount. The optic adjustment in the “Z” direction is +/- 1.5mm angular adjustment is +/- 1.degree.
CN-0004 86 Critical Design Document (CDD)
Figure 51: Kinematic optical mount detail. The optics rest against stainless steel balls to minimize surface contact and maintain repeatability during thermal cycling of the system. Spring loaded “clamps” position the optic against the balls. The clamps are aligned with the balls so that the spring force is against the ball. The Tip/Tilt/Z Adjustment is accomplished by using a screw to push a tooling ball against spring tension to align the optic. The tooling ball locates in a socket in the backing plate of the optic mount. Once the optic is aligned the lock nut is tightened to lock the position. Each optic mount has three adjustment screws for tip/tilt/z adjust. Adjustments are around the center of the optical face. The tension springs are retained by a dowel pin that rests in a slot centered on the thru hole for the spring.
To check the kinematic mirror alignment scheme we have modeled the optics as a vertical plate with the nominal mirror thickness supported by two spherical ball constraints from the bottom. In addition a small horizontal force pushes the corners of the mirror against 3 fixed ball supports on the back of the mirror surface near its edge. This mounting scheme yields a maximum horizontal deflection of our largest mirror of less than 87 nm. The maximum vertical deflection of any point on the mirror surface is less than 4 nm. These are all well within our mirror figure tolerance requirements. Another major constraint is the cool down time of these mirrors. To estimate this cooling time, some experiments have been done in a test dewar to estimate the mirror cooling time with our spring-loaded edge conductive coupling and emissive back mirror surface. In all cases we found that the optics cool to below 100 K within the nominal 2-day CryoNIRSP cool down time. To reduce the mass of the mirrors their back surface is generally not perpendicular to the optical axis, but parallel to the center tangent of the mirror front surface. Since each mirror is fiducialized from the front by conical touch points there is no loss in optical reference for aligning the optics and considerable reduction in mirror thermal and gravitational mass.
CN-0004 87 Critical Design Document (CDD)
Figure 52: Model results for mirror deflection caused by the mounts.
Figure 53: Thermal and mount tests with a prototype mount using the final mount design.
CN-0004 88 Critical Design Document (CDD)
Figure 54: Cool down times of the test configuration.
Figure 55: Optics mounting scheme (left, middle) and substrate shape (right) (SM5 components are shown here)
CDD-4.1.1 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
CN-0004 89 Critical Design Document (CDD)
4.1.2 Adjustable Mounts
Fixed mounts for mirrors that do not require remote motorization will be adjustable with lockable micrometer screws.
Requirements Angular accuracy [] Spectrograph ± 0.06 Context imager ± 0.03 Angular resolution [] Spectrograph 0.025 Context imager 0.015 Decenter accuracy [mm] Spectrograph ± 2 Context imager ± 1 Spacing accuracy along optical axis [mm] ± 0.5 Spacing resolution along optical axis [mm] 0.1 Table 28: Fixed mount requirements
CDD-4.1.2 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
4.1.3 Analyzing Beam-splitter Mount
This assembly is a machined aluminum support for fixed mirrors. The prism will be mounted on an adjustable and lockable rotation stage from Sigma Koki (model KSPS 406M) and a xy-axis stage from Newport (model M-DS25- XY). The spectrograph camera mount is attached directly on this assembly. A light trap mounted to the analyzer housing will capture all the light that would otherwise be reflected back into the dewar.
Requirement Alignment fixed Angular accuracy [°] 0.1 Table 29: Beam-splitterfixed mirror mounts requirements
Requirement Angular accuracy [°] ±0.1 Angular resolution [°] 0.02 Decenter accuracy [mm] ±0.1 Translation resolution [mm] 0.02 Translation accuracy [mm] ±0.02 Table 30: Beam splitter prism mount requirements.
CDD-4.1.3 Source: ISRD-5.2, 5.4, 5.6 Verification: Analysis, Inspection
CN-0004 90 Critical Design Document (CDD)
Figure 56: Beam-splitter mechanical design. The left image shows the xy and rotational stages to adjust the prism and the SIDCAR ASIC that is connected to the FPA. Right, the three fold mirrors, the two wire grid polarizers and the 90-degree prism form our polarization analyzer.
Figure 57: Compact and lockable x, y stage to mount the prism.
CN-0004 91 Critical Design Document (CDD)
Figure 58: Sigma Koki KSPS 406M rotation stage.
Figure 59: The light trap that is attached to the polarization analyzer assembly will capture all the light that would otherwise be reflected back into the dewar.
CN-0004 92 Critical Design Document (CDD)
4.1.4 Camera sub-assembly
Both cameras from the context imager and the spectrograph are identical. Therefore their sub-assemblies are also identical. Each camera and its dual ASIC side car are vertically attached to the same mounting platform (see Figure 60). These assemblies are electrically isolated fixed mount, hence the FPA are electrically isolated from the rest of the system.
Name SCAM/CCAM Requirement Axial accuracy [mm] ±0.25 Angular accuracy [°] ±0.03 Decenter [mm] ±0.25 Mass [lbs} 4.8 Table 31: Camera mount requirements
CDD-4.1.4 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis, Inspection
Figure 60: FPA and ASIC mount (front view).
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Figure 61: FPA and ASIC mount (rear view).
4.1.5 Pick-off Mirror Assembly
This custom pivot type assembly allows the pick-off mirror and the slotted pick-off mirrors to be moved in and out of beam path. It is designed to fit within available space. Microdot switches are used for position reference.
Name PO Requirement Mount type pivot Travel range 360 rotation: 0 deg. rotation = context imager 90 deg. rotation = spectrograph 180 deg. rotation = single slit slotted mirror 270 deg. rotation = triple slit slotted mirror Number of axis 1 Angular repeatability [] ±0.25 Angular resolution [] 0.005 Time to move to next position [sec] < 7 Mass [kg] 6.6 Table 32: Pickoff mirror assembly requirements
CN-0004 94 Critical Design Document (CDD)
Figure 62: Pickoff mirror assembly.
CDD-4.1.5 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis, Inspection
4.1.6 Focus Mirror Stages
The focus mirror stages are designed as a cross roller slide with precision ball screw for maximum accuracy and repeatability. Microdot switches are used for position reference and limits.
Name CM5 SM5 Requirement Mount type Cross roller slide Travel range [mm] 20 Number of axis 1 Linear resolution [mm] 0.02 Linear repeatability [mm] +/-0.02 Time to move to next position [sec] < 30 Mass [lbs] 56.4 22.5 Table 33: Focus mirrors requirements
CN-0004 95 Critical Design Document (CDD)
CDD-4.1.6 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
Figure 63: Focus mirror stage design. Here the mount for SM5 is shown.
4.1.7 Context Imager Filter Wheel Assembly
The assembly has a common filter wheel design box and provides two filter wheels implementing up to 18 positions. It is designed to fit in the working volume and beam path of the context imager. Spacers will be used to accommodate different filter thicknesses. Wheels will have detent switches to register home position and active cells. Name CW CWF 1 & 2 Requirement Mount type Fixed Motorized Travel range 81 combinations 360 rotation, 2× 9 positions Number of axis n/a 2 × 1 Angular repeatability ±100 micro rad Angular resolution 50 micro rad Housing alignment ±0.25 Time to move to next/ farthest position [sec] n/a < 60 / < 60 Mass [lbs] 13 Maximum filter thickness [mm] n/a 10 Table 34: Context imager filter wheels assembly requirements
CN-0004 96 Critical Design Document (CDD)
Figure 64: Context imager filter wheel assembly.
CDD-4.1.7 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
CN-0004 97 Critical Design Document (CDD)
4.1.8 Spectrograph Filter and Slit Wheel Assembly
One assembly provides both the filter wheel and the slit wheel implementing up to 16 identical positions each. The dual wheel filter box, holding slits and filters is designed to fit in the working volume and beam path of the spectrograph. This design maximized filter and slit locations in the given space. Spacers will be used to accommodate different filter thicknesses. Wheels will have detent switches to register home position and active cells. Multi-slit option: The ATST SRD (3.2.8) implies that an 80 s cadence is required for significant improvement in nanoflare science. Intensity and spectroscopy observations are possible with this cadence and arcsec spatial resolution using the FeXIII line if the 120” CryoNIRSP field-of-view can be spectrally sampled about 3 times more efficiently. CryoNIRSP achieves this cadence and resolution goal with a “triple-slit” at wavelengths near 1075nm. To accommodate this goal one slit wheel position is reserved for a multi-slit and one filter wheel position is reserved for a narrow-band (1.2 nm) FeXIII filter.
Name SW SWF SWS Requirement Mount type Fixed Rotational Rotational Travel range 360 rotation, 360 rotation, 360 rotation, 256 combinations 16 positions 16 positions Number of axis n/a 1 1 Angular repeatability ±100 micro rad ±10 micro rad Angular resolution 50 micro rad 10 micro rad Angular housing alignment ±0.25 Time to move to next / < 60 / < 60 farthest position [sec] Mass [lbs] 31 Maximum Filter/Slit n/a 10 mm 10 mm thickness [mm] Table 35: Spectrograph filters and slits wheel assembly requirements.
Figure 65: Filter and slit wheels assembly. CDD-4.1.8 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
CN-0004 98 Critical Design Document (CDD)
4.1.8.1 Filter Wheel Test The filter wheel assembly for the context imager has been delivered with our test dewar. Speed, accuracy and repeatability tests are described in our motion control analysis (Section 7.14).
4.1.9 Grating Turret
The grating turret holds two gratings kinematically mounted onto a structure that rotates 360 degrees around a vertical axis. A full travel range of 360˚ is needed not only to accommodate 2 gratings but also to be able, for diagnostic and alignment purpose, to rotate the grating to ‘zero order’, that is the position that would reflect the beam either back to the entrance slit or off to the detector, as if the grating were a normal flat mirror. Hall effect sensors and a high resolution, 23bit encoder will be used for home reference position and positioning. The grating drive mechanism pierces the dewar separately from the encoder shaft which mounts rigidly to the grating platform.
Name SGT Requirement Travel Range 360 rotation Angular repeatability [microrad] 4 Angular resolution [microrad] 1 Rotation axis alignment[microrad] 10 Time to move to any other position with the same grating [s] < 70 Time to move to any other position on the other grating [s] < 70 Mass [lbs] 102 Table 36: Grating turret requirements CDD-4.1.9 Source: ISRD-5.2, 5.4, 5.6 Verification: Design Review, Analysis
Figure 66: Grating travel range.
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Figure 67: Grating turret details.
CN-0004 100 Critical Design Document (CDD)
4.1.10 Deployable Cold Stop Mount
The cold stop mask in front of the mirror SM4 is mounted on a custom motorized stage which moves the stop in and out of the spectrograph beam.
Requirement Angular resolution [°] 0.225 Angular accuracy [°] 0.5 Time to move to other position [s] 5 Table 37: Spectrograph cold pupil mount specifications.
Figure 68: Deployable cold stop (aperture colored in magenta) in the spectrograph context.
Figure 69: Deployable cold mask mechanism.
CN-0004 101 Critical Design Document (CDD)
CDD-4.1.10 Source: ISRD-5.16 Verification: Design Review
4.1.11 Baffling
Primary optical baffling consists of anodized black shroud and corrugated cylinder assemblies. The baffling consists of the following primary components: A corrugated cylinder that extends from the pick-off mirror assembly towards the entrance window A box around the pick-off mirror assembly Corrugated rectangular baffles extending in both directions from the spectrograph filter and slit wheel assembly A cylindrical housing around the spectrograph grating A corrugated cylinder that extends from the context imager filter assembly to the detector mount Rectangular housing around the analyzing beam splitter assembly Light trap that absorbs light reflected out of the analyzing beam splitter assembly Additional cold-pupil baffles are mounted at the entrance to the grating turret assembly, on CM4 and on SM4. The pupil on SM4 is actuated to flip in and out of the beam during disk observing where the higher spectral resolution requires full grating illumination and out-of-field thermal emission is not a concern.
Figure 70: Cross section of the baffling on the spectrograph slit and filter wheel assembly and the pick-off assembly.
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Figure 71: Cylindrical housing around the spectrograph grating.
CDD-4.1.11 Source: ISRD-5.16 Verification: Design Review
4.1.12 Motors
4.1.12.1 Location External motors are used to improve the reliability and to decrease the internal heat load and wiring complexity of the dewar system (see Figure 72). Only one type of motor will be used for all motorized mounts. The choice for the stepper motors is based on previous knowledge of the manufacturer reliability, the power class, duty rating (aligned on the most demanding component: the grating) and ability to interface with proposed electronics. All stages are customized parts because of the cryogenic thermal environment and tight mechanical requirements. Ferrofluidic mechanical feed- through pierce the outer dewar surface and light-baffled re-entrant mechanical baffles isolate the feed-throughs into the inner cold working space.
CDD-4.1.12.1 Source: ISRD-3, 5.4 Verification: Design Review
CN-0004 103 Critical Design Document (CDD)
Figure 72: Steppers, feed-through, and mounts.
4.1.12.2 Description Steppers are the chosen type of motor for all motorized mounts. Motors will be connected with a drive shaft thru the case wall with a Ferrofluidic feed-through. See section 7 for the detailed control design layout.
CDD-4.1.12.2 Source: ISRD-3, 5.2, 5.4 Verification: Design Review
CN-0004 104 Critical Design Document (CDD)
4.2 ROOM TEMPERATURE INSTRUMENT COMPONENTS: FEED OPTICS
4.2.1 Adjustable Mirror Mounts
Fixed custom mounts for mirrors that do not require remote motorization will be adjustable with lockable micrometers.
Requirements Angular accuracy [] ± 0.05 Angular resolution [] 0.025 Decenter accuracy [mm] ± 2 Spacing accuracy along optical axis[mm] ± 0.5 Spacing resolution along optical axis[mm] 0.1 Table 38: Fixed mounts requirements
CDD-4.2.1 Source: ISRD-5.2, 5.9 Verification: Analysis
4.2.2 Scanning Mirror Motorized Stage
The scanning mirror will be mounted in a dual axis gimbal mount from Aerotech. The vertical axis will be motorized to scan the field of view. The other axis will be locked. The smallest slit size is 52 microns hence the minimal step size should be ¼ of the slit size to ensure overlap.
Name FM1st Requirement Mount type 2 axis gimbal/flex pivots Angular travel range [micro rad] 8500 Angular repeatability [micro rad] 3.7 Angular resolution [micro rad] 7.38 Number of axis 2 Horizontal axis manual micrometer Vertical axis motorized micro-positioner Time to move to the next position 200 ms Table 39: Scanning mirror requirements.
CDD-4.2.2 Source: ISRD-5.9 Verification: Design Review, Analysis
CN-0004 105 Critical Design Document (CDD)
Figure 73: Scanning mirror mount.
Model AOM130M-9 @ 4000 Steps/Rev 0.04 µrad (0.009 arc sec) Resolution @ 1000 Steps/Rev 0.18 µrad (0.037 arc sec) Clear Aperture 218.95 mm Range (Mechanical) 360° AZ/EL Range (Motor-Driven) ±4° AZ/EL Unidirectional Repeatability 3.64 µrad (0.75 arc sec) Component Diameter (Max) 228.6 mm Component Thickness (Max) 41.4 mm Component Weight (Max) 15 kg Maximum Slew Rate 6.2°/min Mount Weight Using Brushless Motors 20.2 kg Vacuum Capability (Optional) 10-3 or 10-6 mbar Material Aluminum
Table 40: Aerotech gimbal mount specifications.
CN-0004 106 Critical Design Document (CDD)
4.2.3 Focusing Mirror Motorized Stage
The focusing mirror stage will use the same custom design as the cryogenic mirrors. A separate stepper motor will drive the cross roller slide with precision ball screw for maximum accuracy and repeatability. Microdot switches are used for position reference and limits.
Name FM2 Requirement Mount type Cross roller slide Travel range [mm] 25 Number of axis 1 Linear resolution [mm] 0.02 Linear repeatability [mm] +/-0.02 Time to move to next position [sec] < 30 Table 41: Warm focusing mirror requirements.
CDD-4.2.3 Source: ISRD-5.2, 5.9 Verification: Design Review, Analysis
4.2.4 Warm Filter Wheel Assembly
The assembly has a common filter wheel design box and provides one filter wheel implementing up to 7 positions. It is designed to fit in the working volume and beam path of the warm optics. Spacers will be used to accommodate different filter thicknesses. The wheel will have detent switches to register home position and active cells.
Name FW F1 Requirement Mount type Fixed Motorized Travel range 7 combinations 360 rotation, 1× 7 positions Number of axis n/a 1 Angular repeatability ±0.25 ±0.5 Angular resolution 0.25 Time to move to next/ farthest position [sec] n/a < 60 / < 60 Maximum filter thickness [mm] n/a 10 Table 42: Context imager filter wheels assembly requirements
4.2.5 Optical Safety Mechanism
A rapid pneumatic shutter assembly outside of the dewar will be directly controlled by photodiode that detects high- scattered light levels reflected from the context imager detector array. If the filter and slit positions are operating in “open” position and full sunlight illuminates the CryoNIRSP context imager detector, the safety interlock will block the beam within 50 ms before the entrance of the dewar by activating a gate valve maintained in open position by a solenoid. The shutter is black to avoid reflecting the blocked beam into the Coudé room. The mechanism can be armed/disarmed and opened/closed manually at the instrument control panel or remotely by the operator.
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Name FSM Requirement Mount type Pneumatic actuated slider Number of axis 1 coating black air pressure [bar] 1.015 - 7 Time to be activated 50 ms Table 43: Safety mechanism requirements.
Figure 74: Safety mechanism design.
CDD-4.2.5 Source: ISRD-4.6 Verification: Design Review, Analysis
4.2.6 Window Mount
The entrance window will be inserted in an optical port machined into the dewar case and placed using a large retainer ring with soft compression interface.
Requirement Angular accuracy [] 0.25 Decenter [mm] ± 1.0 Angular position Inclined by 1.67 Table 44: Window mount requirements. CDD-4.2.6 Source: ISRD-5.2, 5.9 Verification: Analysis
CN-0004 108 Critical Design Document (CDD)
4.2.7 Baffling
Black anodized shields will be placed around the scanning mirror FM2 to prevent damage caused by beam misdirection. At other places if necessary. CDD-4.2.7 Source: ISRD-5.16 Verification: Design Review
4.2.8 Motors
We use stepper motors to drive the feed optics focusing stage. This is the same type as used to drive the cold optics stages. See section 7 for the detailed control design layout.
CDD-4.2.8 Source: ISRD-5.2 Verification: Design Review
CN-0004 109 Critical Design Document (CDD)
4.3 VACUUM SYSTEM
Conventional roughing and turbo-pump systems will be used to evacuate the dewars. The roughing pumps will be located in the electronic racks. A filter will be mounted to the roughing pump exhaust to retain ejected particles. A water cooled turbo pump is directly attached to each dewar. A software alarm system will monitor the dewar. A burst valve on the dewars provides a pressure release mechanism for catastrophic dewar vacuum or cryogenic failure. The spectrograph and the context imager dewar will be connected by a vacuum bellow. Brackets are used to guarantee that the dewars are not pressed together when the vacuum pumps are turned on. A gate valve on the spectrograph dewar allows the two vacuum enclosures to be isolated. The gate valve needs compressed air with 4 to 7 bar pressure.
Equipment Specification Vacuum port Interface ISO-160 Gate Valve Turbo pumping station V 750 TWISTORR Rough Vacuum Pump TriScrol 600 Vacuum controller Agilent turbo controller (model 969-9525) Vacuum gauge Pfeiffer PKR 251 cold cathode Gauge controller Pfeiffer TPG 262 Vacuum Pump Isolator Varian (VPI) Getter Activated Coconut Charcoal Table 45: Vacuum system specifications.
CDD-4.3 Source: ISRD-5.4 Verification: Design Review
4.3.1 Context Imager Vacuum System
The Context imager is equipped with a KF-50 vacuum vale and a Turbo-V 750 TWISTOR ISO160 turbo pump mounted directly on the case bottom cover. An ISO160 gate vale is mounted between the turbo pump and the dewar case for pump isolation. The Context cryostat should reach a vacuum pressure of less than 1 x 10-4 Torr at 300K. We consider a pressure of 10-4 torr as the maximum operating pressure for the dewars. With running turbo pumps the pressure should drop to 10-6 with cryo pumping. If the turbo pumps were switched off the combination of cryo pumping and active charcoal gatherers should keep the pressure below 10-4 torr for several days. The exact time will be determined early. Once a year the gatherers should be warmed to room temperature to recover. The context Imager shares a common guard vacuum with the spectrograph imager while in operation. Viton O-Rings are used at all vacuum interfaces.
4.3.2 Spectrograph Vacuum System
The spectrograph is equipped with a KF-50 vacuum vale and a Turbo-V 750 TWISTOR ISO160 turbo pump mounted directly on the case bottom cover. An ISO160 gate vale is mounted between the turbo pump and the dewar case for pump isolation. The Context cryostat should reach a vacuum pressure of less than 1 x 10-4 Torr at 300K. We consider a pressure of 10-4 torr as the maximum operating pressure for the dewars. With running turbo pumps the pressure should drop to 10-6 with cryo pumping. If the turbo pumps were switched off the combination of cryo pumping and active charcoal gatherers should keep the pressure below 10-4 torr for several days. The exact time will be determined early. Once a year the gatherers should be warmed to room temperature to recover. The spectrograph shares a common guard vacuum with the context imager while in operation. Viton O-Rings are used at all vacuum interfaces.
CN-0004 110 Critical Design Document (CDD)
4.4 CRYOGENIC SYSTEM
The CryoNIRSP instrument is comprised of two cryostats, the context imager and the spectrograph. The two instruments are positioned next to each other and share a common guard vacuum. This approach was chosen over one single cryostat for ease of manufacturing resulting in reduction of cost and weight and because the requirements for the inner radiation shield temperature of the context imager could be relaxed without scientific impact. CryoNIRSP is a closed cycle cryostat that has multiple cold heads (GM style refrigerators) mounted on vibration minimizing interfaces. The vacuum jacket (case) material is made with 6061 aluminum with the exterior surfaces anodized. The upper part is removable for instrument access. All optics for the context and spectrograph instruments are accessible when the upper part of the dewar is removed. All mechanical and electrical vacuum case penetrations occur on the bottom cover of each system to allow for easy assembly and disassembly. Due to the size and weight of each instrument they are mounted on a permanent stand with casters. Once the instruments are on site and aligned, the stands will be anchored to the floor decking to secure the instrument to the telescope. The interface between the two systems is via a flexible bellows and a single pneumatic gate valve. The size and shape of the instruments were driven by the optical design for the Teledyne Imaging Sensors (TIS) H2RG detector and ASIC.
Figure 75: Dual dewar design of CryoNIRSP.
Physical information Context imager dewar Spectrograph dewar CryoNIRSP total Mass 706 kg 1101 kg 1807 kg Load per leg 177 kg 276 kg - Size (LWH in inches) 73 x 34 x 52 75 x 46 x 60 100 x 83 x 60 removable top case mass 208 kg 250 kg - Cold mass 106 kg 315 kg - Table 46: Dewar physical characteristics.
CN-0004 111 Critical Design Document (CDD)
CDD-4.4 Source: ISRD-5.4 Verification: Design Review
4.4.1 Handling System
The dewar will be supported by a cart mounted on clamps as defined in ICD 1.1/3.1.3. Internal weld flange for G10 rigid support will be installed to allow the removal of the top cover. Welded mounting block on sides of the case will be installed for lifting the complete assembly. Clevis and tow attachments will be added for mechanized truck for movement. Cross bars will be provided as part of the handling system to lift the box separately (lid removed).
4.4.2 Context Imager
The optical design layout is the driving factor for the design of the context imager cryostat along with the operational requirements provided by the CryoNIRSP team. The context imager will be a separate cryostat and joined to the spectrograph via a bellows and a single gate valve interface. The context imager is capable of running alone without the spectrograph if needed, this concept will allow for easier setup and testing of the context imager. The context imager will operate with a single Sumitomo RDK-408s cold head (provisions have been made to add an additional cold head if needed for lower back ground temperatures). The cryostat will also have its own vacuum valve and turbo pump mounted to the bottom case cover allowing it to operate separately from the spectrograph. The Bellows and gate valve act as the interface between the two systems and are also the port that will allow the optical beam to be sent from the spectrograph to the context optics and array. A test dewar has been manufactured and delivered to the IFA at the University of Hawaii, this test dewar will serve as the cryo engine of the context imager.
Figure 76: Context imager dewar (left), top section and shields removed (right).
CN-0004 112 Critical Design Document (CDD)
Figure 77: Left, test dewar cryo-engine. Right, cross section through cryo-cooler cold fingers.
4.4.2.1 Context Imager Cold Head Model: Sumitomo RDK-408s The context imager has a calculated thermal heat load of 139 Watts on the outer active region and 10 Watts of heat on the inner region including H2RG and SideCAR ASIC during operation. The required operating temperature of the array is 30 K with a background temperature of 190 K. The required operating temperature of the SideCAR ASIC is 70 K. The Sumitomo model RDK-408S was selected for the context imager cryostat based on its robust cooling power and proven performance on previous systems that Universal Cryogenics has built that were similar in size and operating requirements.
Power Supply 50 Hz 60 Hz
2nd Stage Capacity 5.4 W @ 10 K 6.3 W @ 10 K
1st Stage Capacity 35 W @ 45 K 40 W @ 45 K
Minimum Temperature <3.5 K
Cool down Time to 10 K 60Minutes
Weight 17.2 kg (37.9 lbs.)
Maintenance 10,000Hours
Table 47: Sumitomo RDK-408s info.
CN-0004 113 Critical Design Document (CDD)
Figure 78: Sumitomo RDK-408 load map.
4.4.2.2 Context Imager Thermal Components
Components Description Interface Power Temperature specification Operations Passive shield Passive Material 0.125" thick Flange mounted using Total Shield 6061-T6 aluminum hardware external surface thermal load area of 57,600 cm2 bare aluminum 60W 280K Outer 1st Stage Material 0.125" thick Flange mounted using Radiation shield 6061-T6 aluminum 2 hardware external surface Shield piece radiation shield bare aluminum Total on first stage area of thermal load Average 40,000 cm2 139W 180K Inner 2nd Stage Material 0.125" thick Flange mounted using Radiation shield 6061-T6 aluminum 2 hardware external surface Average Shield piece radiation shield. bare aluminum - 20K Table 48: Context imager thermal components.
CDD-4.4.2.2 Source: ISRD-5.4 Verification: Design Review, Analysis
4.4.2.3 Context Imager Thermal Analysis The passive shield gets a net radiation heat load from the case of 60 Watts and has a steady state temperature of 280 K. The 1st stage receives a net 110 W thermal radiation from the passive shield and 11 W by residual gas conduction from the dewar case. An additional 10 W make their way through the 20 G10 supports (0.5” thick and 4” wide). The dewar
CN-0004 114 Critical Design Document (CDD) entrance window yields a heat load of 8 W. So the 1st stage has a total heat load of 139 W. Using the SRDK-408S cryo-cooler to cool the 1st stage, the 1st stage cold tip of the cooler will be at 140K. The temperature difference between the cold tip and the center of the cold plate is 10K. This design is based on 700 copper straps (6” x 0.002” of which 4” are flexible) with a conductivity of 8 Wcm-1K-1. The edges of the cold plate will be at 165K and the farthest point of the 1st stage radiation shield has a temperature of 195K. Using the SRDK-408S cryo-cooler to cool the 2nd stage, the 2nd stage cold tip of the cooler will be at 12K. This has already been verified with the test dewar. The total cold mass on the context imager 1st stage is 88 kg and it will take 72 hours to cool the outer regions to bellow 180K. The total cold mass on the context imager 2nd stage is 18 kg and it will take 48 hours to cool the outer regions to 20 K.
4.4.2.4 Context Imager Vibration Isolation The cold head is mounted with a bellows assembly designed to minimize the translation of vibrations from the cold head to the optical components. The concept is the guard vacuum force is applied to both sides of the cold head mounting interface to facilitate the cold head resting in a neutral position during operation reducing and or eliminating the transfer of vibrations into the system. The counter balance bellows size and quantity are determined by the amount of force incurred by the main bellows under vacuum plus or minus the weight of the cold head and thermal links, depending on the orientation of the assembly during use. This design concept is new and was specifically created for this project. The test results show that vibrations off the cold working surfaces are reduced to below 2 µm RMS.
Main bellows inside dia: 127mm
Counter balance bellows inside dia: 49.22mm
Number of balance bellows: 5
Cold head weight: 17.2 kg
Thermal link weight: 9.13kg
Main bellows pressure: 290 Psi
Balance bellows Pressure: 46 Psi *5=230 Psi Figure 79: Floating anti-vibration mount design.
CN-0004 115 Critical Design Document (CDD)
4.4.2.5 Context Imager Thermal Link
Figure 80: Thermal links for the context imager cryo-cooler (flexible copper ropes are drawn straight in the CAD model but are actually bent).
The thermal links are made from oxygen free copper and connected by welding flexible copper rope between the plates to act as the thermal path from the cold head to the instrument stage plates. The welded copper rope remains very flexible and compliant after welding and reduces the transfer of vibrations wile thermally linking the cold head to the system. The base flange of each assembly is bolted to the cold head and the top flange of each assembly is bolted to its respective stage. This thermal link design has been used in several of our cryostats and is quite effective for thermal transfer with minimal vibration translation. Other cryostat manufactures use this welded rope scheme in their low vibration systems as well.
Figure 81: Thermal link between the 1ststage of test dewar and the 1st stage of context imager.
CN-0004 116 Critical Design Document (CDD)
4.4.2.6 Context Imager Electrical Isolation
Figure 82: Context imager electrical isolation.
The system requires electrical isolation of the dewar stages from the vacuum case, to accomplish this we use 4 G-10 stand offs to mount the fixed plate of the cold head mount to the case cover plate. The interface between the ISO bellows and the case cover has a G-10 centering ring that electrically isolates the assembly from the vacuum case of the dewar. Universal Cryogenics uses this design for all of cold head mounts requiring electrical isolation and it has been proven to work effectively and reliably with no issues.
4.4.2.7 Context Imager Vacuum Case The vacuum case is comprised of “panels” that will be welded in to frames to create the main case body, the O-ring interfaces will be on these frames. The top and bottom covers will be made from solid plate. Using SolidWorks simulation an FEA study was performed to validate the case material thickness and weight reduction pockets. This method of manufacturing for large vacuum cases is a common practice at Universal Cryogenics and has performed well in the past. All case panel welds are internal and will be verified by Helium leak check.
CN-0004 117 Critical Design Document (CDD)
Figure 83: Displacement of top case cover under vacuum is 2.3 mm.
Figure 84: Context Imager case panels and frames:
CN-0004 118 Critical Design Document (CDD)
Figure 85: Welded context imager top case assembly. The top cover measures 1,803 x 813 x 35 mm , the case flange measures 1,803 x 813 x 31.75 mm, the case walls are 19 mm thick and the case weldment (without cover) measures 1,803 x 813 x 504 mm .
Figure 86: Bottom view of the context imager dewar.
CN-0004 119 Critical Design Document (CDD)
4.4.2.8 Context imager Cold Stages
Figure 87: Context imager cold stages are isolated by G-10 stand-offs shown in green.
Context imager stage plates are made from 6061 aluminum. The optic mounts reside on the 1st stage on the system except for the dual filter wheel and the FPA/ASIC mount, these are located on the 2nd stage of the test dewar which is installed as the cryo-engine of the context imager. G-10 supports are used to mount the stages to the bottom cover of the instrument. G-10 is used because of its poor thermal conductance and high strength. The supports are positioned “radially” around the center of contraction of the system, with the face of the G-10 facing to the center of contraction.
Figure 88: The support assembly consists of two aluminum blocks and G-10 strap material. The aluminum blocks have threaded holes for mounting of the stage plates.
CN-0004 120 Critical Design Document (CDD)
4.4.2.9 Context imager Thermal Shields The radiation shields are made of 6061 aluminum panels welded in to 6061 aluminum frames. The frames are the mounting surfaces and have a labyrinth light sealing interface to keep each region “light tight”.
There are 3 levels of radiation shielding: Passive shield is the outer most layer of the thermal shielding located between the vacuum case and the 1st stage shield. This shield is not actively cooled. The 1st stage shield that is mounted to the perimeter of the 1st stage cold plate and cooled from the first stage of the cold head. The 2nd stage shield is mounted on the perimeter of the 2nd stage cold plate and is cooled by the 2nd stage of the cold head.
The 1st and 2nd stage shields mount on stepped down areas of their respective stage plates to facilitate a light tight seal. Each shield has a removable lid for access into the shielded area. All shield penetrations are baffled to minimize/eliminate stray light. UCryo has polished shields and has achieved emissivities of polished aluminum surfaces at 0.05. These shields will be polished to maximize the surface reflectivity.
Figure 89: Modular thermal heat shield design.
CN-0004 121 Critical Design Document (CDD)
Figure 90: Cross section of the context imager. The thermal heat shield is shown in blue.
4.4.2.10 Test Dewar Cool Down Time We used our test dewar to determine cool down times. During this experiment no thermal heat shields were mounted and the first stage reaches a temperature of 32 K after 10 hours. The 48 hours cool-down time from the full-dewar thermal analysis is consistent with the test dewar cool-down time. The measured cool down times for a dewar, similar to the context imager, confirm the results of the thermal analysis.
280
230
180 1st stage
130 filter wheel base temperature temperature [K]
80
30 0 5 10 15 20 25 30 35 40 45 50 time [hours]
Figure 91: Cool down times measured with the test dewar. The temperature bump at 18 hours is due to the cooler being switched off for vibration measurements. The cooler was turned off after 21 hrs.
CN-0004 122 Critical Design Document (CDD)
350
300
250
200 2nd stage
150 inner
temperature temperature [K] shield 1st stage 100
50
0 0 5 10 15 20 25 30 time [hours]
Figure 92: Cool down time for a dewar that has a similar size as the context imager, which was built by UCryo for the Big Bear Observatory.
4.4.3 Spectrograph
The optical design layout is the driving factor of the size and shape of the spectrograph instrument as well as the thermal requirements. The CryoNIRSP spectrograph cryostat is a separate unit that will be connected to the context imager with an ISO 160 bellows and gate valve. The two systems share a common guard vacuum but can be isolated from one another and run independently, this approach allows for testing and set up of each system independently. The spectrograph will use Sumitomo GM style cold heads, there are two model RDK-408s two stage coolers and one model RDK-400B single stage cooler. The single stage RDK-400B cooler cools the 1st stage of the spectrograph cooling the outer shield. The 1st stages of the RDK-408s coolers will be connected to the 2nd stage plate of the spectrograph and cool the optics and shield on the 2nd stage. The 2nd stage of the RDK-408s will be dedicated to the FPA and ASIC. The second RDK-408s is only needed during cooling down the instrument. It is connected in the same way to dewar as the first unit. This guarantees a redundant cooling operation for the spectrograph.
CN-0004 123 Critical Design Document (CDD)
Figure 93: Spectrograph dewar.
4.4.3.1 Spectrograph Cold Heads Model: Sumitomo RDK-408s and RDK-400B The spectrograph has a calculated thermal heat load of 260 Watts on the 1st stage region and 30 Watts on the 2nd stage region with an additional heat load of 4 Watts generated by the H2RG and SideCAR ASIC during operation. The required operating temperature of the array is 30 K with a background temperature of 70 K. The required operating temperature of the SideCAR ASIC is 70 K. The Sumitomo model RDK-408S and model RDK-400B were selected for the spectrograph cryostat based on their robust cooling power and proven performance on previous systems that Universal Cryogenics has built that were similar in size and operating requirements. Information on the RDK-408S model can be found in section 4.4.2.1.
Power Supply 50 Hz 60 Hz
1st Stage Capacity 54 W @ 40 K 70 W @ 40 K
Minimum Temperature <25 K
Cool down Time to 40 K 30Minutes
Weight 16.0 kg (35.3 lbs.)
Maintenance 10,000Hours
Table 49: Sumitomo RDK-400s information.
CN-0004 124 Critical Design Document (CDD)
Figure 94: Sumitomo RDK-400 load map.
4.4.3.2 Spectrograph Thermal Components
Components Description Interface Power Temperature specification Operations Passive shield Passive Material 0.125" thick Flange mounted using Total Shield 6061-T6 aluminum hardware external surface thermal load area of 80,600 cm2 bare aluminum 85 W 280 K Outer 1st Stage Material 0.125" thick Flange mounted using Radiation shield 6061-T6 aluminum 2 hardware external surface Shield piece radiation shield bare aluminum Total on first stage area of thermal load Average 63,000 cm2 309 W 180 K Inner 2nd Stage Material 0.125" thick Flange mounted using Radiation shield 6061-T6 aluminum 2 hardware external surface Shield piece radiation shield bare aluminum Total on second stage area of thermal load Average 59,000 cm2 44 W 60 K Table 50: Spectrograph thermal components.
CDD-4.4.3.2 Source: ISRD-5.4 Verification: Design Review, Analysis
4.4.3.3 Spectrograph Thermal Analysis The passive shield gets a net radiation heat load from the case of 85 Watts and has a steady state temperature of 280 K. The 1st stage receives a net 260 W thermal radiation from the passive shield and 26 W by residual gas conduction from the dewar case. An additional 15 W make their way through the 20 G10 supports (0.5” thick and 4” wide). The dewar
CN-0004 125 Critical Design Document (CDD) entrance window yields a heat load of 8 W. So the 1st stage has a total heat load of 309 W. Using the SRDK-400B cryo-cooler to cool the 1st stage, the cold tip will be at 140 K. The temperature difference between the cold tip and the center of the cold plate is 10 K. This design is based on 700 copper straps (6” x 0.002” of which 4” are flexible) with a conductivity of 8 Wcm-1K-1. The edges of the cold plate will be at 165 K and the farthest point of the 1st stage radiation shield has a temperature of 195 K. The 2st stage receives a net 30 W thermal radiation from the 1st stage shield and 8 W by residual gas conduction from the dewar case. An additional 5 W make their way through the 20 G10 supports (0.5” thick and 4” wide). The dewar entrance window yields a heat load of 1 W. So the 2nd stage has a total heat load of 44 W. Using two SRDK-408S cryo-cooler to cool the 2nd stage, the 1st stage cold tip of the cooler will be at 43 K. The temperature difference between the cold tip and the center of the cold plate is 5 K. This design is based on 500 copper straps (6” x 0.002” of which 4” are flexible) with a conductivity of 16 Wcm-1K-1. The edges of the cold plate will be at 50 K and the farthest point of the 1st stage radiation shield has a temperature of 70 K. The 2nd stage of the SRDK-408S cryo-coolers will be used to cool the detector array. The total cold mass on the spectrograph 1st stage is 90 kg and it will take 35 hours to cool the outer regions to bellow 180 K. The total cold mass on the spectrograph 2nd stage is 225 kg and it will take 48 hours to cool the outer regions to bellow 180 K.
4.4.3.4 Spectrograph Vibration Isolation The cold head is mounted with a bellows assembly designed to minimize the translation of vibrations from the cold head to the optical components. The concept is the guard vacuum force is applied to both sides of the cold head mounting interface to facilitate the cold head resting in a neutral position during operation reducing and or eliminating the transfer of vibrations into the system. The counter balance bellows size and quantity are determined by the amount of force incurred by the main bellows under vacuum plus or minus the weight of the cold head and thermal links, depending on the orientation of the assembly during use. This design concept is new and was specifically created for this project. The test results show that vibrations off the cold working surfaces are reduced to below 2 µm RMS.
Main bellows inside dia: 127mm
Counter balance bellows inside dia: 49.22mm
Number of balance bellows: 5
Cold head weight: 17.2 kg
Thermal link weight: 9.13kg
Main bellows pressure: 290 Psi
Balance bellows Pressure: 46 Psi *5=230 Psi
CN-0004 126 Critical Design Document (CDD)
4.4.3.5 Spectrograph Thermal Link
Figure 95: Thermal links for the spectrograph RDK-400 cryo-cooler (flexible copper ropes are drawn straight in the CAD model but are actually bent).
The thermal links are made from oxygen free copper and connected by welding flexible copper rope between the plates to act as the thermal path from the cold head to the instrument stage plates. The welded copper rope remains very flexible and compliant after welding and reduces the transfer of vibrations wile thermally linking the cold head to the system. The base flange of each assembly is bolted to the cold head and the top flange of each assembly is bolted to its respective stage. This thermal link design has been used in several of our cryostats and is quite effective for thermal transfer with minimal vibration translation. Other cryostat manufactures use this welded rope scheme in their low vibration systems as well.
CN-0004 127 Critical Design Document (CDD)
4.4.3.6 Spectrograph Electrical Isolation
Figure 96: Spectrograph electrical isolation.
The system requires electrical isolation of the dewar stages from the vacuum case, to accomplish this we use 4 G-10 stand offs to mount the fixed plate of the cold head mount to the case cover plate. The interface between the ISO bellows and the case cover has a G-10 centering ring that electrically isolates the assembly from the vacuum case of the dewar. Universal Cryogenics uses this design for all of cold head mounts requiring electrical isolation and it has been proven to work effectively and reliably with no issues.
4.4.3.7 Spectrograph Vacuum Case The spectrograph vacuum case is split in to two sections a lower “permanent” case section and an upper case section. Each case section is comprised of “panels” that will be welded in to frames to create the main case body, the O-ring interfaces will be on these frames. The top and bottom covers will be made from solid plate. Using SolidWorks simulation an FEA study was performed to validate the case material thickness and weight reduction pockets. This method of manufacturing for large vacuum cases is a common practice at Universal Cryogenics and has performed well in the past. All case panel welds are internal and will be verified by Helium leak check.
CN-0004 128 Critical Design Document (CDD)
Figure 97: Spectrograph top case cover mass is 248.4 kg.
Figure 98: Spectrograph upper case.
CN-0004 129 Critical Design Document (CDD)
Figure 99: Spectrograph permanent lower case (96 kg).
Figure 100: Spectrograph dewar bottom view showing the electrical feed throughs and vacuum ports.
CN-0004 130 Critical Design Document (CDD)
Figure 101: Electrical feed throughs on the test dewar.
Figure 102: Spectrograph dewar with the top case and radiation shields removed.
CN-0004 131 Critical Design Document (CDD)
4.4.3.8 Spectrograph Cold Stages The spectrograph has two stages the 1st stage will operate at a temperature of 180 K and serve as a shield for the 2nd stage the second stage temp will be 43 K.
Figure 103: Spectrograph design showing the G-10 stand-offs in green.
The 1st stage will be made from 12.7 mm (.500”) thick 6061-T6 aluminum plate and the 2nd stage is made from 19.05 mm (.750”) 6061-T6 aluminum plate. The mass of the 2nd stage with optic mounts is 222 kg.
Figure 104: The support assembly consists of two aluminum blocks and G-10 strap material. The aluminum blocks have threaded holes for mounting of the stage plates.
CN-0004 132 Critical Design Document (CDD)
4.4.3.9 Spectrograph Thermal Shields
The radiation shields are made of 6061 aluminum panels welded in to 6061 aluminum frames. The frames are the mounting surfaces and have a labyrinth light sealing interface to keep each region “light tight”. There are 3 levels of radiation shielding: Passive shield is the outer most layer of the thermal shielding located between the vacuum case and the 1st stage shield. This shield is not actively cooled. Passive shield is the outer most layer of the thermal shielding located between the vacuum case and the 1st stage shield. This shield is not actively cooled. The 1st stage shield that is mounted to the perimeter of the 1st stage cold plate and cooled from the first stage of the cold head. The 2nd stage shield is mounted on the perimeter of the 2nd stage cold plate and is cooled by the 2nd stage of the cold head.
The 1st and 2nd stage shields mount on stepped down areas of their respective stage plates to facilitate a light tight seal. Each shield has a removable lid for access into the shielded area. All shield penetrations are baffled to minimize/eliminate stray light.
4.4.4 Cryo-cooler Compressors
The cryo-coolers used to cool the system are each driven by a water cooled compressor. The supports on the mounting brackets for the Helium supply lines are vibration isolated. The only connection of the lines to the instrument will be on the cryo-cooler which is mounted isolated from the rest of the dewar.
Closed Cycle Coolers Gifford-McMahon Style Compressors 4 x Sumitomo F-50L, water cooled electrically isolated from dewar Compressor dimensions (L x W x H) 24 × 18 × 24 in (60 × 45 × 59 cm) Compressor electrical rating 480VAC, 3Ph, 60Hz or 200VAC, 3Ph, 60Hz Compressor power consumption 7.5 to 8.3 kW at 60 Hz Compressor cooling water flow rate 4 to 10 liters per minute at 4 to 28°C Compressor weight 120 kg Compressor maintenance 30,000 hours He lines maintenance None specified by manufacturer Table 51: Cryocooler compressor specifications
CDD-4.4.4 Source: ISRD-5.4 Verification: Design Review
CN-0004 133 Critical Design Document (CDD)
4.4.5 Temperature control
Four Lakeshore Cryogenic temperature monitors will be needed (each monitoring 8 channels). Temperature zones will measure the tip of the SRDK-408S2s, and the SRDK-400B and at the warm end of the copper straps to measure the cooler performance. In addition diodes at temperature zones on the detector mount, radiation shields, cold plates and selected optic mounts (grating, large collimating mirror) will also be monitored with the possibility of sampling the temperature at 32 points. A Lakeshore model 336 PID controller will control the temperature of both the imaging and spectrograph detectors. The imager and spectrograph detectors are thermally isolated from the 2nd stage cold plate and their ASIC controllers. Each detector is PID temperature regulated using separate channels of the 336 and associated 25 W heaters. The ASICs each generate 5 W and are thermally linked to the cryo-coolers with isolated electrical grounds. Additional heaters are placed on the 1st and 2nd stages of the context imager and spectrograph to speed up the warm- up procedure. Built-in thermistors ensure that the critical warm up rate for the detector arrays is never exceeded.
CDD-4.4.5 Source: ISRD-5.4 Verification: Design Review
4.4.6 Vibration control
Cryo-cooler mechanical interface mounting is by means of a combination of welded bellows assemblies working in conjunction to balance the pressure force when dewar – cryo-cooler assembly is working under a vacuum environment. When the assembly is under vacuum the cryo-cooler mounting plate is in a “floating” state and only the static weight of the cryo-cooler is fixed to the dewar cover. Dampening grommets are used in series to the cryo-cooler’s support legs to minimize any vibration coupling thru this attachment path. The supports on the mounting brackets for the Helium supply lines are vibration isolated. The only connection of the lines to the instrument will be on the cryo-cooler which is mounted isolated from the rest of the dewar.
Figure 105: Vibration control system on cold heads of cryo-coolers.
4.4.6.1 Vibration Tests Our test dewar uses the expected cryo-cooler mounting designed by Universal Cryogenics for CryoNIRSP. We bolted the test dewar to a solid board. In order to decouple the board from vibrations of the lab environment we mounted it
CN-0004 134 Critical Design Document (CDD) kinematically on three solid pins. The vacuum system vibrations through the floor were avoided by placing the pumping station on rubber feet To determine the vibrations on different parts of the dewar we used our Leica 901 laser tracker unit which is going to play a vital part in the overall alignment of our components. It can measure the absolute position of reference points (corner cube reflectors) with an accuracy of 15 µm. But relative position changes can be detected on the 3 µm level (or better). The coordinate system is chosen such that the z-direction follows gravity and represents up and down movements of the reference points. The y-direction corresponds to a movement towards or away from the tracker through the entrance window. Variations along the x-axis represent lateral displacement. We were able to track relative movements of reference points on the test dewar with 1 ms temporal resolution. A 3 µm peak to peak variation is seen even in the baseline data with the vacuum system and the chiller turned off. The baseline power spectrum shows a 30 Hz component which could be associated with the servo system of the laser tracker or another noise source. When turning the vacuum system on, the peak to peak variations do not increase but a few more frequency components show up at about 6.2, 12, 50 and 62 Hz. Beat frequencies of the chiller are not seen in the time series and the power spectra anymore. Previously reported vibrations were due to light baffles, which were touching other stages once the dewar was evacuated. Although the peak to peak variations and RMS values double in size, they are still lower than required by our design. Our concerns that the vibrating entrance window to the dewar might introduce a laser tracker beam displacement did not prove to be true. After introducing the window to the beam path and keeping it still, the variations stay the same. Even when we were tapping the window mount at a rate of 1 Hz, we were not able to detect spikes in the relative displacement measurements.
Conclusion The anti-vibration mount used in the test dewar efficiently reduces vibrations transmitted to the cold working and detector surfaces. Our concerns at CryoNIRSP Readiness review about cold head vibration have been solved by modifying the optical baffling hardware around the cold head thermal feed throughs.
Direction Location Peak to RMS Frequencies Description Peak [µm] [Hz] [µm] z 2nd stage 6 1.2 30, 62 y 2nd stage 6 0.8 10, 30, 62 x 2nd stage 6 0.7 10, 12, 30 z 1st stage 8 1.5 30 y 1st stage 8 1.1 6.2, 10, 30, 62 vacuum pump on, x 1st stage 6 0.8 6.2, 10, 30 cooler on z Dewar 7 1.0 30 y Dewar 6 0.9 5, 6.2, 12, 30, 62 x Dewar 6 0.8 1,2, 2,2 , 6.2, 12, 30 z Optical table 4 0.6 6,2, 30, 50, 62 y Optical table 3 0.3 30, 50, 62 x Optical table 5 0.6 30, 50 Table 52: Vibration tests summary.
CN-0004 135 Critical Design Document (CDD)
Figure 106: Test dewar set up to measure vibrations.
Figure 107: Time series of the movements measured in the z-direction on the 2nd stage.
CN-0004 136 Critical Design Document (CDD)
Figure 108: Power spectrum generated from the 2nd stage z-direction movement time series. The dominant excitation near 30Hz is present when there is no mechanical excitation of the dewar. Mechanical resonances at higher frequencies have been eliminated by modifying the cold baffle.
4.4.7 Operational constraints
The cool-down and warm-up procedures must follow strict safety rules. They are not software driven, although the engineering interface will continuously monitor temperature status and report feedback. These procedures will be detailed in the operator manual. Cryostat warm-up cycle “engineering servicing” . Turn off cryo-coolers . Turn on Lakeshores on outer stage (336-S-O) and inner stage (336-S-I) regions. (Each 336-S has two temp sensors located by one control heater in middle of region). This will monitor temperatures of those regions for feedback of heater loops. . Turn on TDK power supply’s full power 1.5kWof the 1st stage and 3.0kWof the 2nd stage regions. . Thermal region temperature will rise at a rate of 5.0 K per hour using temperature control of power output of TDK power supplies and temperature feedback of 336-S. . The heaters have integrated thermal fuses to avoid too high temperatures which could destroy the detector. The set point at which the fuse disables the heater is at 80° F. Once the heater cools down, it is enabled again. . A cold trap will always be at a lower temperature than the detector to avoid condensation on the array. Cool-down procedure . Begin turbo pumping dewars. . Monitor guard vacuum level of dewars. Once 103 Torr is reached turn on cryo-coolers.
CN-0004 137 Critical Design Document (CDD)
. Keep turbo pumping. When steady state is reached start measurements. Steady state is when no temperatures change more than 0.1 K over three hours. Depending on the cryo-cooling performance, the turbo pumps might be switched off.
4.5 DUST CONTROL
The instrument will be sealed whenever possible and all work with the open dewar assembly and optics will be undertaken in a class 10000 clean tent at the manufacturing site as well as at the ATRC optical laboratory. The environment in the Coudé laboratory is clean enough so that the instrument can be opened (if necessary the clean tent can be used). The lift capacity of the Coudé crane is sufficient to lift the CryoNIRSP dewar tops.
CDD-4.5 Source: ISRD-5.4 Verification: Design Review
4.6 EARTHQUAKE COMPLIANCE
CDD-4.6 Source: ISRD-5.4 Verification: Design Review
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5 CALIBRATION PROCEDURES
Ideally, experiment or observation-level calibrations are executed at night, before sunrise, or at twilight, typically before and after individual observing runs. They can also be run independently when necessary. System calibrations are performed after any major telescope or instrument change and generally follow the observation calibration, but are performed for all CryoNIRSP instrument modes. All images produced by each calibration task are available to other subsystems (DHS/quicklook/...). A general hierarchy of calibrations is illustrated in Figure 109 below.
Figure 109: CryoNIRSP calibration hierarchy.
System level calibrations are intended to be executed infrequently whenever the telescope or instrument hardware has been changed. System level calibrations encompass the full range of wavelengths for all modes.
These include:
Wavelength Baseline Polarization Baseline Scattered Light Baseline Alignment Focus
A full system calibration of all CryoNIRSP modes and wavelengths will require approximately 6 hours of solar observations. Stellar observations needed to obtain PSF and full aperture ATST system polarimetric Mueller calibration will require an additional 6 hours of night-time observations.
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Type ATST Mode CryoNIRSP Mode Duration Output [MB] [hrs] PSF Night-time; Context/SpectoP Multi 3 PSF table [0.1] Stellar wavelength Align GOS mask; Context/SpectroP. Ref. 0.5 Registration table Disk center wavelength, 3-mode pick [0.1] off Focus GOS mask; Context/SpectroP. Multi- 1 Baseline and baseline Disk center wavelength, multi- “delta” Focus table pickoff [0.1] Wavelength NA Warm calibrator 1 Baseline and baseline “delta” wavelength tables [1] Polarization Various [see 1.7] Table 53: System level calibration procedures.
Observing-level calibrations are done at the beginning and possibly in the middle or end of an observing period. These calibrations are applicable to specific wavelength and configuration. These include: Instrument Dark Thermal Dark Telescope Thermal Dark Scattered Light Precision Disk Flat Field Dither Disk Flat Field Wavelength Polarization
System-level and observing-level calibrations effectively use the same observing modes and procedures described below, but the observing-level calibrations will utilize a subset of the overall system calibration package. Thus, depending on the stability of the telescope, it should be possible to begin observations with saved system level calibrations. Observing-level calibrations will be used to monitor changes in the telescope and instrument performance during an observing run, and to allow rapid mode changes of the instrument-telescope without requiring a complete, and possibly lengthy, recalibration. Thus, the observing- level calibration procedures should function to provide calibration “deltas” that yield calibration parameters expressing the fast-time variability of the telescope and the operating mode variability.
5.1 CRYONIRSP POLARIMETRIC CALIBRATION (POLCAL)
Polarization calibration is required of the ATST mirror train from the telescope M1 to the instruments at GOS level, to the CryoNIRSP warm calibration unit and to the final CryoNIRSP detectors. These three Mueller matrix elements have very different properties and are calibrated using different techniques. The ATST system polarization calibration issues are also addressed by Elmore in ATST-TN-0137. Here we represent the total system (telescope optics and CryoNIRSP) Mueller calibration matrix by M(t,Ω,λ). Here the full calibration is dependent on time (t), telescope and Coudé table pointing (Ω), and wavelength and instrument mode, (λ). It is convenient to divide the polarization calibration into three Mueller matrix pieces:
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1) A Mueller matrix for mirrors M1 and M2 which do not change orientation (T), 2) the optics between M2 and the CryoNIRSP warm calibrator, which experience a variable geometry determined by telescope pointing and Coudé table rotation (O), and 3) the CryoNIRSP optical system from the warm calibrator through the modulator, CryoNIRSP analyzer and CryoNIRSP detectors (C). We can isolate the rapid time dependence, pointing, and wavelength/mode dependence by noting that T=T(λ), O=O(Ω,t,λ), and C=C(λ). Slow variations due to changing mirror coating properties, dust, etc. can be expected for each of these terms. We define our calibration procedure by assuming a Mueller matrix model of the form: M(t,Ω,λ) = T(λ)·O(Ω,t,λ)·C(λ) A slow time dependence is expected for each term but the fastest (minute) timescale changes are in O, while the largest wavelength (and instrument mode) dependence is from C. In principle we expect the T and O terms of this calibration to be an ATST facility task, but because of the strong polarimetric coupling between instrument and telescope, our system calibration for CryoNIRSP depends on CryoNIRSP tasks for each element. The CryoNIRSP has some advantage for the system calibration task in that it is a standalone instrument – it does not operate with AO or other optical beam disturbances. It also is designed for low- light conditions and can take advantage of full-aperture nighttime stellar polarimetric calibration opportunities. CryoNIRSP polarization calibration procedures depend on known input polarized sources that are injected into the optical path at three places: 1) (P1) from astronomical sources into the full aperture of the telescope, 2) (P2) from ATST-supplied polarization calibrators at the GOS, and 3) (P3) from the CryoNIRSP warm calibration unit upstream of the CryoNIRSP polarization modulator on the Coudé table. Thus P1 calibrators are sensitive to terms T, O, and C; P2 calibrators are sensitive to O and C; P3 calibrators are sensitive to C.
Combining P1, P2, and P3 calibrators allows each term of a wavelength differential, or absolute Mueller matrix model for the ATST-CryoNIRSP system to be developed. Many of the CryoNIRSP calibrators are fast-enough that the PolCal procedures may be used as system-level or observation level calibrators depending on the, as yet, unknown variability of the ATST polarization. The most accurate calibrations will follow from P1 procedures, but these are not always available for all observing modes and must be merged with P2, and P3 results to account for temporal and wavelength variability. Several different P1 calibrators are needed to “cover the Poincare” sphere and to account for the depolarizing (or polarizing) properties of the ATST system – depending on the observing program and required polarimetry. No facility telescope has ever been polarimetrically calibrated to the requirements of the ATST and the exact procedure for merging and interpolating the P1, P2, and P3 calibrators to derive the T, O, C expressions needed to capture the time dependence of the ATST will be finalized during the instrument commissioning phase. The calibrators, ATST system requirements, and expected performance are described in the table and summary below.
We note that it can be shown that for a weakly polarizing or depolarizing optical system (as the ATST is) the system Mueller matrix is simply a rotation in QUV space, i.e. that the input polarization can be recovered as a simple 2-angle rotation from the measured Stokes-QUV state. We (Harrington and Kuhn, 2013) have demonstrated that this calibration works on the Haleakala 3.6 m AEOS telescope with an accuracy of about 2% when only two known linearly, or partial linearly polarized input states are measured. As the AEOS telescope is highly variable with Mueller matrix elements that approach 100% we expect somewhat better performance using this technique with ATST. Thus even though some of our P1 calibrators described below only provide Q/U input polarization, this is sufficient for obtaining the full Mueller calibration to the level of about 1%. Obviously multiple P1 (Q/U) sources are important for validating this technique. For the discussion below we will denote this as the “Dual Linear Polarization Calibration” technique (DLPC).
Atmospheric Rayleigh scattering is an important continuum calibration source. Swindle et al. (2013) have used atmospheric modeling (MODTRAN) and direct all-sky polarization measurements (WAASP) to verify the input linear polarization state. DLPC works to calibrate the telescope by measuring the output polarization state for two Sun-sky positions.
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Some P1 sources are useful spectral line calibrators and can be used for highly accurate wavelength differential Mueller calibration. Disk observations based on the Kuhn et al. sunspot umbra line symmetry technique are just one example. In the corona, where the dominant polarization is by far linear in the line profile a similar line profile technique allowed SOLARC cross-talk calibration to the 10-5 level. We denote these techniques as “Line Symmetry Calibration” (LSC).
Some P1 sources are important continuum polarization sources, both for corona and disk measurements. The solar limb is weakly polarized tangential to the limb at the level of 5x10-4, and the K-corona is similarly polarized at the level of 0.1 (in the absence of scattered light).
Finally, stellar sources are important polarimetric calibrators both as continuum and line sources and they also offer the possibility of Stokes V input calibrators (e.g. using magnetic A/B stars). ). TN-0137 also illustrates a list of ‘standard’ stars with few-percent linear polarization suitable for CN ATST full-aperture calibration.
Source Class ATST mode Technique Stokes Line or Req. [sensitivity] continuum observing time [hr] Rayleigh sky P1 Corona, sky off- DLPC QUV [1%] C 1 point 20-90 deg K-corona P1 Corona, 0.1 deg DLPC QUV [1%] C 0.5 Solar limb P1 Limb Validation QU [0.05%] C 1 Sunspot P1 Disk LSC QUV [0.1%] L 1 E-corona P1 Corona LSC QU [0.05%] L 1 Std. stars P1 Night-time Validation QUV [0.05%] L/C 6 GOS P2 Disk center Direct QUV [0.05%] C 1 Warm optics P3 Disk center Direct QU [0.05%] C 1 Table 54: CryoNIRSP spectropolarimetry calibration modes.
5.2 PHOTOMETRIC CALIBRATIONS
5.2.1 Dark
The dark calibration is required to measure the emissivity of the system and check the detector performance under different conditions. A dark calibration can be performed either in coordination with the telescope dark operational mode or as a stand-alone instrument operational mode.
Configurations for this task will be automatic and include exposure times, number of measurements and number of repeats. They are based on each observing task, for both the spectrograph and the context imager.
This task may be diagnostic (taking less than 2 minutes) or it may be full comprehensive in order to obtain high signal-to-noise calibration data (requiring up to an hour or longer).
CryoNIRSP will have the following stand-alone dark tasks. This calibration task can be done either at day or night:
System dark – to measure the internal emissivity of the instrument and bias and dark levels in the detectors. It is performed using the internal filter wheel dark stop positions. It can be performed either at day or night.
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Sky dark - to determine sky and telescope emission. In this case, the telescope must be pointed far away from the Sun. Three sub-modes shall be available: . Instrument Dark, where the instrument cold-stop is closed. . Thermal Dark, that measures the thermal emission of the mirrors by off-pointing the telescope on the sky . Telescope Thermal Dark, where the dome slit is closed and M1 is covered.
5.2.2 Gain
A spatial gain calibration should be performed to measure the spatial photometric gain variation. Gain data will be acquired when the telescope resides in the Gain operation mode. Configuration for this task will be automatic and include exposure times, number of measurements and number of repeats. This task can be executed during the day using sunlight or executed before sunrise using twilight sky conditions with longer exposure times.
CryoNIRSP will have the following gain procedures:
Flat field mode for both spectrograph and context imager. This operation is done by taking displaced images of the Moon or Sun using the Kuhn et al. flat-field algorithm. This mode requires nominal telescope pointing. Flat field mode for the spectrograph. This operation requires either the continuum lamp or the solar disk. For spatial and spectral calibration tasks, offset images of the Sun are acquired and displaced on the camera. This task can also be performed by moving the scanning mirror. Spectrally shifted images obtained by incrementing the grating while observing known sources (like the warm calibration optics) will be used for spectral direction detector gain calibration. Quick flat field mode for both spectrograph and context imager is realized by looking at an out-of- focus translucent white screen located with the CryoNIRSP warm pre-optics. This operation can be performed either at day or night.
5.2.3 PhotoCal
CryoNIRSP requires a suite of telescope and instrument system diagnostics that measure the system photometric integrity, including the sky and instrument (pre- and post-slit) scattered light. These procedures are to be done on a regular (approx. monthly) cycle in order to carefully measure the photometric throughput and system point spread function (both due to geometrical and diffractive optical contributions and any spurious large-angle contributions) over a broad wavelength range (0.5-5 µm). Degradation of any system optical component, including mirror emissivity and reflectivity changes, will be monitored through these photometric calibration procedures.
CryoNIRSP has the highest photometric dynamic range and broadest wavelength sensitivity of all ATST instruments so we expect the telescope system photometric calibration from CryoNIRSP to be useful to most ATST instruments. The sensitivity and limitations of these photometric techniques will be determined during CryoNIRSP integration. Wide-field, spurious, scattering will be measured and monitored from sensitive bright star observations as will the small-angle point-spread function be obtained from high dynamic range stellar observations.
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5.3 GEOMETRY CALIBRATION
A CryoNIRSP context imager and spectrograph spatial registration task will be provided for initial alignment and diagnostic purposes. A non-redundant pinhole mask developed by the CryoNIRSP instrument team will be deployed at the ATST GOS for system level alignment of the CryoNIRSP context and spectrograph detectors. A two-minute slit scan observing sequence using the GOS mask will provide the map between the slit and the context imager for each pickoff mirror mode. This calibration also yields a large spatial scale I+P and I-P slit registration mask for the spectropolarimetry data. The slit wheel includes a linear pin-hole slit that provides accurate spatial registration of the I+P and I-P polarized spectra from the polarizing beam splitter to the detector. This mask and alignment are independent of the GOS and telescope systems. There is no daily CryoNIRSP alignment required.
Figure 110: Geometry calibration illustration. The slit overlaps a non-redundant pinhole pattern that precisely defines the slit location and orientation in the CryoNIRSP FOV. One 2-D mask pattern is illustrated above and a schematic showing how the slit overlaps mask pinholes to define its position and orientation. A single spectrograph and context imager observation of the Gregorian mask will yield the relative alignment of the two optical systems to 0.1 arcsec accuracy.
A hole pattern consisting of approximately 540, 150 micron diameter, through-holes in a 0.2 mm thick stainless steel membrane allows registration of the context imager field of view with the spectrograph slit with an accuracy of 0.1arcsec. Figure 110 illustrates a sample mask solution for the Gregorian 180 x 240 arcsec CryoNIRSP field-of-view.
5.4 FOCUS CALIBRATION
Two basic focus tasks are available: a full diagnostic focus mode and a quick/table-lookup focus adjustment mode. The full diagnostic task will use night-time stellar point source observations. The table-lookup task will be initialized from point source observations indexed by wavelength and any other required ATST system parameters (like telescope temperature or pointing information).
CryoNIRSP focus is dependent on instrument filter configuration and possible slow system temperature drift. Focus values will be evaluated for the system for a standard filter configuration of the imager and
CN-0004 144 Critical Design Document (CDD) spectrograph and focus deltas will be table-recorded for alternate CryoNIRSP operations using alternate image and spectrograph filters. Because CryoNIRSP does not have AO corrected images, good night-time seeing conditions will be utilized for diagnostic focus task zero-points.
The focus procedure involves three steps:
1) Find optimum telescope focus condition with CryoNIRSP pre-optics which maximizes the GOS mask point-source throughput through the narrow slit in the standard filter at the spectrograph detector, (this is a facility task – no interaction with the telescope is required by CryoNIRSP) 2) Adjust the spectrograph focus to maximize the spectral signature in the spectrograph detector while observing disk center spectra 3) Adjust the imager focus to maximize the GOS mask spot brightness in the imaging detector at the reference filter.
During commissioning, daytime diagnostic focus verification will be evaluated by sharpening solar limb observations with a similar algorithm. Table-lookup focus delta settings will be evaluated during instrument commissioning. CryoNIRSP shall be focused prior to each experiment. This is expected to be done using solar limb observations following the above stated three-step procedure.
This task is included in the Sunrise setup operation. It can also be run autonomously but in coordination with the telescope. It shall use the last valid focus position for the referenced line first, followed by fine tuning.
5.5 WAVELENGTH CALIBRATION
Wavelength calibration will be obtained using a spectral lamps. A Th-Ar lamp in the warm optics assembly slides into the optical path while the warm filter wheel deploys a diffuser which uniformly illuminates the CryoNIRSP slit. Standard observing scripts yield spectra that define the wavelength mapping onto the dual image plane.
CN-0004 145 Critical Design Document (CDD)
6 DATA HANDLING
The purpose of this section is to provide the critical design definition for the CryoNIRSP Data Processing Pipeline software system. Due to its large size, it is described in a separate document, the CryoNIRSP Software Design Document (CN-0005).
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7 CONTROL SYSTEM AND MOTION CONTROLL
The CryoNIRSP system consists of three computers. The main computer will execute all communications with the ATST system and will implement the CSF and run the Instrument Control System (ICS). The engineering interface will be available on this computer too. All commands to be executed by the motorized components will be issued from this computer and sent to the motion controller. All external equipment to the dewar (thermal, vacuum) will also be controlled from this computer. Two other computers will be dedicated to the communications with cameras. Hardware interface will be via Ethernet wherever possible and inter-computer communication is by Ethernet. Figure 112 shows the overall system layout of the CryoNIRSP. Table 55 summarizes the CryoNIRSP equipment list. The software part of the control system for the CryoNIRSP instrument is described in the CryoNIRSP Software Design Document (CN-0005).
7.1 INSTRUMENT HEALTH
The CryoNIRSP instrument control system will report on health of the instrument and its components. It will log all performance and configuration information (or at least pass it on to a higher authority) and store this information permanently. Unusual and fault conditions will be reported via the Operations GUI, email, SMS and/or audio alerts and passed on to the appropriate entity as described in the CryoNIRSP Operations Manual.
Figure 111: CryoNIRSP health reporting flow diagram.
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Figure 112: CryoNIRSP system layout diagram.
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 1 0 context 706 kg 73 x 34 x 52 NA not required not required 0 0 0 0 NA no servicing no imager inches required other than dewar scheduled instrument maintenance 1 0 spectrograph 1101 kg 75 x 46 x 60 NA not required not required 0 0 0 0 NA no servicing no dewar inches required other than scheduled instrument maintenance - 2 ferrofluidic 0.5 kg diameter: 25 mm NA not required not required 0 0 0 0 NA NA no feed through length: 70 mm
- 0 o-ring (one - - NA not required not required 0 0 0 0 NA NA no spare for every seal) 2 1 vacuum 32 kg diameter: 355 1 phase: operation in not required 3000 3000 3000 3000 via Check monthly: yes roughing mm 50/60Hz; cooled rack controller - Bearing grease pump: length: 471 mm 100-115 or replenishment and Agilent dry 200-230 tip seal scroll pump VAC replacement when TriScroll 600 3 phases: the pump base 50Hz/200- pressure has risen 230, 380-415 to an unacceptably VAC; 60Hz/ high level. 200-230, 460 - Bearings, rotary VAC seals and O-rings should also be replaced if the pump exhibits humming or grinding noises from the bearings.
2 1 low vacuum 15 kg diameter: 40 mm NA not required not required 0 0 0 0 NA no servicing no hose: length:15 m required
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 2 1 vacuum 15.7 kg diameter: 209 see dissipation: not required 0 0 0 0 via no servicing yes turbo pump: mm controller 300 W controller required Agilent length: 242 mm (48 VDC) flow: 100 Turbopump l/hour (0.89 V-750 GPM) TWISTORR temperature: ISO 160K +15 to +30 flange degree Celcius pressure: 3 to 5 bar (45-75 psi) connection: 1/8 G 2 1 Agilent turbo 6.2 kg 295.5 x 212.95 x 100 - 240 operation in not required 900 900 900 900 RS-232, no servicing yes controller: 110.5 mm VAC cooled rack RS-485 required 969-9525, 50 - 60 Hz (rack max 450 VA mounted) 320 W pump ramp up 300 W water cooling 2 1 vacuum 1.06 kg 71 x 129 x 310 90 - 250 operation in not required 60 60 60 60 RS-232 no servicing yes gauge mm VAC cooled rack required controller: 18 W pfeiffer TPG 262 4 1 vacuum 0.7 kg diameter: 64 mm powered by not required not required 0 0 0 0 via annually yes gauge: length: 101 mm controller controller - replace the Pirani Pfeiffer PKR element when 251 cleaning the gauge.
3 1 Pfeiffer SVV 9 kg 237 x 70 x 680 24VDC, not required min. 4 bar (60 17 17 17 17 position 20,000 cycles yes 160 PA gate mm 5.4W psi) indicator valve max. 7 bar (105 psi) 4 1 manual ISO- 1 kg diameter: 25 mm NA not required not required 0 0 0 0 NA no servicing no K-25 valves length: 70 mm required to isolate vacuum gauges
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 1 0 cryo-cooler 16 kg 160 x 294 x 357 powered by not required not required 0 0 0 0 via 10,000 hours yes head: mm compressor compress Sumitomo or SRDK-400 3 1 cryo-cooler 17.2 kg 160 x 294 x 520 powered by not required not required 0 0 0 0 via 10,000 hours yes head: mm compressor compress Sumitomo or SRDK-408 8 1 Sumitomo 40 kg 20 m NA not required not required 0 0 0 0 NA 30,000 hours no superflex He line 4 1 cryo-cooler 120 519 x 588 x 450 200 VAC operation in not required 33200 22500 22500 0 volt/ohm monthly: check He yes compressor: mm 60 Hz cooled rack reading pressure Sumitomo F- 26 A 15 pin 30,000 hours: 50L operating “D-sub” Replace current Compressor 160 A Adsorber starting current 35 min. circuit ampacity 60 A max fuse or circuit breaker 8.3 kW max power consumption 7.5 kW steady power consumpt. 1 1 Airpel 0.1 kg diameter: 16 mm NA not required min. 1.015 bar 0 0 0 0 2 position no servicing yes Double- length: 300 mm max. 7 bar indicators required Acting Front, Rear Stud Mount base model # E16DS 1 1 Macvalve 0.2 kg 10 x 20 x 60 mm 24 DC not required not required 2.5 2.5 2.5 2.5 2 position no servicing yes 44C-DDA- indicators required G-DFA-1BA 1 1 Hamamatsu 0,05 kg diameter: 10 mm NA not required not required 0 0 0 0 via safety no servicing yes Si length: 20 mm circuit required Photodiode S1087-01
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 12 2 stepper 1.75 kg NEMA 23 max 80 VDC not required not required 0 0 0 0 via driver no servicing yes motors: provided by (dissipation required Parker driver is < 2.5 W LV231 Drive per motor; Current motors will Series:3.87A be powered (pk) 2.74 off when not A(rms) needed) Parallel:7.74 (pk) 5.47A(rms) 1 1 stepper 1.9 kg NEMA 34 max 80 VDC not required not required 0 0 0 0 via driver no servicing yes motor: provided by (dissipation required Parker driver is < 8 W per LV341 Drive motor; motor Current will be Series:3.87A powered off (pk) 2.74 when not A(rms) needed) Parallel:7.74 (pk) 5.47A(rms) 13 1 Stepper 0.6 kg 135.9 x 83.5 x 20-75VDC operation in not required 0 0 0 0 via no servicing yes Drivers STP- 41.8 mm Input cooled rack DeltaTau required 075-07 5A con't, 7A (optional Analogic Peak RS-232) Corp powered by (Copley TDK supply Motion) Digital Drive for Stepper Motors 20- 75VDC Input, 5A con't, 7A Peak 13 1 STP-CK 0.1 kg length: 500 mm NA not required not required 0 0 0 0 NA no servicing no Analogic required Corp. (Copley) Connector Kit for StepNet 1 1 Copley 0.5 kg length: 500 mm NA not required not required 0 0 0 0 NA no servicing no Serial Cable required
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 3 1 TDK-Lamda 8.5Kg W: 422.8, H: 100V to operation in not required 0 4500 0 0 RS-232, periodic cleaning yes Gen 1500W 43.6, D: 432.8 240V, single cooled rack RS-485 Power phase, Supply for 47~63Hz. stepper motor drivers (50VDC - 30 A model) 1 0 Delta Tau see see Delta Tau see Delta operation in not required 0 0 0 0 100baseT periodic cleaning yes Power Delta rack Tau rack cooled rack RJ45- UMAC Base Tau Ethernet Unit w/ rack interface Power PC TCP and (2A @ +5V UDP (+/-5%) (10 protocols W))EP460E is X Full included 32/64-bit USB too architecture, 1.0GHz CPU, 2GB DDR2 RAM, 1GB Flash, No EtherCAT, 1 Slot Front/Top/B ottom Plates (Black) Power PMAC IDE included 1 0 Delta Tau 17.0 lbs 431.8 mm x 222.2 100V to operation in not required 300 300 300 300 via periodic cleaning yes Integrated mm x 132.1 mm 120V and cooled rack PowerPC UMAC Rack 200V to w/18-Slot 240V Backplane, (autoranging 21 slot rack ) Input & power Current Max supply, 2.9 Arms Black Plates
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 1 1 Delta Tau see see Delta Tau see Delta operation in not required 0 0 0 0 via periodic cleaning yes ACC-24E3 Delta rack Tau rack cooled rack PowerPC 2-axis Tau Digital rack (PWM) amplifier, quadrature & serial encoder feedback w/ removable Terminal Block connectors, Black plates (2 slots) 3 1 Delta Tau see see Delta Tau see Delta operation in not required 0 0 0 0 15 pin periodic cleaning yes ACC-24E3 Delta rack Tau rack cooled rack DB 4-axis Tau Female Digital rack (PWM) amplifier, quadrature & serial encoder feedback w/ removable Terminal Block connectors, Black plates 1 1 Delta Tau see see Delta Tau see Delta operation in not required 0 0 0 0 via periodic cleaning yes ACC-14E,48 Delta rack Tau rack cooled rack PowerPC bits of High- Tau Speed I/O, rack TTL discrete digital I/O points at 5V level. Two 50-pin IDC headers plates, Screws and 2 Card Guides (1 slot)
CN-0004 154 Critical Design Document (CDD)
Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 2 1 Delta Tau see see Delta Tau see Delta operation in not required 0 0 0 0 via periodic cleaning yes ACC- Delta rack Tau rack cooled rack PowerPC 36E,16- Tau Channel 12- rack Bit A/D Converter board with RTB Connections BLACK Front, Top, Bottom plates, Screws and 2 Card Guides (1 Slot) 2 1 Lakeshore 7.6 kg 435 mm W × 89 100, 120, operation in not required 480 480 480 480 Ethernet, periodic cleaning yes 336 (16.8 lb) mm H × 368 mm 220, 240 cooled rack USB and Controller D (17 in × 3.5 in VAC, ±10%, IEEE- × 14.5 in), full 50 or 60 Hz, 488.2 rack 250 VA
2 1 Lakeshore 3 kg 216 mm W × 89 100, 120, operation in not required 480 480 480 480 IEEE- periodic cleaning yes 218 Monitor (6.6 lb) mm H × 318 mm 220, 240 cooled rack 488.2, D (8.5 in × 3.5 in VAC, (+6%, RS-232 × 12.5 in), half -10%), 50 or rack 60 Hz, 18 VA 2 0 TDK-Lamda 8.5Kg W: 422.8, H: 100V to operation in not required 0 0 0 3000 RS-232, periodic cleaning yes Gen 1500W 43.6, D: 432.8 240V, single cooled rack RS-485 Power mm phase Supply for heaters 24 5 DT-670B- 0.01 kg 8 x 4 x 1 mm powered by not required not required 0 0 0 0 NA no servicing yes SD-12 Lakeshore required Lakeshore devices silicon Diodes 12 2 100ohm 0.05 kg 1-inch diameter powered by not required not required 0 0 0 0 NA no servicing yes Watlow Fire TDK power required Rod Heater supply 2 2 25ohm 0.03 kg 1/4-inch diameter powered by not required not required 0 0 0 0 NA no servicing yes Watlow TDK power required Heater supply
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 1 1 IOLAN SCS 2 kg 1U - 19" rack, 100-240v operation in not required 18 18 18 18 RS-232, periodic cleaning yes Console AC cooled rack Ethernet Server (RS 232 to Ethernet) 3 0 Dell 7.0 kg 197.92 x 544.32 x 100, 120, operation in not required 900 900 300 900 Ethernet periodic cleaning yes PowerEdge 50.35mm 220, 240 cooled rack M620 VAC, ±10%, 50 or 60 Hz, 250 VA 450 W 1 0 Acopian 3 kg 1U - 19" rack, 110 V operation in not required 15 15 0 15 NA periodic cleaning no +5V linear cooled rack power supply 1 0 Acopian +- 3 kg 1U - 19" rack, 110 V operation in not required 50 50 0 50 NA periodic cleaning no 12V linear cooled rack power supply 1 0 Acopian 3 kg 1U - 19" rack, 110 V operation in not required 50 50 0 50 NA periodic cleaning no +24V linear cooled rack power supply 1 1 Newport 20 kg 5.68" x 12" x 16" 110V, 4A operation in not required 400 400 0 400 RS-232 no servicing yes Universal cooled rack required Arc Lamp Power Supply, Modelnr. 69907; 1 1 Thorium 0.2 kg diameter: 30 mm powered Dissipation: not required 0 0 0 0 via lamp no servicing no Argon length: 100 mm by Newport 70 W; water power required hollow Universal cooling see supply cathode lamp Arc Lamp housing Power Supply (69907) 1 1 Quartz 0.2 kg diameter: 30 mm powered Dissipation: not required 0 0 0 0 via lamp no servicing no Tungsten length: 100 mm by Newport 100 W; power required Halogen Universal water supply lamp Arc Lamp cooling see Power housing Supply (69907)
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Equipment Mass Size Electrical Cooling Compressed Total Power Total Power Total Power Total Power control Maintenance cycle manua
requirements requirements air consumption consumption consumption consumption interfaces l requirement cool down normal power saving warm up [W]
spare [W] operation mode [W] quantity [W] 2 0 Oriel 2 kg 16 x 7 x 7 inches NA water not required 0 0 0 0 NA no servicing yes PhotoMax cooling required lamp possible housing, flow: 0.25 modelnr liters per 60100 minute temperature: <18°C (65°F) pressure: 2 bar (30 PSI)
1 0 Power NA NA NA NA NA 1500 1500 0 1500 Contingency 41372.5 35172.5 28057.5 11172.5
Table 55: Equipment list.
CN-0004 157 Critical Design Document (CDD)
7.2 MOTION CONTROLLER
There are 9 stepper motors for internal components, 4 stepper motors for external components, a solenoid valve to control the pneumatic safety shutter and a pneumatic gate valve between the dewars. Digital I/O connections for the limit switches and the safety mechanism and analog signals from the cryo-coolers will also be managed through a Delta Tau controller, as recommended by the ATST control system standards (TN-0127). The Delta Tau Power PMAC will be used to control all mechanisms of the CryoNIRSP. This controller is a 19”x3U rack mount unit. It will be directly connected to the CryoNIRSP main computer. It will be composed of:
Delta Tau Power PMAC board 3Delta Tau ACC-24E3 4-channel axis interfaces. This will provide interfaces for 12 motors and their amplifiers. 1 Delta Tau ACC-24E3 2-channel axis interface. This will provide an interface to one motor and its amplifier. The other channel is needed to connect to TRADS. ACC-R1, integrated UMAC rack 18-slot backplane 1 ACC-14E, 48 channel digital I/O board 2 ACC-36E, 16 channel, 12 bit A/D board
7.3 MOTOR DRIVERS
The motor drives receive low-level PWM signals from the axis expansion cards and provide the electrical energy to move the stepper motors. All of the CryoNIRSP stepper drivers will be Copley STP-075-07 (20- 75DVC Input, 5A cont., 7A Peak) which is compatible with the PWM signals provided by the Delta Tau. 3 TDK Genesys 50-30 power supplies will feed the stepper drivers. The de-rating factor for the 11,000 ft. elevation at Haleakala for motor drives is 75% so the STP-075-07 will be capable of 3.7 A continuous and 5.2 A peak current making it a good match to our Parker stepper motors.
7.4 MOTORS
We use 12 NEMA 23 (200 or 500 steps per revolution) and 1 NEMA 34 (500 steps per revolution) stepper motors to drive our motorized stages. Our motion control analysis has shown that we meet the resolution, repeatability and time requirements with these motors. In some cases we need the micro stepping capability of the motors to achieve the desired resolution. We do not plan to use more than 8 micro steps per native motor step if there is no encoder mounted on the stage. If we need even higher resolution, we will be using high precision planetary gear heads. All motors will be equipped with a motor break. Thus, the motors can be unpowered once the motion stage is in position to avoid a heating of the motor.
CN-0004 158 Critical Design Document (CDD)
Figure 113: Motor speed and torque performance curves (blue dashed 24 VDC, blue solid 48 VDC, and red 75 VDC).
Parameter LV231 LV341 Static torque [Nm] 85 550 Rotor inertia [kg cm2] 0.128 1.402 Drive current [A] series 1.98 rms (2.8 pk) 2.74 rms (3.87 pk) parallel 3.96 rms (5.6 pk) 5.47 rms (7.74 pk) Phase inductance [mH] series 2.41 15.44 parallel 0.6 3.96 Resistance [Ohm] series 0.77 2.01 parallel 0.19 0.50 Detent torque [Nm] 0.021 0.103 Thrust load [kg] 5.91 11.36 Radial load [kg] 6.82 17.73 (0.79 inch from face) Table 56: Motor specifications.
7.5 LIMIT SWITCHES, HOME MARKERS AND ENCODERS
Limit switches, home markers and the grating encoder will be connected to and controlled by the Power PMAC.
7.6 TEMPERATURE SENSORS
Temperature sensors (silicon diodes from Lakeshore) are connected to Lakeshore temperature controllers and monitors. These units connect to the control computer through Ethernet.
7.7 VACUUM EQUIPMENT
The two turbo pumps, two roughing pumps and vacuum gauges will use RS-232 connections to communicate with the control computer.
CN-0004 159 Critical Design Document (CDD)
7.8 CRYOGENIC EQUIPMENT
The cryo-compressors and cold heads can be controlled and monitored using the analog and digital I/O cards of the Power PMAC.
7.9 POWER SUPPLIES
All power supplies are controlled by the CryoNIRSP computer by an RS-232 connection.
7.10 CALIBRATION LAMPS
The calibration lamp power supplies are controlled by the CryoNIRSP computer by an RS-232 connection.
7.11 POWER FEED
CryoNIRSP will be provided a 480 VAC three phase circuit, a 208 VAC three phase circuit and a 208 VAC three phase UPS battery power backup circuit. A power distribution panel mounted to one of the racks provides the power to the CryoNIRSP components. The power feed can be controlled either manually on the panel or by the instrument computer. Status LEDs indicate which components are powered. Details on the panel are described in the Electrical and Wiring Diagrams document (CN-0022).
7.12 ELECTRICAL LAYOUT AND WIRING
Figure 115 and Figure 116 show the rack layout for our electronics and the major cable connections. The wiring and grounding of our instrument is described in the Electrical and Wiring Diagrams document (CN- 0022).
7.13 SAFETY INTERLOCK
A photo diode measures the stray light reflected from the CryoNIRSP context imager detector array. A safety interlock circuit (see CN-0022) reads this signal and closes the safety shutter if unexpected high levels of sunlight are fed to CryoNIRSP. The circuit can be armed and disarmed by the digital I/O card of the Power PMAC. The status of the shutter is read by the same card. Periodical testing of the safety mechanism should be performed. Safe but above saturation solar light level will be used with array electronics off, to test the shutter/detector mechanism. Details on this task will be described in the maintenance manual. All our heaters have built-in thermistors to protect the detector arrays from over temperature conditions. If the temperature is higher than a room temperature threshold, the heater thermal fuse breaks the circuit. In addition, to prevent contaminant condensation on the arrays there is a large heat capacity “cold finger” that prevents the array from cooling too fast or having a temperature that is cooler than its immediate surroundings. We identified electrical shocks as one of our main hazards. To mitigate this risk we implemented a main power switch on the power distribution panel and an emergency power off button close to the instrument. The ICS will handle all interlock related events as described in TN-0102, Rev C. When an interlock occurs, the ICS will abort any actions currently being performed by the CryoNIRSP instrument, and block any additional actions from being started. The ICS will maintain this state until the interlock is cleared, at which point it will continue normal operations. For more information on the ICS handling of interlocks please refer to the ICS design document.
CN-0004 160 Critical Design Document (CDD)
Of particular interest to CryoNIRSP are alarms and interlocks concerning compressed air and liquid coolant systems. Faults in these systems could compromise the safety and/or health of the instrument. Events produced by the GIS (if available) will be used by CryoNIRSP to protect the relevant hardware components. This may include GUI alarm indications for operators to shut down power to the detectors.
CN-0004 161 Critical Design Document (CDD)
Figure 114: Power feed diagram.
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Figure 115: CryoNIRSP rack layout.
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Figure 116: CryoNIRSP cable connections.
CN-0004 164 Critical Design Document (CDD)
7.14 MOTION CONTROL ANALYSIS
Table 57 below gives an overview of our speed, repeatability and resolution analysis for our 14 motorized stages. The motor type, micro-stepping and gear heads are chosen to meet the requirements specified in the opto-mechanical design.
7.14.1 Motor Speed
Our motor controllers each use a simple constant acceleration ramp up/down to speed. Warm and cold motor tests were performed with our test dewar to establish our minimum step time and maximum acceleration. Using the available NEMA 23 single stack stepper motor with 8 micro steps per native step, we were able to drive the context imager filter wheel with an acceleration up to 20,000 steps per s2. The start velocity was set to 10 steps per second and maximum velocity was at 72,000 steps per second. With the current motor and gear train this maximum speed would correspond to 90 motor shaft revolutions per second. However the longest distance the wheel has to travel is 1 revolution (7,200 steps or 9 motor shaft revolutions) and full speed can never be achieved. The motor speed linearly ramps up to 12,000 steps per second before decelerating at the same rate. It takes 0.6 seconds to accelerate to max speed or 1.2 seconds to complete a full revolution. Thus our maximum usable motor speed is 7.5 revolutions per second. This speed maximum is applicable when micro-stepping is used. If we use no micro-stepping, the available torque is higher and a speed of 10 revolutions per second can be used. Using these speeds, all design requirements are met.
7.14.2 Positional Repeatability
The positional repeatability was measured in the warm and cold state of the filter wheel. We used our laser tracker to determine the position of a corner cube reflector that was placed in one of the filter inserts of the wheel. The nominal home of the cube was at the 3 o’clock position. We will present our results in terms of radial and tangential deviations with respect to the filter wheel plane. A baseline was established by repeatedly measuring the cube position without moving the wheel. The data taken over half an hour is highly repeatable with a one sigma standard uncertainty of 1µm. Then the filter wheel was rotated clockwise with varying numbers of steps and the positions were recorded. After overshooting the last position, we rotated the wheel counterclockwise back to the “same” positions. After overshooting the last position we repeated this procedure several times. For the warm measurements – 8 micro-steps per native motor step – the average tangential micro-step size was 89 µm which agrees very well with the calculated 87 µm. The one sigma standard uncertainty is 5 µm or 6% of the step size. The same measurements performed at cryogenic temperatures yield an average tangential micro-step size of 79 µm with a one sigma standard uncertainty of 5 µm or 6% of the step size. We were not able to detect a significant difference between the warm and cold measurements. With a unidirectional repeatability of better than 20% of the nominal design step size we will meet all the repeatability requirements.
7.14.3 Backlash
Comparing the clock- and anticlockwise measurements gives us an idea of the backlash in our gear train. At room temperature the average tangential difference between CW and CCW positions is 1033 µm with a one sigma standard uncertainty of 15 µm or 17% of the step size. At cryogenic temperatures the difference is 794 µm with a one sigma standard uncertainty of 48 µm or 61% of the step size The smaller backlash at cold temperatures was expected since the design accounts for contractions and gives the gears more play at room temperatures. With appropriate step motion algorithms we will be able to reduce the backlash uncertainty to the order of the nominal design step size.
CN-0004 165 Critical Design Document (CDD)
Mechanism Mechanism Positioning Limit Home-ing Motor Motor Micro Motor Motor Additional Final Final Nomina Time to Encoder Power Type Details Types Types Step Step Gearhead Angular Gearing Resolution Repeatability l Design move Resolution -off Size (usteps / (ratio) Resolution Step between motor step) Speed positions brake FM1: Scan 2-Axis Gimbal Angular 2x limit 1x home Stepper 1.8° N N 1.8° / step Vendor 0.9 µrad 3.6 µrad 6.2°/mi 5 ms (one NA yes Mirror - Vertical Axis Travel switches marker NEMA 23 Spec nute step) Motorized Total Range = (plus hard Single 1:34951 (1.8 0.5° stops) Stack µrad/m (8.5mrad) s)
FM2: Focus Linear Stage Axial Travel 2x use one of Stepper 1.8° N N 1.8° / step Ball Screw 5.0 µm 20 µm 10 rps 2.5 s (full NA yes Mirror with Total Range = microdot the NEMA 23 (1mm/rev = 2000 range) Precision Ball 25 mm switches microdot Single pitch) step/sec Screw (plus hard switches Stack stops) as home marker Calibration Linear Stage 2 Position 2x use one of Stepper 1.8° N N 1.8° / step Ball Screw 5.0 µm 20 µm 10 rps 50 s (full NA yes Lamps with Axial Travel microdot the NEMA 23 (1mm/rev = 2000 range) Selection Preloaded Total Range = switches microdot Single pitch) step/sec Nut 500 mm (plus hard switches Stack stops) as home marker Heat Rotating 7 Positions 1x detent 1x home Stepper 1.8° N N 1.8° / step Spur Gear 15.4 30 arcmin 10 rps 0.35 s NA yes Rejection Selection Angular switch marker NEMA 23 Train arcmin/step = 2000 (180°) Filter Wheel with Travel Single (7:1 gear radial 1.5 step/sec Selection Detented 360° Stack ratio) µm/step Positions Continuous tang. 672 µm/step (150 mm arm) Safety Pneumatic 2 Position 2x use one of Airpel NA N N NA NA NA NA NA 50 ms NA NA Shutter linear Axial Travel microdot the pneumati Mechanism actuator; Total Range = switches microdot c actuator Solenoid 230 mm (plus hard switches valve stops) as home marker Pick-off Rotary Stage 4 Position 2x limit use one Stepper 1.8° 4 N 0.45° / step Worm Gear 0.0045° 0.018° 7.5 rps 7 s (180°) NA yes Mirror with Worm Angular switches position as NEMA 23 (100:1 = 6000 Stage Gear Drive Travel to home Single ratio) step/sec Total Range = determin Stack 360° e position Spectrograp Rotating 16 Positions 1x detent 1x home Stepper 0.72° 8 10 0.009° / Spur Gear 2.03 arcsec / 11.3" 7.5 rps 11 s NA yes h Slit Wheel Selection Angular switch marker NEMA 23 step Train step (backlash = (180°) Wheel with Travel Single (16:1 gear radial 6 pm / gearhead) 30,000 Detented 360° Stack ratio) step step/sec Positions Continuous tang. 1.2 µm / step (120 mm arm) Spectrograp Rotating 16 Positions 1x detent 1x home Stepper 0.72° 8 10 0.009° / Spur Gear 2.03 arcsec / 11.3" 7.5 rps 11 s NA yes h Filter Selection Angular switch marker NEMA 23 step Train step (backlash = (180°) Wheel Wheel with Travel Single (16:1 gear radial 6 pm / gearhead) 30,000 Detented 360° Stack ratio) step step/sec Positions Continuous tang. 1.2 µm / step (120 mm arm)
CN-0004 166 Critical Design Document (CDD)
Mechanism Mechanism Positioning Limit Home-ing Motor Motor Micro Motor Motor Additional Final Final Nomina Time to Encoder Power Type Details Types Types Step Step Gearhead Angular Gearing Resolution Repeatability l Design move Resolution -off Size (usteps / (ratio) Resolution Step between motor step) Speed positions brake SM4: Rotary Stage 2 Position 2x use one of Stepper 1.8° 8 N 0.225° / 1:1 0.225° 0.5° 7.5 rps 35 ms NA yes Deployable with Spur Angular microdot the NEMA 23 step = (90°) Cold Mask gear Travel switches microdot Single 12,000 Total Range = (plus hard switches Stack step/se 90° stops) as home c marker Grating Rotary Stage 2 Positions no limit 1x home Stepper 0.72° 16 10 0.0045° / Worm Gear 0.162" / step 8.64" 7.5 rps 67 s 23 bit yes Mount with Worm Angular switches marker NEMA 34 step (100:1 = (180°) rotary Gear Drive Travel Single ratio) 60,000 encoder = 360° Stack step/sec 0.15" Continuous SM5: Mirror Linear Stage Axial Travel 2x use one of Stepper 1.8° N N 1.8° / step Ball Screw 5.0 µm / step 20 µm 10 rps 2 s (full NA yes Focus with Total Range = microdot the NEMA 23 (1mm/rev = 2000 range) Precision Ball 20 mm switches microdot Single pitch) step/sec Screw (plus hard switches Stack stops) as home marker CM5: Linear Stage Axial Travel 2x use one of Stepper 1.8° N N 1.8° / step Ball Screw 5.0 µm / step 20 µm 10 rps 2 s (full NA yes Mirror with Total Range = microdot the NEMA 23 (1mm/rev = 2000 range) Focus Precision Ball 20 mm switches microdot Single pitch) step/sec Screw (plus hard switches Stack stops) as home marker Context Rotating 9 Positions 1x detent 1x home Stepper 0.72° 8 10 0.009° / Spur Gear 3.6 arcsec / 20" (backlash 7.5 rps 6 s (180°) NA yes Imager Selection Angular switch marker NEMA 23 step Train step gearhead) = Filter Wheel Wheel with Travel Single (9:1 gear radial 15 pm 30,000 #1 Detented 360° Stack ratio) / step step/sec Positions Continuous tang. 1.7 µm / step (100 mm arm) Context Rotating 9 Positions 1x detent 1x home Stepper 0.72° 8 10 0.009° / Spur Gear 3.6 arcsec / 20" (backlash 7.5 rps 6 s (180°) NA yes Imager Selection Angular switch marker NEMA 23 step Train step gearhead) = Filter Wheel Wheel with Travel Single (9:1 gear radial 15 pm 30,000 #2 Detented 360° Stack ratio) / step step/sec Positions Continuous tang. 1.7 µm / step (100 mm arm) Table 57: CryoNIRSP motion control summary.
CN-0004 167 Critical Design Document (CDD)
8 HAZARD ANALYSIS
The CryoNIRSP hazard analysis and the resulting safety plan have been moved to separate documents: Safety Plan (CN-0006), Hazard Analysis Plan (CN-0007) and Preliminary Hazard Analysis (CN-0008).
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9 DESIGN TRADE STUDIES SUMMARY
Several design trades were undertaken during the design phase for CryoNIRSP. Some of the most significant decisions and reasons for these choices are summarized here. Dual camera collimator optical spectrograph design: A Littrow configuration would have simplified the spectrograph optics but removed flexibility in achieving output image scale and wavelength resolution. Decision: Anamorphic dual collimator and spectrograph camera optics must be implemented to achieve angular and spatial resolution requirements Reflective versus refractive design: A refractive design would have resulted in a smaller dewar but proved impossible to implement without many more focus mechanisms and with much greater ghosting and scattered light. Final decision: Reflective design is a very high priority and will be implemented wherever possible Internal (cold) stepper motors vs. external: Internal motors would have eliminated the need for mechanical feed-throughs and potentially decreased the dewar size. Internal motors are more expensive, cannot be debugged without a 2 weeks dewar cycle time, increase the heat load in the dewar, and increase the wiring complexity. Final decision: External motor drives with well baffled mechanical feed-throughs in the internal radiation shields will be implemented. Polarizing beam-splitter vs. reflective membrane beam-splitter design: The broad wavelength coverage could not be accommodated without multiple polarizing beam-splitters and additional mechanisms. Final decision: implement a broadband polarizing beam-splitter that uses wire grid polarizer and prototype this component early in the design. Grating choice – echelle vs. low order: Large grating polarization may degrade the CryoNIRSP spectropolarimetric efficiency. Based on Richardson Lab Inc. grating polarization estimates we minimize the grating polarization using commercial echelles. We will obtain reliable measurements of the grating polarization at primary wavelengths to quantify the CryoNIRSP net spectropolarimetric efficiency. Dual vs. single grating assembly: Dual gratings do not significantly increase the mechanical complexity but allow significant flexibility for achieving optimum throughput (observing near the blaze angle) over a broader wavelength range and with echelle style gratings that minimize grating polarization effects over the full wavelength range. Two or three Gifford-McMahon cryo-coolers: Maximum cooling capacity and long MTBF is achieved with this choice of chiller. The flexibility of adding a third SRDK-400B means the detector can be operated at 35K (or lower) if required because of uncertain IR FPA characteristics. Aluminum cryogenic optical bench vs. invar: Invar metering for all internal cryogenic components would still require careful room and cold temperature alignment procedures. The expense of an invar optical bench was significant and provided no qualitative simplicity in the optical alignment approach. Final decision: implement aluminum optical bench Aluminum mirrors vs. Low Expansion Glass: The residual temperature gradient across the optical bench and mirrors with the large CTE of aluminum would yield mirror distortion that exceeded the diffraction limited performance of the context imager. Decision: implement Low Expansion Glass- style cryogenic optical glass-ceramic mirrors. Use of fixed vs. adjustable optical alignments: The significant thermal contraction and somewhat uncertain thermal gradients across the cryogenic bench make it difficult to design fixed optics into the 2 m-scale aluminum structure. Tolerance analysis shows that de-space is not critical (except for focus) and so the minimal adjustable degrees of freedom imply that manual tilt of the primary mirror optics is adequate to allow final alignment at cryogenic temperatures using the Leica Laser Tracker and Romer Coordinate Measuring Machine. Only three actuated focus mechanisms are required (one for the imager, one for the spectrograph and one for the feed optics). This also means that no gross focus alignment will be required by moving the CryoNIRSP dewar assembly.
CN-0004 169 Critical Design Document (CDD)
Silver vs. protected silver mirror coatings: The protective overcoat on silver mirrors is not needed within the sealed dewar environment and may increase the wavelength dependence of the instrumental polarization cross-talk – thereby complicating polarization calibration. Haleakala is above the inversion layer and is reputed to be a low sulfur environment that is amenable to long-lifetime silver coatings. Trial coatings placed on the summit of Haleakala early in the project were monitored for degradation. These samples are being measured but our preliminary results indicate that we must use protected silver for all exposed mirrors Studies of the pellicle indicate that it has good cryogenic wavefront properties but has significant vibration sensitivity for the reflected beam. Our conclusion is that we must use a slotted fixed mirror for the beam splitter CryoNIRSP mode. Laser-tracker tests indicate that in-dewar cold vacuum metrology will be sufficient for primary alignment tasks. Multiple cool-down cycles for empirical cold optical alignment will not be necessary. Mechanical “floating” cold head coupling to the dewar cold space versus software veto or active mechanical filtering to mitigate GM chiller vibration noise on optic: A series of cold tests have established that the “floating head” design of our GM cold heads satisfies our vibration isolation requirements when the radiation baffling around the cold head coupling is properly mechanically isolated.
CN-0004 170 Critical Design Document (CDD)
10 CONSTRUCTION PHASE PLANNING
10.1 FABRICATION PLAN
The dewar, internal components, external electronic components and cryogenic and vacuum systems will be assembled at Universal Cryogenics Llc. (UCL). Mechanical alignment and functional tests of all vacuum, thermal and opto-mechanical systems will take place at UCL. Acceptance of the dewar will be based on these tests at which point the dewar assembly and all electronic, vacuum, and thermal systems will be shipped to the ATRC on Maui. UCL will provide on-site training and installation of the dewar systems at the ATRC and will provide continued support of the internal dewar systems throughout all phases of alignment and testing. All of our reflective optics could be produced by a vendor on Maui. Their quote is competitive and the proximity of their manufacturing site will allow close supervision of the optics fabrication. For example the optics can be tested before acceptance at the ATRC in Maui or at the vendor’s facility or we may use our Laser Tracker for optics metrology in their local facility. Alignment testing at the level of bore-sighted external light (laser and continuum) will be done at ATRC. The warm feed optics, optical scanning and safety light shutter testing and acceptance will be done at the ATRC. Final acceptance and integration of the IR detectors will be done at the ATRC. Software development, calibration and system integration activities will be done at the ATRC. The CryoNIRSP system has several fabrication phases which are described and illustrated in the management plan Gantt diagram (see CN-0010). Because the dewar system has two independently operable elements (the context imager and the spectrograph) and we will be using two facilities for fabrication and testing (UCryo and ATRC), we will be fitting optics, aligning and doing final testing at ATRC while the second unit is being manufactured at UCryo. The test dewar now at the ATRC will be used for camera and cold component testing before the first dewar arrives in Maui (the spectrograph). This test dewar at ATRC then gets disassembled and integrated into the context imager dewar when that is completed at UCryo.
10.2 MANUFACTURABILITY
For the two dewars we have partnered with Universal Cryogenic Llc. in Tuscon AZ. This company has successfully delivered many large dewar systems, e.g. Cyra to Big Bear Observatory. The test dewar delivered to the CryoNIRSP team contains many of the critical systems that will be used in the final instrument and has convinced us that UCL is capable of manufacturing the CryoNIRSP dewars. Discussions with different potential optics manufacturers have shown that none of the mirrors and filters are considered demanding. All off axis conic mirrors can be made as free forms to reduce material costs. Two companies provided quotes for the mirrors. The filters are available from several vendors and we have refreshed our earlier filter quotations to verify cost and availability. The IfA has extensive design and construction experience with astronomical IR systems. The PI has built four IR systems, including the first sensitive IR imaging camera used in solar astronomy. He is responsible for the large Coudé HiVIS spectropolarimeter on the 3.6 m AEOS telescope, which includes a 1-2.5 micron high resolution spectrograph. Our project manager and mechanical designer (Tim Bond) has been a key member of the design team for several IR instruments, including the SPeX IRTF IR spectrograph. Our instrument scientist (Andre Fehlmann) has had extensive cryogenic and solar instrumentation experience with ground and space experiments before joining the CryoNIRSP project. The CryoNIRSP software team has experts in hardware control, data systems, and system management. With these two staff and an experienced consultant (Isabelle Scholl) the CryoNIRSP software design and implementation are in-hand. In addition the IfA and its engineering and technical staff are all part of a larger “Job Order System.” This engineering expertise is available, at an hourly rate, to help with design (electrical and mechanical), or manufacturing (e-tech, machine shop, or software) needs.
CN-0004 171 Critical Design Document (CDD)
10.3 LONG LEAD ITEMS
The two dewars are our only real long lead items. Our partner Universal Cryogenic has provided the detailed SolidWorks models of all parts they will manufacture in their shop. Thanks to the dual dewar CryoNIRSP design, the dewars will be delivered in two batches. Thus, testing and optics integration can be done at ATRC while the second dewar is built in Tucson (see Management Plan CN-0010 for schedule). The largest two mirrors – FM2 (fore optics focus mirror) and CM5 (context imager focus mirror) – require a little bit longer fabrication times due to their size and off axis parabolic shape. The drawings for these optics are provided in the documents CN-0020 and CN-0021.
Figure 117: CM5 solid model drawing.
10.4 PACKAGING AND TRANSPORTATION PLAN
10.4.1 Tucson to ATRC
The dewar will be transported by ground in a custom container from Tucson to the West coast. It will be then shipped by sea to Maui, and put on a truck to be delivered at IfA/Maui. Optics will be disassembled from the dewar and packed separately before leaving Tucson. The test dewar has already been shipped from Tucson to ATRC without any problems or damages. Shipping will be commercially insured.
10.4.2 ATRC to ATST site
After alignment and system testing is completed, the dewar will be transported by flat-bed truck to the summit to be installed in the ATST building for ISRD verification and calibration. The IfA has extensive experience transporting delicate and/or heavy instrumentation between the ATRC and the summit. It is a great manufacturing advantage to be able to bring a well-tested instrument directly from our labs just 20 miles to the ATST. The major electronic and cryogenic components will be disassembled from the dewars, but each of the dewar assemblies with optics will be vibration-isolated and loaded on a flat-bed. The slow trip to the summit from ATRC will take less than 3 hours under carefully controlled vibration conditions.
CN-0004 172 Critical Design Document (CDD)
10.5 QUALITY CONTROL AND QUALITY ASSURANCE PLAN
The quality assurance and control plan has been moved to the separate document: Quality Control/Assurance Plan (CN-0011).
10.6 VERIFICATION TEST PLAN
Our verification test plan has been moved to the separate document: Quality Control/Assurance Plan (CN- 0011).
CN-0004 173 Critical Design Document (CDD)
11 SCHEDULE, RISKS AND COST ESTIMATES
11.1 PROJECT SCHEDULE, WBS, COST ESTIMATES
These sections of the CDD have been moved to a separate document: Management Plan (CN-0010).
11.2 RISK ASSESSMENT AND MITIGATION STRATEGIES
These sections of the CDD have been moved to separate documents: Risk Analysis (CN-0012) and Risk Register (CN-0016).
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