AN INTRODUCTION TO THE NRAO Very Large Array R. M. Hjellming * National Radio Astronomy Observatory Socorro, New Mexico 87801 Operated by Associated Universities, Inc. under contract with the National Science Foundation First Edition November 1978 The NRAO VLA in September 1978. CONTENTS Page No. CHAPTER I. THE BASIC INSTRUMENT 1. The History and Concept of the VLA.......... ..... 1-1 2. The Basic Antenna...............................1-8 3. The Antenna Feed System........................ .1-12 4. Array Geometry................................ .1-17 5. Location of Major System Components............ .1-25 CHAPTER II. THE THEORETICAL BASIS OF INTERFEROMETRY AND APERTURE SYNTHESIS 1. Introduction.................................. .II-l 2. Some Definitions.............................. .II-4 3. Interferometer Response to a Point Source........II-7 4. System Correction and Calibration.............. .11-17 5. Amplitude and Phase Noise...................... .11-26 6. Geometry and Coordinates....................... .11-32 7. Visibility Function for a Point Source......... .11-38 8. Visibility Function for an Extended Source.......11-40 9. u-v Plane Coverage..............................11-43 10. Radio Image Reconstruction by Direct Transform... 11-46 11. Point Source Images for the Full VLA........... .11-47 12. Radio Image Reconstruction with the FFT Algorithm 11-52 13. Map Sizes and the Number of Points per Synthesized Beam................. .11-55 14. Gridding, Cell Sizes, and the Sampling Theorem... 11-58 CHAPTER III. THE VLA ELECTRONICS SYSTEM 1. Introduction.................................. .III-l 2. A Dataflow Description of the VLA Electronics System............... .111-10 3. Technical Descriptions of VLA Electronics........111-19 Page No, CHAPTER IV. ARRAY CONTROL AND DATA ACQUISITION 1. Introduction.................................. IV-1 2. System Control Files........................... IV-7 (a) Role of the Control Files................. IV-7 (b) The ARRAY File............................ IV-8 (c) Subarray Files: SUBn = SUB1, SUB2, ... SUB5 IV-9 (d) ANTENNAS File............................ IV-13 (e) The ROT Files with Front End and Subreflector Parameters......... IV-14 (f) IF Files with Gain Calibration Parameters... IV-16 (g) Source List Files......................... IV-16 3. The MODCOMP Computer System.................... IV-23 (a) Major Tasks Running in the On-line System... IV-23 (b) On-line Computations................ ..... IV-25 (1) Ephemeris Calculations............... IV-25 (2) Time Calculations.................... IV-26 (3) Antenna Location Corrections......... IV-26 (4) Refraction Calculation............... IV-27 (5) Phase Command Calculations........... IV-28 (6) Pointing Command Calculations........ IV-29 (c) Observing Modes........................... IV-30 (d) Correlator Data Processing........ ....... IV-31 CHAPTER V. PREPARATION FOR OBSERVING AND THE OBSERVING PROCESS 1. VLA Capabilities in Late 1978.................. V-l (a) Antennas and Configurations............... V-l (b) Sensitivity, Resolution, and Confusion..... V-7 (c) Polarization............................. V-8 2. Planning for Observing......................... V-9 (a) Before Observing.......................... V-9 (1) The VLA "Friend” Program............. V—9 (2) Materials to Bring Along............. V-9 Page No. (3) Arrival at the Site.................. .V-10 (4) First Things to Do................... .V-10 (b) Calibration.............................. .V-ll (c) Lists of Phase and Amplitude Calibrators.... V-12 (d) Planning Source Scheduling................ .V-13 (1) Subarray Operation................... .V-13 (2) Detailed Source Scheduling........... .V-13 (3) Punching Source Input Cards.......... .V-17 3. Preparing and Modifying Source List Files - OBSERV and SEDIT..................... .V-18 (a) Introduction............................. .V-18 (b) Running OBSERV in the MODCOMPs............ .V-18 (c) List of OBSERV Commands with their Syntax and Purpose.............. .V-23 (d) List of OBSERV Identifiers................ .V-25 (e) An Example Using the OBSERV Program.........V-27 (f) Modifying Old Source List Files........... .V-35 4. Activities During Observing.................... .V-38 (a) Responsibilities of the Array Operators.....V-38 (b) Repair of Malfunctioning Equipment........ .V-39 (c) The Start-up Process...................... .V-39 (d) Operation of a Test Subarray.............. .V-40 (e) On-line Programs and Displays............. .V-40 (f) Real Time Filling of Visibility Data Base... V-42 (g) Monitoring Calibrators.................... .V-42 (h) Monitoring Source Results................. .V-44 (i) Useful Practices...........................V-45 (1) Copies of ANTENNAS and Source List Files............................V-45 (2) Tables of Properties of Usable Antennas V-46 CHAPTER VI. OFF-LINE DATA PROCESSING 1. Theory of Off-line Visibility Data Processing.... VI-1 Page No. 2. Off-line Computer Hardware..................... VI-7 (a) The DEC-10............................... VI-7 (b) The PDP 11/40 Map Processing System........ VI-11 (c) The PDP 11/70 Map Making System........... VI-12 3. Running Programs in the DEC-10................. VI-14 (a) Logging On and Logging Off................ VI-14 (b) The DEC-10 File System.................... VI-16 (c) Monitor System Commands................... VI-16 (d) Special Control (CTRL) Characters......... VI-19 (e) Running Programs.......................... VI-19 (f) Editing Text Files - The SOS Program....... VI-21 (g) Batch Execution of Programs............... VI-25 (h) Handling Magnetic Tapes................... VI-30 4. Visibility, Monitor, and Map Data Bases - The Standard Data Files.............. VI-33 (a) Introduction............................. VI-33 (b) Visibility Data Bases..................... VI-33 (c) Monitor Data Bases........................ VI-37 (d) Map Data Bases............................ VI-37 5. Summary of the Major Off-line Data Reduction Programs in the DEC-10............... VI-41 6. Standard Program Syntax and Methodology........ VI-44 (a) Introduction......... .................... VI-44 (b) Commands Common to All Standard Programs.... VI-45 (c) Standard Data Selection Commands Common to Many Programs.............. VI-47 (d) Units of Command Parameters............... VI-49 7. DANEEL Control vs. Separately Run Programs...... VI-51 8. Program Documentation.......................... VI-53 (a) Introduction............................. VI-53 (b) ANTSOL................................... VI-54 (c) AVGVIS................................... VI-58 Page No. (d) BASFIT................................... .VI-60 (e) CLNMAP................................... .VI-62 (f) DANEEL................................... .VI-66 (g) DBCON.................................... .VI-68 (h) FILANT................................... .VI-70 (i) FILLER................................... .VI-72 (j) FLAGER................................... .VI-74 (k) GTBCAL................................... .VI-78 (1) GTBCOR................................... .VI-82 (m) GTTSYS................................... .VI-88 (n) LISTER................................... .VI-90 (o) MAKMAP................................... .VI-98 (p) MONLST................................... .VI-106 (q) MONPLT................................... .VI-108 (r) SEEMAP................................... .VI-112 (s) SETJY.................................... .VI-118 (t) VISPLT................................... .VI-122 9. The Process of VLA Data Reduction.............. .VI-130 (a) Importance of a Systematic Approach.........VI-130 (b) Data Evaluation and Editing............... .VI-131 (c) Correction and Calibration - Preparing the Gain Table............. .VI-135 (d) Evaluation of Calibration................. .VI-145 (e) Mapping and Map Display................... .VI-148 (f) Archiving or Carrying Away Results on Magnetic Tapes............ .VI-162 10. The Interactive Map Processing System - IMPS.....VI-163 (a) Introduction to IMPS and Associated Hardware..............................VI-16 3 (b) Producing Maps for IMPS................... .VI-165 (c) The IMPS Menu Structure and IMPS Functions................... .VI-166 Page No. (1) The Evolving Nature of IMPS Menus......VI-166 (2) The IMPS Top Level Menu.............. .VI-167 (3) 1. Utility Functions................ .VI-167 (4) 2. COMTAL Image Loading Functions.....VI-169 (5) 3. COMTAL Image Display Modification Functions........VI-171 (6) 4. Data Plotting Functions.......... .VI-172 (7) 5. Data Processing Functions..........VI-173 (8) 6. DICOMED Output Functions......... .VI-175 (9) 7. VERSATEC Output Functions........ .VI-175 (10) 8. Execute User Coded Function........VI-175 (11) 9. Exit from IMPS................... .VI-175 (11) VLA Data Reduction at NRAO in Charlottesville.... VI-176 APPENDIX 1. THE LOGISTICS OF TRAVEL AND STAYING AT THE VLA SITE.. A-l 1. Transportation................................ .A-l 2. Telephone Use and Special Telephone Numbers......A-6 3. Living Quarters at the Site.................... .A-7 4. Meals......................................... .A-8 5. Library Facilities............. ............... .A-9 APPENDIX 2. PROCEDURES FOR PUBLICATIONS INVOLVING NRAO INSTRUMENTS..........................A-10 PREFACE The purpose of this INTRODUCTION TO THE NRAO VLA is to provide astronomers who wish to observe with the VLA most of the basic information they need to use the instrument. It is described as an introduction rather than a user's manual because no attempt is made to provide completeness