Status of the James Webb Space Telescope (JWST) Observatory

ESTEC Mark Clampin JWST Observatory Project Scientist mark.clampin@nasa.gov Goddard Space Flight Center JWST and its Precursors HUBBLE JWST SPITZER

0.8-meter 2.4-meter T ~ 5.5 K T ~ 270 K

6.5-meter 123” x 136” T ~ 40 K λ/D1.6μm~ 0.14” 312” x 312” 324” x 324” λ/D5.6μm~ 2.22” λ/D24μm~ 6.2” 114” x 84” 132” x 164” λ/D20μm~ 0.64” λ/D2μm~ 0.06” Wavelength Coverage 1 μm 10 μm 100 μm

HST

JWST

Spitzer

JWST Clampin/GSFC JWST: How It Works

Integrated Science Cold Side: ~40K Instrument Module (ISIM) Primary Mirror 5 Layer Sunshield

Secondary Mirror

Solar Array Spacecraft Bus

Hot Side Sun JWST Clampin/GSFC MirrorMirror MeasuredMeasured UncertaintyUncertainty TotalTotal RequirementRequirement (RMS(RMS SFE) SFE) (RMS(RMS SFE) SFE) (RMS(RMS SFE) SFE) (RMS(RMS SFE) SFE) 1818 Primary Primary Segments Segments 23.723.7 nm nm 8.18.1 nm nm 25.025.0 nm nm 25.825.8 nm nm (Composite(Composite figure) figure) ’ SecondarySecondary 14.514.5 nm nm 14.914.9 nm nm 20.820.8 nm nm 23.523.5 nm nm JWST s Optical SystemTertiaryTertiary 17.517.5 nm nm 9.49.4 nm nm 19.919.9 nm nm 23.323.3 nm nm FineFine Steering Steering 14.714.7 nm nm 8.78.7 nm nm 17.117.1 nm nm 17.517.5 nm nm Telescope Optics • Completed the AOS (w/Tertiary and Fine Steering Mirrors)

➡ 3 cryogenic cycles: alignment measurements completed.

➡ AOS provides stray light mitigation, and radiator panels

JWSTJWST Full Full scale scale modelmodel

JWSTJWST Clampin/GSFCClampin/GSFC Aft-Optics System MirrorMirror MeasuredMeasured UncertaintyUncertainty TotalTotal RequirementRequirement (RMS(RMS SFE) SFE) (RMS(RMS SFE)SFE) (RMS(RMS SFE)SFE) (RMS(RMS SFE)SFE) 1818 Primary Primary Segments Segments 23.723.7 nm nm 8.18.1 nmnm 25.025.0 nmnm 25.825.8 nmnm (Composite(Composite figure) figure) SecondarySecondary 14.514.5 nm nm 14.914.9 nmnm 20.820.8 nmnm 23.523.5 nmnm Measured Uncertainty Total Requirement Mirror TertiaryTertiary 17.517.5 nm nm 9.49.4 nmnm 19.919.9 nmnm 23.323.3 nmnm (RMS SFE) (RMS SFE) (RMS SFE) (RMS SFE) FineFine Steering Steering 14.714.7 nm nm 8.78.7 nmnm 17.117.1 nmnm 17.517.5 nmnm

JWST Clampin/GSFC 18 PM Segments (Composite Figure) 23.6 8.1 25.0 25.8

Secondary 14.7 13.2 19.8 23.5 Tertiary 18.1 9.5 20.5 23.2

JWSTJWST Full Full scale scale FSM 13.9 4.9 14.7modelmodel 18.7

JWSTJWST Clampin/GSFCClampin/GSFC JWST Clampin/GSFC Predicted Image Quality

Diffraction Limited: Strehl > 0.8 (WFE ≤ 150 nm) F115W F200W F444W F070W

Log Linear Scale Scale

JWST Clampin/GSFC Encircled Energy

• F115W • F200W

JWST Clampin/GSFC Observing Constraints

l Field of Regard is an annulus with rotational symmetry about the L2-Sun axis, 50° wide

l Sun angle constraints yield 35% instantaneous sky coverage

➡ Full sky coverage achieved North Ecliptic Pole Continuous over a sidereal year Coverage Zone North 5° 45 l Observations interrupted for: °

7 ➡ Orbit maintenance Toward the Sun ➡ 360 station-keeping burns ° ➡ Momentum management

➡ reaction desaturation burns Continuous Coverage Zone South

JWST Clampin/GSFC Gersh-Range and Perrin: Improving active space telescope wavefront control using predictive thermal modeling

(a) too restrictive since the constraints limit the field of regard for long observations. Since changes in the WFE are directly related to changes in the telescope temperature, the scheduling constraints are derived Observing Constraints from temperature restrictions; the basic principle is to generate schedules that do not cause the telescope temperature to expe- rience extreme swings or deviate from a specified range. Limiting the WFE change during an observation, for example, l Field of Regard is an annulus with rotational symmetry about the corresponds to defining a range of allowable final temperatures Sun based on the initial temperature and the observation duration. L2-Sun axis, 50° wide Similarly, ensuring that the total WFE change remains below a specified threshold corresponds to requiring that the temper- ature remain at all times within a range determined by the refer- l Sun angle constraints yield 35% instantaneous sky coverage ence temperature (for which there is no WFE). In each case, the temperature limits determine the maximum and minimum equi- librium temperatures, which correspond to the minimum and ➡ Full sky coverage achieved (b) maximum allowable sun angles, respectively, for the next space- craft attitude in the schedule. As a result, the scheduling con- over a sidereal year straints are derived by relating the desired WFE condition to restrictions on the final temperature, determining the limiting equilibrium temperatures, and calculating the corresponding l Observations interrupted for: sun angles. Sun As an example, to ensure that the WFE change during an observation does not exceed a desired threshold τ, we require 8 ➡ Orbit maintenance that

ΔRMS W tf · W tf − W t0 · W t0 ≤ τ; (5) j j ¼ ð Þ ð Þ ð Þ ð Þ ➡ station-keeping burns pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

(c) where t0 and tf are the times at which the observation begins and ➡ ends, respectively. Using Eq. (4), we can rewrite this condition Momentum management as

Tf − T0 ➡ −τ ≤ pw · w ≤ τ; (6) reaction desaturation burns T T Sun hot − cold ffiffiffiffiffiffiffiffiffiffiffi where Tf is the temperature at the end of the observation. Solving for Tf, we find that Eq. (5) is satisfied if

JWST Clampin/GSFCTf ∈ Tmin;Tmax ; (7) Fig. 2 Attitude range. For the simulations, we assume that the space- ½ Š 20 craft attitude range is identical to that of JWST (a). The hottest atti- where tude corresponds to a sun angle of 85 deg (b), and the coldest attitude corresponds to a sun angle of 135 deg (c). Since thermal changes are τ T − T T ð hot coldÞ T (8) predominately due to changes in the sun angle ϕ, we concentrate max pw · w 0 here on the effects of ϕ alone. ¼ þ and ffiffiffiffiffiffiffiffiffiffiffi 3 Limiting the WFE Using Schedule −τ T − T Restrictions T ð hot coldÞ T : (9) min ¼ pw · w þ 0 Since the WFE perturbations are driven by changes in the sun angle, they are a byproduct of the observing schedule, which we For an observationffiffiffiffiffiffiffiffiffiffiffi of duration tf − t0, these limiting values for know and determine in advance. As a result, we can control the Tf correspond to equilibrium temperatures of WFE evolution passively by introducing scheduling constraints

as part of the schedule generation process. This type of approach τ Thot − Tcold has been studied for managing the spacecraft momentum,22 Te;max min ð Þ T0;Thot (10) ¼ 1 − e−k tf −t0 pw · w þ which also depends on the sun angle, and the same or similar ½ ð ފ  constraint mechanisms in the scheduling software could be and ffiffiffiffiffiffiffiffiffiffiffi extended to consider the WFE. These constraints can in princi- −τ T − T ple either limit the WFE change during an observation or ensure T max ð hot coldÞ T ;T ; (11) e;min −k t −t 0 cold that the total WFE change never exceeds a specified limit. ¼ 1 − e ð f 0Þ pw · w þ For the thermal model we consider, both approaches are suitable ½ Š  for typical observations, allowing most if not all of the sky. respectively, where the additionalffiffiffiffiffiffiffiffiffiffiffi restrictions ensure that the However, in practice limiting the total WFE change may be temperature remains within the range T ;T . Substituting ½ cold hotŠ

Journal of Astronomical Telescopes, Instruments, and Systems 014004-4 Jan–Mar 2015 • Vol. 1(1)

Downloaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 10/09/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx Optical Stability

Optical stability modeling based on worst case hot-cold slew

Wavefront error changes will be smaller when the telescope executes a real observing program.

e.g.Gersh-Range Gersh-Range and Perrin: Improving and active Perrin space telescope (2015) wavefront control using predictive thermal modeling

(a) Hot-Cold slews (b) Simulated observing program

140 140 130 120 120 110 100 100 90 80 80 Pointing Angle (Degrees) 0 5 10 15 20 25 Pointing Angle (Degrees) 0 50 100 150 200 250 Time (Days) Time (Days)

50 50 40 40 30 30 20 20

RMS WFE (nm) 10 10 RMS WFE (nm) 0 0 0 5 10 15 20 25 0 50 100 150 200 250 Time (Days) Time (Days)

JWST Fig. 5 Mission schedules. To evaluate the performance of the wavefront control algorithms, we consider Clampin/GSFC two types of schedules: square wave schedules that represent repeated worst-case slews between the hottest and coldest attitudes (a), and hypothetical mission schedules based on the SODRM schedules for JWST23 (b). Fifteen such SODRM schedules were provided to us by the JWST planning and scheduling system developers.

worst-case slews, with the observatory oscillating between the spacecraft [Fig. 6(a)]. This correction consists of the additive the hottest and coldest attitudes with a period of 1 to 56 days inverse of Wm7: [Fig. 5(a)]. Since we assume that the attitude changes occur instantaneously, these schedules consider the worst-case thermal W if pW · W τ; u − m7 m7 m7 ≥ (21) changes for each period. In contrast, the SODRM-based sched- c ¼ 0 otherwise: ules simulate more realistic hypothetical mission scenarios  ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi based on a detailed population of candidate observations. We consider 15 realizations of the sample mission schedules, which This algorithm is analogous to the classical feedback control laws that are typically used to actively control large ground represent different orderings of the same underlying pool of 24–27 observations; an example is shown in Fig. 5(b). telescopes; it is similar to a proportional controller with a logic-driven gain operating on a 2-week timescale rather than continuously. It may seem inefficient or overly simplistic to sim- 4.2 Control Schemes ply discard six out of seven measurements. However, given the time-variable wavefront evolution as noted above, it is For an active space telescope, the WFE evolution depends on the control scheme in addition to schedule parameters such as not straightforward to combine measurements from different the sun angle changes and the observation durations. Control times, and how to do so for JWST has not yet been specified. schemes that use a sequence of wavefront measurements to This scenario intentionally represents a simplest possible correct excursions at regular intervals, for example, perform algorithm against which we can compare more sophisticated differently than schemes that preemptively correct the wavefront approaches. before the error exceeds a desired limit. To investigate the effec- The averaging algorithm, on the other hand, uses all of the tiveness of each approach, we have developed three control wavefront measurements taken during the last control period. algorithms: baseline and averaging algorithms that correct every These measurements are used to construct a vector of the 2 weeks as needed, and a predictive algorithm that uses an average wavefront coefficients during the last 2 weeks, Wavg, internal model to determine in advance when corrections will and a correction is issued if the corresponding RMS WFE be needed. exceeds τ [Fig. 6(b)]. This correction consists of the additive inverse of Wavg: 4.2.1 Baseline and averaging algorithms mean Wm11 ;Wm21 ;Wm31 ;:::;Wm71 meanðW ;W ;W ;:::;W Þ For the baseline and averaging algorithms, we use a control ð m12 m22 m32 m72 Þ 6 Wavg 2 . 3; (22) scheme that is similar to the baseline scheme for JWST. The ¼ . WFE is measured every 2 days, and the measurements taken 6 7 6 mean W ;W ;W ;:::;W 7 during the last 2-week period are used to determine if a correc- 6 ð m1n m2n m3n m7n Þ 7 tion is needed. For the baseline algorithm, only the most recent 4 5 measurement W is used. At the end of each control period, m7 W if W · W τ; the RMS WFE from W is compared against the correction u − avg avg avg ≥ (23) m7 c ¼ 0 otherwise: threshold τ, and if the error exceeds τ, a correction is sent to  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

Journal of Astronomical Telescopes, Instruments, and Systems 014004-7 Jan–Mar 2015 • Vol. 1(1)

Downloaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 10/09/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx Sky Background

JWST should be zodi-limited at λ < 10 μm

➡ Background levels will include contribution from stray light

➡ Meet requirements @ 20 μm: 174 MJy/Sr vs 200 MJy/Sr req.

JWST Clampin/GSFC Pointing

Predicted performance for offset precision ➡ from 0 - 45” currently expecting < 5.3 mas

Fine steering mirror offsets will be employed for offsets of <60 mas to deliver ~mas precision ➡ Pixel response function mapping ➡ Coronagraphy ➡ Slit mapping

JWST Clampin/GSFC Telescope Structure

Pathfinder Backplane Flight Backplane

JWST Clampin/GSFC

JWST Clampin/GSFC Pathfinder Backplane

The pathfinder is a backplane section with a secondary mirror support structure (SMSS)

➡ Verify SMSS deployment

Tests integration of primary mirror segment installation with two flight spare mirror segments, plus flight-spare secondary mirror

Pathfinder is scheduled for three cryogenic tests during 2015 in Chamber-A at JSC

➡ Verify optical test equipment

JWST Clampin/GSFC

Primary Mirror Integration

Ambient Optical Ambient Optical Alignment Stand Alignment Stand

Mirror Installation dry-run

JWST Clampin/GSFC

Mirror Installation dry-run

JWST Clampin/GSFC JWST Clampin/GSFC Sunshield Membranes

Five flight-like Template Membrane layers manufactured

➡ Template layers tensioned to flight-like configuration

- 3-D membrane shapes measured by Lidar

- Critical for layer-to-layer spacing ➠ thermal performance

- Edge alignment ➠ thermal performance & stray light

Flight membranes under construction (#3 completed)

Tensioned to 3x flight tension for shape measurement by Lidar

(3x tension counteracts gravity sag)

JWST Clampin/GSFC JWST Clampin/GSFC Spacecraft Bus Structure Complete

GIMBALED +J3 ANTENNA ASSY

-J2 PANEL SOLAR ARRAY

LOW GAIN ANTENNA

BUS HARNESSES (2) Cryocooler Radiator Panel INERTIAL REFERENCE UNIT (2X) BUS STRUCTURE WITH INTEGRATED PROPULSION COMPONENTS

+J2 PANEL

+J2 STAR TRACKER ( 3X)

REACTION WHEEL WITH +J1 Star Tracker ISOLATOR ASSY (6X) Radiator Panel FINE SUN SENSOR (2X)

JWST Clampin/GSFC

In addition, the color-coding of Figure 1 indicates the location and responsible organization (NASA, Northrop Grumman, ITT Exelis, or ESA/Ariane) for each of the principal I&T activities. The sequence shown depicts the typical hierarchical process by which such a complex system is developed, with lower level elements developed and tested in parallel, then integrated into higher level assemblies and tested again, e.g.: subsystems Science Instruments; four Science Instruments + Electronics + Structure ISIM; ISIM + OTE OTIS; OTIS + Spacecraft Element Observatory. Philosophically, the JWST I&T program strives to verify performance requirements at the most appropriate level of assembly, attempting to identify problems at the lowest level of assembly possible (when they are easiest to fix), while providingJWST independent Integration cross-checks of key requirements: at thePath higher levels toof test toLaunch confirm that nothing went wrong in the assembly process.

Figure 1.The overall I&T flow for the JWST observatory, depicting its buildup from its principal constituent elements, JWST culminating in integration to the Ariane 5 launch vehicle in Kourou, French Guiana, for launch in 2018. Clampin/GSFC In this paper, we focus on the upper half of the flow shown in Figure 1, the integration and test of the Science Instruments in the ISIM Element, of the Optical Telescope Element, and of the combined OTIS system, culminating in the end-to-end optical and thermal test in the cryo-vacuum chamber at JSC. Space precludes comparable discussion of the Sunshield and Spacecraft I&T; some information on those vital portions of the JWST observatory can be found in Clampin and Bowers (2012) in this volume.2

2. I&T OF THE INTEGRATED SCIENCE INSTRUMENT MODULE (ISIM) 2.1 ISIM and its cryo-test configuration The Integrated Science Instrument Module, shown schematically in Figure 2, is comprised of the four JWST Science Instruments (NIRCam, NIRSpec, FGS/NIRISS, and MIRI), the ISIM structure that holds them (these elements operating at 35-40K, except for the MIRI, which is cooled by a two-stage mechanical cooler to a focal plane temperature of <7K), the ISIM Electronics Compartment (IEC) that houses (at room temperature) control electronics for the Science

Proc. of SPIE Vol. 8442 84422K-2

Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 10/07/2014 Terms of Use: http://spiedl.org/terms How Do We Test the Telescope ?

Cryogenic Optical Test will be conducted at JSC’s Chamber A

Goals of Cryogenic Optical Test

➡ Optical workmanship - check on assembly of the telescope e.g. mechanical interference

➡ Optical alignment - are we inside the capture range of the telescope’s active optics ?

➡ Thermal balance - will the telescope cool to 40K ?

JWST Clampin/GSFC OTIS Test Preparations

Chamber Isolator Units

Cryo Position Metrology

Center of Curvature Optical Assembly

JWST Clampin/GSFC Dry Run testingPreparations w/Chamber-A for OGSE-2 Proceed Apace ➡ ✔ 1) Dry run test- phasing■ Here is the two flight mirrors Aft Optical System on pathfinder mounted to the Pathfinder

6 ➡ 2) Dry run imaging with AOS ✔

➡ 3) Test thermal monitoring equipment

JWST Clampin/GSFC JWST Clampin/GSFC Overall Commissioning Schedule

Month 1 Month 2 Month 3 Month 4 Month 5 Month 6

Observatory Deployments

Telescope Phasing

Observatory& Instrument Commissioning

• Phased Instrument power-on with Temp. • Observatory check-out & calibrations ➡ Attitude control, acquisition, thermal .... • Instrument check-out and calibration

JWST Clampin/GSFC JWST will do transformational science and change our view of the Universe

JWST ➠ Launch 2018

JWST Clampin/GSFC JWST Clampin/GSFC Where To Follow JWST

Web pages Social Media www.jwst.nasa.gov webbtelescope.org

Webcam ibook ➠ itunes

www.jwst.nasa.gov/webcam.html

JWST Clampin/GSFC

iBook for JWST free on iTunes (search for “James Webb Space Telescope”) JWST Clampin/GSFC JWST Operations

Communications Coverage Provided For all Critical Events SOC Available 24 Hours, 7 Days per Week Observatory Deployments Until Telescope Phased - Solar Array - High Gain/ Medium Antennas L2 Point Ariane 5 Upper Observatory – Upper Stage - Sunshield Stage Injects JWST Separation - Optical Telescope Element L2 Lissajous Into Direct Transfer Orbit Trajectory

L2 Transfer S-Band Tlm Link ( 2Kbps) Trajectory Ariane 5 S-Band Cmd Link (0.25 Kbps) Launch S-Band Ranging System Ka-Band Science Link ( Selectable 7, 14, 28 Mbps) S-Band Tlm Link (Selectable 0.2 - 40 Kbps) S-Band Tlm Link ( 2Kbps) S-Band Cmd (Selectable 2 and 16 Kbps) S-Band Ranging S-Band Ranging Nominal 4 Hour Contact Every 12 Hours

Deep Space Network Communications Services for Launch Space Telescope Science Institute (TDRS, ESA, Malindi) NASA Integrated Services Network Science & Operations Center

Ariane PPF S5 GSFC Flight Dynamics Facility

JWST Clampin/GSFC Phasing the Telescope

OTE Deployment / First Light (Image Mosaic)

SM Focus Sweep

Segment Identification NIRCam First After Segment-Image After Global Light Array Alignment Segment Search

Segment-Image Array

DHS (20 edges) Global Alignment After Image Stacking

Commissioning After Coarse Image Stacking Phasing

Coarse Phasing (DHS)

Single Field Fine Phasing After SF Fine Phasing Pupil Imaging Multi-Field Fine Phasing

Wavefront Monitoring & Maintenance

JWST Clampin/GSFC