Status of the James Webb Space Telescope (JWST) Observatory ESTEC Mark Clampin JWST Observatory Project Scientist [email protected] 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 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 Þ 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 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 hot 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.