1. Co-phasing JWST during commissioningusingDispersed Hartmann SensingorDispersed Fringe Sensing Anand Sivaramakrishnan Telescopes Branch, Instrument Division Space Telescope Science Institute methods developed by Paul Atcheson, Scott Acton (Ball), Fang Shi (JPL) DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

2. From Atcheson (BATC) Commissioning Process – Operational Performance Assumptions PM/SM Deployment • OTE - 0.5 arcsec 1-s pointing • PM segments - <100 mm, 1 arcmin wing deployment • <100 mm ROC, 0.5 arcmin tilt, 100 mm piston • SM - <3 mm linear position, 5 arcmin tilt Control Sense C 1. Coarse Alignment <100 mm seg piston, <200 nm rms WFE >150 mm seg piston, >250 nm rms WFE S Commission 2. Coarse Phasing/ Fine Guiding C Accomplished in transit to L2 (20 days) <1 mm rms WFE, <8 marcsec LOS jitter >1 mm rms WFE range, <10 nm accuracy S C 3. Fine Phasing <100 nm rms WFE setpoint 150 nm rms WFE worst case decay Maintenance >>150 nm rms WFE range, <10 nm measurement accuracy As required (~1 hr weekly or monthly) S 4. Wavefront Monitoring Sensing requirements at each step determine interface parameters – FOV, etc. DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

3. From Atcheson (BATC) Where does Coarse Phasing fitin JWST commissioning? Before coarse phasing At end of Coarse Alignment < 100 micron segment piston < 200 nm RMS wavefront error (WFE) At end of Coarse Phasing < 1 micron RMS WFE < 8 mas Line of Sight (LOS) jitter After guiding After coarse phasing DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

4. Actuator Single actuator Gears Large coarse motion Small Fine motion Cryogenic Excellent repeatability DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

5. Radius of Curvature actuatoron Ball testbed telescope segment(TBT: same basic design as JWST, smaller size) DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

6. Bipod Actuation Flexures attach actuator to back of segment DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

7. Single segmenton Ball testbed telescope segment 3 bipods = 1 hexapod RoC actuator in middle (hanging free from back surface) DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

8. What is Fringe Sensing? Circular aperture Cut it horizontally across a diameter Offset one half by some piston error (a ‘step’ across the cut) What is the PSF at various wavelengths? PSFs for same piston at different wavelengths DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

9. From Fang Shi (JPL) How is the Fringe Formed? L L x x Segmented Telescope DFS Grism DFS Fringe l Increases Dark Band Dispersion Wavefront Pistoned Piston Corrected Final data strip on detector DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

10. From Fang Shi (JPL) DFS fringes from Keck tbd tbd Red dots show 12 subaperture locations, each seeing two segments DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

11. What is DHS? Place a PRISM over the edge between two adjacent segments prism tilts the resulting spot away from ‘on-axis’ location on detector When sent through GRISM the spot is smeared out in l same as a DFS fringe On a different pair of segments use a diffent tilt prism this spot does not overlap the first spot a second DFS-like fringe is created Measure relative piston between all pairs of adjacent segments using a single exposure DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

12. From Scott Acton (BATC) DHS Simulations -30 –20 –10 –5 0 5 10 20 30 microns • Simulation code allows user to specify: • Center wavelength • Width of light spectrum • Number of discrete wavelengths in the simulation • Percent full well in the images • Read noise • Dark current • Magnitude of star (affects noise due to dark current) • Dispersion in X and Y directions • Image sampling parameters Assume pupil is 35 mm dia. DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

13. DHS observing sequence - summary 1 sensing, 1 check observation (best case) ~5 worst case • Slew to target • Lock with FGS (on 5x blur) • Take two ~100s NIRCam exposures with two pupil wheel positions • Downlink data, analyze, determine segment updates • Upload actuator commands • Take two ~100s NIRCam exposures with two pupil wheel positions • Adjust all segments • Take two ~100s NIRCam exposures with two pupil wheel positions • Downlink data, analyze --- DONE (with good actuators)! DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

14. From Fang Shi (JPL) DFS for 18-Hex Example: Segment Group Group #5 15 14 13 Group #4 16 5 4 12 Group #3 11 17 6 3 Group #2 18 2 1 10 Group #1 8 9 7 • Two-grism approach provides optimal visibility for all operations • 2 crossed dispersed grisms in NIRcam filter wheel (may have 1 in each of 2 channels) • 7 images • First phases segments into rows • Then phases rows to each other • Fine phasing between each “big move” to remove residual tilts • Position the groups along a 45º line so that fringes from vertical and horizontal dispersion grism won’t collide • The spots are (from lower left): PARKING LOT, Group #1, #2, #3 (middle), #4 and #5 • Spots are separated by 1 arcsec in x and y in the simulations Tilt test segment pairs (cyan & magenta) away from PARKING LOT PARKING LOT DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

15. DFS observing block - summary7 or more iterations (best case) ~20 worst case? • Slew to target • Move N segment pairs out of PARKING LOT • Guide w/FGS on some spot? • Take two NIRCam exposures with grisms in pupil wheel • Command segment tilts to re-arrange segment pairs • Take NIRCam exposures • Repeat till all required segment pairs have been observed • Downlink and analyze, prepare segment updates • Repeat 7 times (or more) till measurement meets spec . . . DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

16. DHS Strengths/weaknesses • Simple FGS operation • Catastrophic pupil misregistration of grism assembly is fatal • Monotonic improvement of PSF with each iteration • Measurement does not require actuation • 5 days to coarse phase if actuators misbehave • Ground verification needs work during I&T • DHS is the baseline approach DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan

17. DFS strength/risks • Strengths • Catastrophic pupil misalignment not an issue • Risks • Non-monotonic image quality during process • Measurement requires actuation • Many more iterations with single grism • 20 days to coarse phase if actuators misbehave (possibly fatal) • DFS is the fallback approach DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan