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

DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan Where does Coarse Phasing fit in JWST commissioning? 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 Before coarse phasing After guiding After coarse phasing From Atcheson (BATC)

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

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

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

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

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

DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan 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 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 -30 –20 –10 – 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. From Scott Acton (BATC) DHS Simulations

DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan 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 DFS for 18-Hex Example: Segment Group 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 Group #1 Group #2 Group #3 Group #4 Group # Tilt test segment pairs (cyan & magenta) away from PARKING LOT From Fang Shi (JPL) PARKING LOT

DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan DFS observing block - summary 7 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 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 Strengths/weaknesses

DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan 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