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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). From Atcheson (BATC). - PowerPoint PPT Presentation
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DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan
Co-phasing JWST during commissioningusing
Dispersed Hartmann Sensingor
Dispersed Fringe Sensing
Anand SivaramakrishnanTelescopes 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 setpoint150 nm rms WFE worst case decay
2. Coarse Phasing/Fine Guiding
<1 m rms WFE, <8 marcsec LOS jitter
Commission
Maintenance
OTE - 0.5 arcsec 1- pointingPM 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. 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 fitin 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 motionSmall Fine motion
Cryogenic
Excellent repeatability
DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan
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
Bipod Actuation
Flexures attach actuatorto 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 apertureCut 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
xL
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
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 prismthis spot does not overlap the first spot
a second DFS-like fringe is created
Measure relative piston between all pairs of adjacent segmentsusing a single exposure
DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan
-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.
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 #5
21
3
45
6
7 8 9
10
11
12
131415
16
17
18
Tilt test segment pairs (cyan & magenta)
away from PARKING LOT
From Fang Shi (JPL)
PARKINGLOT
DHS/DFS description, JWST Informal Monthly, November 2003 Sivaramakrishnan
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
•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
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