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
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
1. Coarse Alignment
C
>150 mm seg piston, >250 nm rms WFE
S
Commission
Accomplished
in transit to L2
(20 days)
<100 mm seg piston, <200 nm rms WFE
2. Coarse Phasing/
Fine Guiding
C
<1 mm rms WFE, <8 marcsec LOS jitter
>1 mm rms WFE range, <10 nm accuracy
S
3. Fine Phasing
C
<100 nm rms WFE setpoint
150 nm rms WFE worst case decay
Maintenance
As required
(~1 hr weekly or monthly)
4. Wavefront Monitoring
S
>>150 nm rms WFE range,
<10 nm measurement accuracy
Sensing requirements at each step determine interface parameters – FOV, etc.
DHS/DFS description, JWST Informal Monthly, November 2003
Sivaramakrishnan
From Atcheson (BATC)
Where does Coarse Phasing fit
in JWST commissioning?
Before coarse phasing
At end of Coarse Alignment
< 100 micron segment piston
< 200 nm RMS wavefront error (WFE)
After guiding
After coarse phasing
At end of Coarse Phasing
< 1 micron RMS WFE
< 8 mas Line of Sight (LOS) jitter
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
From Fang Shi (JPL)
How is the Fringe Formed?
Segmented Telescope
L
DFS Grism
L
x
DFS Fringe
l Increases
Dark Band
Dispersion
x
Final data strip on detector
DHS/DFS description, JWST Informal Monthly, November 2003
Wavefront Pistoned
Piston Corrected
Sivaramakrishnan
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
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
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
DHS/DFS description, JWST Informal Monthly, November 2003
Assume pupil is 35 Sivaramakrishnan
mm dia.
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
From Fang Shi (JPL)
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
15
16
17
14
5
4
6
18
2
8
Group #4
12
11
3
1
7
Group #5
13
10
9
Group #3
Group #2
Group #1
Tilt test segment pairs
(cyan & magenta)
away from PARKING
LOT
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
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
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