Co-phasing JWST during commissioning using Dispersed Hartmann Sensing or Dispersed Fringe Sensing

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)


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)


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)


Actuator

Single actuator

Gears

Large coarse motionSmall Fine motion

Cryogenic

Excellent repeatability


Radius of Curvature actuatoron Ball testbed telescope segment

(TBT: same basic design as JWST, smaller size)


Bipod Actuation

Flexures attach actuatorto back of segment


Single segment on Ball testbed telescope segment

3 bipods = 1 hexapod

RoC actuator in middle(hanging free from back surface)


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


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


DFS fringes from Keck

tbd

tbd

From Fang Shi (JPL)

Red dots show 12 subaperture locations, each seeing two segments


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


-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 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


• Adjust all segments


• Downlink data, analyze --- DONE (with good actuators)!


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


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

.

.

.


•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


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

Documents

Co-phasing JWST during commissioning using Dispersed Hartmann Sensing or Dispersed Fringe Sensing