1. An Unexpected Hysteresis Effect Seen in Hawaii-2RG Detectors
Background
Distortion calibration important for JWST
Coronagraphy, MSA, slits
Self-calibration will be difficult
ACS field observed in 2006
In LMC, in JWST’s CVZ
55: big enough for all detectors
Good density of stars
Many well-measured stars
More than 100,000 good to 1 mas
Good, but…
Not big enough for cross-calibration
No IR color information
17"
Jay Anderson --- TIPS
Feb 21, 2008 J1

2. Ground-based Observations
HAWK-I: new IR camera
Being commissioned on VLT at Paranal
7.57.5 FOV
22 mosaic of 20482048 NIR detectors
“Active optics”
Luigi Bedin (STScI) observed the JWST field
For them: exquisite distortion solution
For us: IR photometry & larger field
We got an additional benefit…
Jay Anderson --- TIPS
Feb 21, 2008 J2

3. HAWK-I distortion: After global correction: Raw residuals One chip J3x x
One chip x x x x x x
Jay Anderson --- TIPS
Feb 21, 2008 J3

4. Periodic residuals Same throughout detector Periodogram Square wave
Phase Diagram
Phase Diagram
Same
throughout
detector
Periodogram
Square wave
removes
periodic
signature
Jay Anderson --- TIPS
Feb 21, 2008 J4

5. Why do we care? Rockwell Scientific HAWAII-2RG Very important to
Prototype for JWST!
Very important to
understand the source
Geometric, like HST?
WFPC2’s 34-row effect
ACS/WFC’s 68-column effect
Check flat fields
No evidence
WFC FLAT FIELDS
HAWK-I
Jay Anderson --- TIPS
Feb 21, 2008 J5

6. Why do we care? Rockwell Scientific HAWAII 2RG
32 amplifiers 
AMP#0
AMP#1
AMP#2
AMP#3
AMP#4
Rockwell Scientific HAWAII 2RG
Prototype for JWST!
Very important to understand the source
Geometric? No!
Readout artifact?
HAWK-I Readout
CTE-like hysteresis
PSF asymmetry?
Look at CRs…
Likely cause
pixel dwell time
All CRs
Even Amps
Odd Amps
inter-pixel capacitance
Jay Anderson --- TIPS
Feb 21, 2008 J6

7. Possible Implications for JWST
JWST will have four amplifiers
four 512-pixel zones
Not clear if effect comes before
or after the amplifiers… Really a problem?
How to deal with hysteresis?
Mild deconvolution? (will amplify noise)
Leave in, but model
Similar to the inter-pixel capacitance
Effective PSF will treat it implicitly as (x,y)
Need to study impact on:
subarray mode, guider, spectra, dithering, noise model
Early NIRCam/U.AZ tests
encouraging!
Jay Anderson --- TIPS
Feb 21, 2008 J7