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
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–
–
–
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…
1) Not big enough for cross-calibration
2) No IR color information
Jay Anderson --- TIPS
Feb 21, 2008
17"
J1
Ground-based Observations
• HAWK-I: new IR camera
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–
–
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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
HAWK-I distortion:
Raw residuals
After global
correction:
x
x
x
x
x
x
x
x
One chip
Jay Anderson --- TIPS
Feb 21, 2008
J3
Periodic
residuals
Phase Diagram
Phase Diagram
•Same
throughout
detector
•Periodogram
•Square wave
removes
periodic
signature
Jay Anderson --- TIPS
Feb 21, 2008
J4
Why do we care?
• Rockwell Scientific
HAWAII-2RG
– Prototype for JWST!
• Very important to
understand the source
WFC FLAT FIELDS
– Geometric, like HST?
• WFPC2’s 34-row effect
• ACS/WFC’s 68-column
effect
– Check flat fields
HAWK-I
• No evidence
Jay Anderson --- TIPS
Feb 21, 2008
J5
32 amplifiers 
Why do we care?
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?
All CRs
• HAWK-I Readout
• CTE-like hysteresis
Even Amps
Odd Amps
– PSF asymmetry?
– Look at CRs…
• Likely cause
inter-pixel capacitance
– pixel dwell time
Jay Anderson --- TIPS
Feb 21, 2008
J6
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