the wfpc2 wf4 anomaly john biretta 15 dec 2005. 2 overview discovery of the anomaly historical...
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The WFPC2 WF4 Anomaly
John Biretta
15 Dec 2005
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Overview
• Discovery of the Anomaly
• Historical Evolution
• Imaging Impacts
• GO Science Impacts
• Image Repair
• Cause / Possible Mitigation Strategy
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Discovery of New Anomaly
• First noticed during study of dark current trends
• Anomalous low dark current for CCD WF4 on 9/23/2005
• 2 of 5 input images were blank in WF4
• Bias levels were zero in the 2 frames
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Dark Current vs. Time
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0.5
1
1.5
2
2.5
1/1/1994 1/1/1996 1/1/1998 1/1/2000 1/1/2002 1/1/2004
Date
Dark Current (DN / 1800s)
PC1
WF2WF3
WF4
Low Dark Currenton 9/23/2005 ??
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Historical Development
• Earliest evidence ~ SM3B (March 2002) -- sporadic images begin appearing with bias level few DN below normal 311 DN
• Late 2004 images with very low bias begin to appear
• Feb. 2005 first zero bias (blank) images • Currently ~all images have low-bias and ~30% have zero bias in WF4• Other three CCDs are OK
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Low Bias
Zero Bias
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Imaging Impacts
Two classes of WF4 image anomaly:
Low-Bias Images• Background streaks• Corrupted photometry• Images probably fixable
Zero-Bias Images• Mostly blank• Images cannot be fixed
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Low Bias Images
• Bias level ~5 to ~290 DN
• Pipeline corrects for low level, but…
• Faint horizontal streaks +/- 0.5 DN
• Photometry up to 70% low for faint targets at very low bias
• ~1800 science images impacted so far
• Images probably fixable
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Low BiasImageExample
8/25/2005
F439W, 700s
Stripes in WF4.
+/- 0.5 DN
WF2 WF3
WF4
PC1
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Low BiasImageExample
8/25/2005
F439W, 700s
Stripes along CCD rows
+/- 0.5 DN
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Normal bias
Faint pixels ~15 DN
Bright pixels ~1000 DN
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Normal bias
Faint pixels ~15 DN
Bright pixels ~1000 DN
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Zero Bias Images
• Bias level falls below A-to-D zero point
• Image is mostly zero (blank)
• Faint images & targets lost
• Only cosmic rays, bright targets visible
• Data unusable for most purposes
• ~200 science images impacted so far
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WF4Zero BiasExample
8/25/2005
F336W, 900s
Mostly zero.
Bright targets,cosmic rays.
Negative imprint of cal files.
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Science Impact
• Modest impact since most targets placed on PC1 or WF3 CCDs
• Large targets / surveys needing entire field-of-view lose 1/3 of area
• GOs notified via STAN, HST website, WFPC2 website, notice with APT release
• ISR on WFPC2 website
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Science Impact (cont.)
• Pending observations reviewed for risk; attempting to move observations early where possible
• WF4 low-bias images probably fixable – preliminary scripts exist to fix images
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Repair of Low-Bias Images: Streak Removal
• Iterative filtering of background very successful in removing streaks
• Preliminary IRAF script being tested
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Before After
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Repair of Low-Bias Images: Photometry
• Correction for non-linearity at low bias
c’ = c + f(bias level)
• Correction for anomalous gain
c’’ = c’ / gain(bias level)
• Can correct existing test data to +/-1%...
• Need testing on broader range of data -- cal proposal 10772 submitted
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Normal bias
Faint pixels ~15 DN
Bright pixels ~1000 DN
Before correction…
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Linearity correction brings all count levels to single curve….
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Photometry vs. Bias Level - Gain 7 (Linearity & Gain Corrections Applied)
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0 50 100 150 200 250 300 350
Bias Level (DN)
Ratio (Counts / Normal Counts)
IFLAT F555W 10s Shadow ~15 DN
IFLAT F555W 10s ~1000 DN
IFLAT F555W 8s ~800 DN
IFLAT F555W 6s ~600 DN
IFLAT F390N 1800s ~70 DN
… then simple gain correction applied as function of bias level.
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Cause of Anomaly
• Exact cause not yet known• Possibly instability / failure of signal amplifier• Bias variations appear driven by temperature
peaks associated with cycling of the WFPC2 “Replacement Heater”
• Tight correlation between WF4 camera head temperature and bias level suggests problem is located in WF4 camera head electronics
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Potential Mitigation Strategy
• Reduce temperatures in WFPC2 camera heads / electronic bays by few deg C
• “Replacement Heater” controlled by WFPC2 Bay 1 temp and software set points at 11 and 15 deg C
• Test planned for January – reduce upper set point from 15 to 12 deg C for 24 hrs and take many internal images
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Mitigation Strategy (cont.)
• 2nd test planned later – reduce both set points – from 11 & 15 deg C to (e.g.) 9 & 10 deg C
• Seen as relatively safe.
• Some potential for change in optical alignment (temp of optical bench).
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Anomaly Review Board
• Ed Cheng (chair), Jim Ries, Renee Taylor, Lisa Mazzuca, Ben Reed, Jeff Travis, Augustyn Waczynski, Mal Niedner, Randy Kimble, Ken Carpenter (GSFC), Tom Bickler, Tom Elliot (JPL), Ed Cheung (J&T), John Bacinski (LMTO), John Biretta (STScI)
• Report due end of January