The WFPC2 WF4 Anomaly John Biretta 15 Dec 2005

2 Overview Discovery of the Anomaly Historical Evolution Imaging Impacts GO Science Impacts Image Repair Cause / Possible Mitigation Strategy

3 Discovery of New Anomaly First noticed during study of dark current trends Anomalous low dark current for CCD WF4 on 9/23/ of 5 input images were blank in WF4 Bias levels were zero in the 2 frames

4 Low Dark Current on 9/23/2005 ??

5 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 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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7 Low Bias Zero Bias

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10 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

11 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

12 Low Bias Image Example 8/25/2005 F439W, 700s Stripes in WF4. +/- 0.5 DN WF2WF3 WF4 PC1

13 Low Bias Image Example 8/25/2005 F439W, 700s Stripes along CCD rows +/- 0.5 DN

14 Normal bias Faint pixels ~15 DN Bright pixels ~1000 DN

15 Normal bias Faint pixels ~15 DN Bright pixels ~1000 DN

16 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

17 WF4 Zero Bias Example 8/25/2005 F336W, 900s Mostly zero. Bright targets, cosmic rays. Negative imprint of cal files.

18 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

19 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

20 Repair of Low-Bias Images: Streak Removal Iterative filtering of background very successful in removing streaks Preliminary IRAF script being tested

21 BeforeAfter

22 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 submitted

23 Normal bias Faint pixels ~15 DN Bright pixels ~1000 DN Before correction…

24 Linearity correction brings all count levels to single curve….

25 … then simple gain correction applied as function of bias level.

26 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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29 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

30 Mitigation Strategy (cont.) 2 nd 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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34 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