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The WFPC2 WF4 Anomaly

The WFPC2 WF4 Anomaly. John Biretta 15 Dec 2005. Overview. Discovery of the Anomaly Historical Evolution Imaging Impacts GO Science Impacts Image Repair Cause / Possible Mitigation Strategy. Discovery of New Anomaly. First noticed during study of dark current trends

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The WFPC2 WF4 Anomaly

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  1. 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 9/23/2005 • 2 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. 2005 first zero bias (blank) images • Currently ~all images have low-bias and ~30% have zero bias in WF4 • Other three CCDs are OK

  6. Low Bias Zero Bias

  7. 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

  8. 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

  9. WF4 Low Bias Image Example 8/25/2005 F439W, 700s Stripes in WF4. +/- 0.5 DN PC1 WF2 WF3

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

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

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

  13. 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

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

  15. 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

  16. 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

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

  18. Before After

  19. 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

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

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

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

  23. 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

  24. 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

  25. 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).

  26. 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

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