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