A Field Guide to WFPC2 HLA Image Anomalies

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A Field Guide to WFPC2 HLA Image Anomalies

• Version August 19, 2008
• Brad Whitmore

• Introduction
• Poor Chip-to-Chip Seams
• WF4 Anomaly (and strange color images)
• Misalignments
• PC Scattered Light
• Reflected Earth Light
• CTE trails
• Field Flattner Ghosts

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1. Introduction

• There is already a very good Instrument
  Science Report describing WFPC2 image anomalies
  at
• http//www.stsci.edu/hst/wfpc2/analysis/wfpc2_anom
  alies.html
• The following is meant to supplement this
  for a few specific examples of additional
  anomalies that have cropped up in recent years,
  have been introduced by the HLA, or which are
  common enough that a reminder might be useful.

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2. Poor Chip-to-chip Seems

• The seams between the four chips are not
  perfectly aligned.
• This is particularly apparent around the
  Planetary Camera (PC) chip, since the smaller
  0.05 arcsec pixels are resampled to 0.10 arcsec
  to match the Wide Field (WF) chips.
• Note that to see the PC (or to download the
  image) in its full resolution you must use
  advanced search and click the WFPC2-PC box.

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3. WF4 Anomaly (and strange color images)

• During the last few years the bias level for chip
  4 has been dropping. Mitigation strategy based on
  changing the temperature have helped, but have
  not alleviated the problem.
• These images can look strange, especially for the
  color images where the program that determines
  the contrast range has problems covering the full
  range, especially the bias stripes on WF4.
• This should get better as part of the WFPC2
  pipeline reprocessing project, that should be
  completed in time to include in Data Release 3
  (Feb. 2009). See http//www.stsci.edu/hst/wfpc2/
  wf4_anomaly.html for more details.

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4. Misalignments

• Images from different exposures can be misaligned
  for several reasons (e.g., failure to reacquire
  the same guide stars after earth occultation).
• In this case the misalignment occurred between
  observations of different filters.
• While we keep the color image (to make it clear
  that there is a misalignment), and the individual
  combined images for each filter, we do not
  combine the filters to make a total or
  detection image in these cases.

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5. PC1 Scattered Light

• The strange arc-shaped figure in the corner of
  the PC is caused by scattered light from a very
  bright star in the missing L-shaped
• region around the PC. In this particular case
  you can actually see evidence of the bright star
  due to the diffraction spikes in the WF chip 2
  just above the PC1 scattered light pattern.
• See Item 5 from
• http//www.stsci.edu/hst/wfpc2/analysis/wfpc2_anom
  alies.html
• for more details.

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6. Reflected Earth Light

• This is due to earth light being scattered off
  the spiders holding the secondary at the top of
  the telescope.
• See item 11 in
• http//www.stsci.edu/hst/wfpc2/analysis/wfpc2_anom
  alies.html
• for details.

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7 CTE Trails

• Charge Transfer Efficiency (CTE) has degraded
  with time. In addition, is is worse for faint
  source, or cases with very low backgrounds (e.g.,
  U band or narrow band).
• This image shows that the CTE trails are longer
  for objects at the top of the chip since they
  must traverse more pixels as they are read out,
  hence loosing more charge and making longer
  trails.
• See the WFPC2 Intrument Handbook or web-site for
  more extensive discussions and how to make
  corrections for point sources.

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8. Field Flattener Ghosts

• Filter ghosts are ring shaped artifacts caused by
  light reflecting within the field flattener. They
  are generally only visible for bright stars.
• The are always on a line between the center of
  the chip and the bright star.
• There are also ghosts from filters which look
  more arc-shaped.
• See item 9 from
• http//www.stsci.edu/hst/wfpc2/analysis/wfpc2_anom
  alies.html
• For details.