1. A “Field Guide” to WFPC2 HLA Image Anomalies
9/13/2018
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

2. 1. Introduction
9/13/2018
There is already a very good Instrument Science Report describing WFPC2 image anomalies at:
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.

3. 2. Poor Chip-to-chip Seems
9/13/2018
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.

4. 3. WF4 Anomaly (and strange color images)
9/13/2018
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 for more details.

5. 4. Misalignments
9/13/2018
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.

6. 5. PC1 Scattered Light
9/13/2018
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:
for more details.

7. 6. Reflected Earth Light
9/13/2018
This is due to earth light being scattered off the spiders holding the secondary at the top of the telescope.
See item # 11 in :
for details.

8. 7 CTE Trails
9/13/2018
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.

9. 8. Field Flattener Ghosts
9/13/2018
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:
For details.