Identifying ACS Image Anomalies
For a comprehensive guide on identifying ACS image anomalies, we refer users to the HLA report, ACS CCD Image Anomalies in the Hubble Legacy Archive. This HLA report provide detailed explanations, as well as visual representations of all the known anomalies present in HRC and WFC images. Below, we list a summary of the image anomalies and artifacts that have been found in all three ACS channels.
ACS image anamolies and artifacts
HRC
- Bleeding along CCD columns from saturated objects
- CTE trails
- Hot and warm pixels
- Occulting finger
- Optical ghosts from reflections in the F660N filter (generated by bright stars)
- Prism issues: red leak / pile-up for blue filters
SBC
- Broken anode disabiling rows 600 to 605.
- Three dark spots smaller than 50 microns at positions (334, 977), (578, 964), and (960, 851)
- Two bright spots at (55, 281) and (645, 102) with fluctuating ratesthat are always less than 3 counts per second
WFC
- Bad CCD columns
- Saturated blobs in darks
- Bias offset between WFC amplifier quadrants
- Bias striping due to 1/f noise on the bias reference voltage
- Bleeding along CCD columns from saturated objects
- CCD amplifier crosstalk (electronic ghosts)
- CTE trails
- Dragon's Breath scattered light from an off-chip star
- Flecks
- Glint from a star in the WFC interchip gap
- WFC interchip gap
- Optical ghosts from reflections in the F660N filter (generated by bright stars)
SBC observations of bright objects may show optical ghosts possibly due to reflection between the back and front sides of the filter. The brightness of the ghost is typically less than 1% of the star brightness. The displacement of the ghosts from the star image is filter dependent:
| Filter | Displacement (pixels) |
|---|---|
| F122M | -50,+155 |
| F125LP | -30,+95 |
| F150LP | -28,+78 |
Note: Displacement is measured from the center of the real object in the undistorted image (i.e. DRZ or DRC).
An example of the optical ghost from observations of white dwarf J132811.4+463050 taken in December of 2015:
- ACS imaging performance, Hartig et al., 2002
- Scatter from the WFC gap, Hartig et al., 2002
- Here Be Dragons: Characterization of ACS/WFC Scattered Light Anomalies B. Porterfield et al. 2016
- A Minor Contamination Event in May 2017 Affecting the ACS/WFC CCDs
Interactive viewers demonstrating the effects of dragons breath and edge glow:
- Correlated noise (crosshatch pattern in sky background)
- Image compression data loss or artifacts
- Residual (unrejected) cosmic rays and artifacts
- Guide star issues: loss of lock, no guide stars
- Cosmic rays: normal, SAA, cascading, residual
- Satellite trails
- Asteroid trails in fixed target images
- Star trails in moving target images
