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.
Description: Sources that are overexposed will saturate pixels, leading to charge that "bleeds" into adjacent pixels in the same column. Saturation typically occurs around 77,400 e- on average. In many cases, reliable photometry can still be performed on saturated sources as shown in the Jupyter Notebook linked below.
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Description: Damage to the CCD detectors leads to the formation of charge traps, which degrades the CCD Charge Transfer Efficiency (CTE). During readout, some charge is trapped and later released, leading to "trails" behind sources in the images. CTE trails are most pronounced in the parallel (y-axis) direction, but are also present at a weaker level in the serial (x-axis) direction. The standard ACS calibration pipeline includes a pixel-based CTE correction algorithm to remove both parallel and serial trails. Alternatively, a correction model has been developed for point sources that can be applied to uncorrected photometric data.
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Description: Warm and hot pixels are individual pixels with elevated dark current. The distinction between warm and hot is somewhat arbitrary, but as of now, hot pixels refer to those that exceed 0.14e-/pix/sec (WFC) or 0.08 e-/pix/sec (HRC), while warm pixels are those between 0.06-0.14 e-/pix/s (WFC) and 0.02-0.08 e-/pix/s (HRC). The number of warm and hot pixels increases with time in orbit.
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Description: A permanently fixed occulting finger designed for coronagraphy and measuring 0.8 \(x\) 5 arcseconds is located in the HRC field of view.
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Description: There are several sources of scattered light:
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Description: When a moving object (typically an artificial satellite) passes across the FOV, it results in a narrow streak across the image. Masking these features before creating final images via Drizzlepac effectively removes them from the final image (Note: without masking the trails, Drizzlepac removes the brightest part of the trails, but typically leaves the faint outer edges). The ACS team has developed two software routines to flag and mask satellite trails, both of which are included in the ACSTOOLS package.
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Description: The WFC bias levels show a range of properties users should be aware of:
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Description: The four WFC CCD amplifiers are read out separately and simultaneously. During read-out, electronic cross-talk between the amplifiers can occur. As a result, a source in one detector quadrant may appear as a faint, mirror-symmetric (often negative) ghost image in the other quadrants. Cross-talk generally has negligible impact for most applications, but is corrected for in full-frame WFC images by the CALACS pipeline.
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Description: In 2017, three new artifacts called "flecks" appeared in WFC calibration data, and appear to be the results of particulates that have fallen onto the surface of the CCD. Another new fleck was found in 2024. Pixels with <50% transmissivity have been added to the bad-pixel tables and flagged in the DQ arrays. The ACS team regularly monitors for new flecks.
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Description: Between 1.5% and 3% of pixels can be impacted by cosmic rays during a 1000s exposure. Dithering observations is an effective means of removing cosmic rays from final images, although the final result depends on exposure time, number of exposures, and parameters used in Drizzlepac.
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Description: A broken anode led to the disabling of rows 600 to 605. The corners of the detector also suffer from vignetting.
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Description: 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).
Loss of lock: If the Fine Guidance Sensors fail to maintain lock on one or both guide stars, the telescope pointing may drift leading to smearing of data (e.g., star trails). Typical losses of lock will be reported in the jitter files associated with an observation.
Undeclared Loss of Lock: In some cases, the fine guidance sensors may lose lock on one guide star but not report it in the jitter files. However, these events can be tracked and observers are notified if an undeclared loss of lock occurs. Typical side effects will be a roll drift that can leading to smearing in the data. In many cases, the level of smearing should be small, but users impacted by undeclared LoL events are encouraged to examine their data closely for issues.
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Please contact the HST Help Desk with any questions.