ACS Data Quality Flags
The following flags are used in the data quality arrays of ACS data, e.g. the *.fits[dq,1] extension, to identify good, bad, and corrected pixels. Some of these flags emanate from the "permanent" bad pixel tables (BPIXTAB), although as of 8 Oct 2004 we are using the BPIXTAB much less than before. Currently, most of these flags emanate from the generic conversion of the science data, or propagate from the data quality arrays of the various reference images used to calibrate and combine the data in the pipeline (CALACS and AstroDrizzle).
Initial processing in CALACS combines the DQ flags for each pixel, from prior calibration processing (BPIXTAB) and from saturation tests, into a single DQ result. If a single pixel is affected by two or more DQ flags, the sum of those flag values is assigned to the corresponding pixel in the final DQ array. For example, a hot pixel (flag 16) that is also unstable throughout an anneal cycle (flag 32) would be given a flag value of 48 (16+32=48). This array then serves as a mask so that CALACS will ignore bad pixels during processing.
| Flag | Definition |
|---|---|
| 0 | Good pixel. |
| 1 | Reed-Solomon decoding error; e.g., data lost during compression. |
| 2 | Data replaced by fill value; e.g., neighboring cosmic ray contaminated pixels. |
| 4 | Bad detector pixel, or vignetted pixel. In the HRC BPIXTAB, this identifies a small detector defect in the "upper right" corner. |
| 8 | Masked by aperture feature, e.g., the HRC occulting finger. |
| 16 | Hot pixel, i.e., pixel with dark current > 0.14 e-/sec, flagged in superdark DQ arrays. |
| 32 | Pixels with unstable dark current; includes random telegraph signal noise and fading hot pixels. See section 3.2 of ACS ISR 2017-05 and ACS ISR 2022-07. |
| 64 | Warm pixel, i.e., pixel with 0.06 e-/s ≤ dark current ≤ 0.14 e-/s, flagged in superdark DQ arrays. |
| 128 | Bias structure, e.g., bad columns. Since 08 Oct 2004, bias structure has been flagged in the superbias DQ arrays. Before 08 Oct 2004, bias structure was only flagged in an increasingly outdated BPIXTAB which was created in July 2002. |
| 256 | CCD full-well saturated pixel (usable at gain = 2). See ACS ISR 2020-02. |
| 512 | Bad pixel in reference file. Used in the flatfield DQ arrays to indicate a portion of the flat which is not defined or not calibrated with the same accuracy as the other regions, often around the detector edges. Used for F892N and WFC polarizer observations, where the filter only subtends a portion of the chip. Used to identify dust mote replacement patches. Used to identify a saturated pixel in superdark reference files. |
| 1024 | Sink pixel or pixel affected by sink pixel charge traps. See ACS ISR 2017-01. |
| 2048 | A-to-D saturated pixel (never usable, even at higher gain settings). See ACS ISR 2020-03. |
| 4096 | Cosmic ray and detector artifact rejected during the combination DITHERED exposure in AstroDrizzle. These flags are not present in the drizzled output images (*drz.fits). Rather, they are propagated "backwards" to the DQ arrays of the calibrated input images (*flt.fits/*flc.fits). |
| 8192 | Cosmic ray rejected during the combination of CR-SPLIT images in the pipeline (CALACS/ACSREJ) -- flagged in the DQ arrays of *crj.fits images. As with the 4096 flag, these values are propagated "backwards" to the DQ arrays of the calibrated input images (*flt.fits/*flc.fits). |
| 16384 | Reserved. Can be used for manual flagging by the user, e.g., for uncorrected detector artifacts, satellite trails, optical ghosts. |
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