Skip to main content

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).

Flag Definition and Source
0 Good pixels
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 beyond aperture. 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 pixels, i.e. pixels where dark current > 0.14 e-/sec)
 
These are 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 pixels, i.e. pixels where 0.06 e-/s ≤ dark current  ≤ 0.14 e-/s
 
These are flagged in superdark DQ arrays.
128 Bias structure (mostly bad or unstable columns). Since 8 Oct 2004, bias structure has been flagged in the superbias DQ arrays. Before 8 Oct 2004, bias structure was only flagged in an increasingly outdated BPIXTAB which was created in July 2002.
256 Full-well saturated pixels (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 dark reference files.
1024 Sink pixels or pixels affected by sink pixel charge traps. See ACS ISR 2017-01.
2048 A-to-D saturated pixels (never usable, even at higher gain settings). See ACS ISR 2020-03.
4096 Cosmic rays and detector artifacts 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).
8192 Cosmic rays 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)
16384 reserved; can be used for manual flagging by the user, e.g. for uncorrected detector artifacts, satellite trails, optical ghosts, etc.
Last Updated: 12/12/2023

Pre-footer

HST Help Desk 

Please contact the HST Help Desk with any questions.