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Data quality flags

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 propogate from the data quality arrays of the various reference images used to calibrate and combine the data in the pipeline (CALACS and MultiDrizzle).

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 (dark current greater than 0.08 e/sec); flagged in superdark DQ arrays.
32 Since 8 Oct 2004, used to flag the CTE tails of hot pixels in superdark DQ arrays. For now, we only flag the first pixel trailing each hot pixel, but this flagging may become more sophisticated and complete as CTE worsens. Note that the full CTE tails are flagged as warm pixels (see flag 64). Before 8 Oct 2004, flag 32 was used to flag (in the BPIXTAB) "blobs" of bleeding around saturated pixels (at the end of bad columns). These blobs are also flagged as hot or saturated pixels, so this was redundant.
64 Since 8 Oct 2004, used to flag warm pixels (between 0.02 and 0.08 e/sec) in superdark DQ arrays. Before 8 Oct 2004, used to flag "permanent" hot pixels which had persisted through the first 4 CCD anneal cycles after launch in 2002. These flags were in the BPIXTAB, which was becoming outdated, and redundant with flag 16.
128 Bias structure (mostly bad columns). Since 8 Oct 2004, bias structure has been flagged in the bi-weekly superbias DQ arrays. Before 8 Oct 2004, bias structure was only flagged in an increasingly outdated BPIXTAB which was created in July 2002.
256 Both full-well (useable at higher gain setting) and A-to-D (never useable) saturated pixels are flagged by ATODCORR, based on the CCDTAB. But A-to-D saturation is also flagged 2048, so it can be distinguished from full-well saturation. Also, since 8 Oct 2004, full-well saturated pixels in superdarks are flagged in their corresponding DQ arrays (note they are also flagged as hot pixels with flag 16). Before 8 Oct 2004, saturated pixels in the superdarks were flagged only in a BPIXTAB created in July 2002 (which was getting outdated).
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.
1024 Charge traps; flagged in the BPIXTAB.
2048 A-to-D saturated pixels (never useable, even at higher gain settings) are flagged by ATODCORR, based on the CCDTAB.
4096 Cosmic rays and detector artifacts rejected during MultiDrizzle image combination. These flags are not present in the drizzled output images (*drz.fits). Rather, they are propogated "backwards" to the DQ arrays of the calibrated input images (*flt.fits). Only associated data from pointing pattern "forms" and CR-SPLITs are automatically combined by MultiDrizzle in the pipeline -- POS TARG patterns are not.
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.
16384 reserved; can be used for manual flagging by the user, e.g. for uncorrected detector artifacts, satellite trails, optical ghosts, etc.
superdark superdark flags

Left: A superdark with warm and hot pixels (and their CTE tails), and saturated columns. Right: The corresponding flagging of dark features in the data quality [DQ] array of the superdark. Click to enlarge images.