Interpretation of the bitmask column

The ccds- and survey- files described on the DR4 files page contain a column bitmask, which is defined as:

Bit	Meaning
0001	this exposure is flagged in the `bad_expid` file
0010	a zeropoint solution was not computed for these exposures, and so the ccdname disagrees with the hdu number
0100	these ccds had outlying values for astrometric or photometric offsets ( $\sqrt($ `ccdrarms` $^2 +$ `ccddecrms` $^2) > 0.1$ OR `ccdphrms` $> 0.2$ )

The CCDs used by legacypipe can be determined via:

photometric == True AND bitmask == 0

Note that photometric, ccdrarms, ccddecrms and ccdphrms are all columns in the ccds- and survey- files described on the DR4 files page.