This page details the meaning of the bitmasks provided with Data Release 8 of the Legacy Surveys. The bits are enumerated as a power (i.e. 7 written in a column of bits means two-to-the-power-of-7).
See also the legacypipe bitmask definitions. Note that the difference between bits 0, 1, 2, 3 for BRIGHTBLOB as compared to bits 1, 11, 12 and 13 for MASKBITS is that BRIGHTBLOB is set based on an initial position (the source detection integer pixel peak), while MASKBITS is set after fitting has finished (i.e., on the final brick pixel position). This can lead to slight differences (~0.15%) in the areas covered by the MASKBITS and BRIGHTBLOB masks.
|0||BRIGHT||source lies in the pixels that define a bright star (Tycho)|
|1||MEDIUM||source lies in the pixels that define medium bright star (GAIA)|
|2||CLUSTER||source lies in the pixels that define a globular cluster|
|3||GALAXY||source lies in the pixels that define an SGA large galaxy|
These definitions apply to both the values in the coadd/*/*/*maskbits* files and to MASKBITS columns in the Tractor and sweeps catalogs. See also the legacypipe bitmask definitions.
|0||NPRIMARY||touches a pixel that is outside the BRICK_PRIMARY region of a brick|
|1||BRIGHT||touches a pixel within the locus of a radius-magnitude relation for Tycho-2 stars or one for Gaia DR2 stars to G < 13|
|2||SATUR_G||touches a pixel that was saturated in at least one \(g\)-band image|
|3||SATUR_R||touches a pixel that was saturated in at least one \(r\)-band image|
|4||SATUR_Z||touches a pixel that was saturated in at least one \(z\)-band image|
|5||ALLMASK_G||touches a pixel that has any of the ALLMASK_G bits set|
|6||ALLMASK_R||touches a pixel that has any of the ALLMASK_R bits set|
|7||ALLMASK_Z||touches a pixel that has any of the ALLMASK_Z bits set|
|8||WISEM1||touches a pixel in a WISEMASK_W1 bright star mask|
|9||WISEM2||touches a pixel in a WISEMASK_W2 bright star mask|
|10||BAILOUT||touches a pixel in a blob where we "bailed out" of source fitting|
|11||MEDIUM||touches a pixel within the locus of a radius-magnitude relation for Gaia DR2 stars to G < 16|
|12||GALAXY||touches a pixel in an SGA large galaxy|
|13||CLUSTER||touches a pixel in a globular cluster|
ANYMASK_X denotes a source that touches a bad pixel in any of a set of overlapping \(X\)-band images whereas ALLMASK_X denotes a source that touches a bad pixel in all of a set of overlapping \(X\)-band images. See, also, the legacypipe bitmask definitions. The ANYMASK and ALLMASK bit masks are defined as follows, mostly from the CP (NOIRLab Community Pipeline) Data Quality bits, which we map to the values in the table.
|0||BADPIX||bad columns, hot pixels, etc.|
|4||CR||single exposure cosmic ray|
|11||OUTLIER||marked as touching an outlier pixel by legacypipe itself|
WISEMASK_W1 and WISEMASK_W2 have identical bit definitions, the only difference is that WISEMASK_W1 applies to masking in the W1 band and WISEMASK_W2 to masking in the W2 band.
|0||BRIGHT||Bright star core and wings|
|1||SPIKE||PSF-based diffraction spike|
|4||LATENT2||Second latent image|
|5||HALO||AllWISE-like circular halo|
|6||SATUR||Bright star saturation|
|7||SPIKE2||Geometric diffraction spike|
Aaron Meisner's unWISE documentation details how these masks were derived.