DR8 bitmasks

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).

BRIGHTBLOB

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.

Bit Name Description
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 LSLGA large galaxy

MASKBITS

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.

Bit Name Description
0 NPRIMARY touches a pixel that is outside the BRICK_PRIMARY region of a brick
1 BRIGHT touches a pixel in a blob containing a bright (Tycho-2) star
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 in a medium-bright (Gaia DR2) star
12 GALAXY touches a pixel in an LSLGA large galaxy
13 CLUSTER touches a pixel in a globular cluster

ALLMASK_X/ANYMASK_X

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 (NOAO Community Pipeline) Data Quality bits.

Bit Name Description
0 BADPIX bad columns, hot pixels, etc. (see the CP Data Quality bit description)
1 SATUR saturated (see the CP Data Quality bit description)
2 INTERP interpolated (see the CP Data Quality bit description)
4 CR single exposure cosmic ray (see the CP Data Quality bit description)
6 BLEED bleed trail (see the CP Data Quality bit description)
7 TRANS multi-exposure transient (see the CP Data Quality bit description)
8 EDGE edge pixel (see the CP Data Quality bit description)
9 EDGE2 edge pixel (see the CP Data Quality bit description)
11 OUTLIER marked as touching an outlier pixel by legacypipe itself

WISEMASK_W1/WISEMASK_W2

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.

Bit Name Description
0 BRIGHT Bright star core and wings
1 SPIKE PSF-based diffraction spike
2 GHOST Optical ghost
3 LATENT First latent
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.