This page details the meaning of the bitmasks provided with Data Release 9 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).
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 within half of the locus of a radius-magnitude relation. Set for Tycho sources with MAG_VT < 13 and Gaia stars with 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. Set for Gaia stars with G < 16. The MEDIUM radius is twice the BRIGHT radius at the same magnitude. |
| 12 | GALAXY | touches a pixel in an SGA large galaxy |
| 13 | CLUSTER | touches a pixel in a globular cluster |
FITBITS
A bit-mask detailing any peculiarities regarding how a source was fit. See also the legacypipe bitmask definitions and the issue that prompted the development of FITBITS.
| Bit | Name | Description |
|---|---|---|
| 0 | FORCED_POINTSOURCE | the source was forced to be type PSF |
| 1 | FIT_BACKGROUND | the source had its background levels independently fit |
| 2 | HIT_RADIUS_LIMIT | the source hit the radius limit during fitting (based on the limits for galaxy models in the Tractor code) |
| 3 | HIT_SERSIC_LIMIT | the source hit the Sersic index limit during fitting (e.g., see the Sersic model limits in the Tractor code) |
| 4 | FROZEN | the source was not fit (all parameters for the source were frozen at the reference catalog values) |
| 5 | BRIGHT | the source is a bright star (see also MASKBITS, above) |
| 6 | MEDIUM | the source is a medium-bright star (see also MASKBITS, above) |
| 7 | GAIA | the source is a Gaia source |
| 8 | TYCHO2 | the source is a Tycho-2 star |
| 9 | LARGEGALAXY | the source is an SGA large galaxy |
| 10 | WALKER | fitting the source shifted its position by > 1 arcsec |
| 11 | RUNNER | fitting the source shifted its position by > 2.5 arcsec |
| 12 | GAIA_POINTSOURCE | the source was a Gaia source that was treated as a point source |
| 13 | ITERATIVE | the source was found during iterative detection |
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 (NOIRLab Community Pipeline) Data Quality bits, which we map to the values in the table.
| Bit | Name | Description |
|---|---|---|
| 0 | BADPIX | bad columns, hot pixels, etc. |
| 1 | SATUR | saturated |
| 2 | INTERP | interpolated |
| 4 | CR | single exposure cosmic ray |
| 6 | BLEED | bleed trail |
| 7 | TRANS | multi-exposure transient |
| 8 | EDGE | edge pixel |
| 9 | EDGE2 | edge pixel |
| 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.
CCD_CUTS
The survey CCDs and forced photometry files include a bitmask ccd_cuts that is used by the legacypipe code to discard CCDs that have low-quality observations before processing imaging for the Legacy Surveys. Any observations with non-zero vales of ccd_cuts are discarded prior to processing.
In this table, the column names are as they appear in, e.g., the survey CCDs file. The overall zero-points (zpt0) are:
- \(g, r\) = [25.74, 25.52] for 90Prime (BASS)
- \(z\) = 26.20 for Mosaic-3 (MzLS)
- \(g, r, z\) = [25.08, 25.29, 24.92] for DECam (DECaLS)
| Bit | Name | Description |
|---|---|---|
| 0 | ERR_LEGACYZPTS | Some error from the legacyzpts code (e.g., the CP had a WCS error, usually due to being very shallow) |
| 1 | NOT_GRZ | Not a \(g\)-, \(r\)- or \(z\)-band observation |
| 2 | NOT_THIRD_PIX | Mosaic-3 one-third-pixel interpolation problem |
| 3 | EXPTIME_LT_30 | Exposure time < 30 seconds |
| 4 | CCDNMATCH_LT_20 | Number of matching CCDs < 20 |
| 5 | ZPT_DIFF_AVG | Zeropoint for CCD is different than the average for the exposure: abs(ccdzpt - zpt) > 0.25 for DECam, 0.1 for 90Prime, 0.1 for Mosaic-3 |
| 6 | ZPT_SMALL | Zeropoint is too low: zpt < (zpt0 - P) where P is [0.5, 0.5, 1.0] mag for [DECam, 90Prime, Mosaic-3] |
| 7 | ZPT_LARGE | Zeropoint is too high: zpt > (zpt0 + Q) where Q is [0.25, 0.18, 0.8] mag for [DECam, 90Prime, Mosaic-3] |
| 8 | SKY_IS_BRIGHT | Sky is too high; ccdskycounts is more than [90, 150, 180] for DECam [\(g, r, z\)]; [200] for Mosaic-3 [\(z\)] (no cuts are made for 90Prime) |
| 9 | BADEXP_FILE | Exposure is listed in the bad_expid.txt file (90prime-bad_expid.txt, mosaic-bad_expid.txt or decam-bad_expid.txt) |
| 10 | PHRMS | Photometric calibration RMS is too large: ccdphrms > 0.2 |
| 11 | RADECRMS | Astrometric calibration RMS is too large: rms (\(\sqrt(\mathtt{ccdrarms^2 + ccddecrms^2})\) > 0.4 arcsec for DECam; 0.2 arcsec for 90Prime; 0.1 arcsec for Mosaic-3 |
| 12 | SEEING_BAD | Bad seeing measure: (seeing < 0 arcsec OR seeing > 3.0 arcsec), where, e.g., seeing = fwhm \(\times\) 0.262 for DECam |
| 13 | EARLY_DECAM | Defined as MJD < 56516 (note that DR8 started with MJD < 56730) |
| 14 | DEPTH_CUT | Depth cut code (this is run after the other cuts) |
| 15 | TOO_MANY_BAD_CCDS | Remove full exposures if more than 70% of the CCDs are cut |
| 16 | FLAGGED_IN_DES | Flagged as bad in DES |
Some of the exact values used to create the CCD_CUTS bitmask are recorded in the legacyzpts code for 90Prime, Mosaic-3 and DECam.