DR9 bitmasks

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 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

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 (NSF's OIR Lab 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.