Bit Masks in DESI

This page describes the bitmasks found in DESI files. For details on working with these values, please see the tutorial on that topic.

Redshift Fitting (Redrock) Masks

ZWARN

The ZWARN bitmask in redshift catalogs indicates known problems with a particular redshift fit or associated QA. ZWARN==0 is good; any non-zero value indicates a potential problem. This mask will be described in more detail in the Redrock paper (Bailey et al. 2023 in prep), as well as Section 5.3.1 of the Survey Operations Paper (Schlafly et al. 2023).

The canonical code location defining these bits is desitarget targetmask.yaml. Bits 0-15 are set by Redrock itself (the redshift fitter), while bits 16-19 are set by DESI-specific post-processing.

ZWARN Mask Locations

File	Table HDU	Column
redrock	REDSHIFTS	ZWARN
emline	EMLINEFIT	ZWARN
zmtl	ZMTL	ZWARN
mtl	MTL	ZWARN
zpix	ZCATALOG	ZWARN
ztile	ZCATALOG	ZWARN
zall	ZCATALOG	ZWARN
lss	LSS	ZWARN

ZWARN Bit Definitions

Bit Name	Bit Number	Description
SKY	0	sky fiber
LITTLE_COVERAGE	1	too little wavelength coverage
SMALL_DELTA_CHI2	2	chi-squared of best fit is too close to that of second best
NEGATIVE_MODEL	3	synthetic spectrum is negative
MANY_OUTLIERS	4	fraction of points more than 5 sigma away from best model is too large (>0.05)
Z_FITLIMIT	5	chi-squared minimum at edge of the redshift fitting range
NEGATIVE_EMISSION	6	a QSO line exhibits negative emission, triggered only in QSO spectra, if C_IV, C_III, Mg_II, H_beta, or H_alpha has LINEAREA + 3 * LINEAREA_ERR < 0
UNPLUGGED	7	the fiber was unplugged/broken, so no spectrum obtained
BAD_TARGET	8	catastrophically bad targeting data
NODATA	9	No data for this fiber, e.g. because spectrograph was broken during this exposure (ivar=0 for all pixels)
BAD_MINFIT	10	Bad parabola fit to the chi2 minimum
POORDATA	11	Poor input data quality but try fitting anyway
LOW_DEL_CHI2	16	DELTACHI2 is lower than 25 for a DESI SV3 target
LOW_DEL_CHI2_BGS	17	DELTACHI2 is lower than 40 for a DESI SV3 BGS target in bright time
BAD_SPECQA	18	QA rejected due to spectrum-level problems
BAD_PETALQA	19	QA rejected due to petal-level problems

Spectroscopic Reduction Masks

The FIBERSTATUS bit mask records the state of individual fibers for issues that impact the entire spectrum, e.g. a broken fiber. The SPECMASK bit mask tracks wavelength dependent isses per spectrum, e.g. masks for cosmic rays.

FIBERSTATUS

The FIBERSTATUS mask is kept as a column in FIBERMAP and related HDUs. Bits 0-7 are set by fiber assignment from focal plane information known before observations; bits 8-24 are set by the spectroscopic pipeline; bits 25-30 are set by the final QA step to set bits for all fibers in a petal (e.g. because sky model noise makes all spectra questionable).

The canonical code location defining FIBERSTATUS bits is desispec.maskbits L55.

FIBERSTATUS Mask Locations

File	Table HDU	Column
frame	FIBERMAP	FIBERSTATUS
sframe	FIBERMAP	FIBERSTATUS
cframe	FIBERMAP	FIBERSTATUS
spectra	FIBERMAP	FIBERSTATUS
coadd	EXP_FIBERMAP	FIBERSTATUS
coadd	FIBERMAP	COADD_FIBERSTATUS
exposure-qa	FIBERQA	QAFIBERSTATUS
tile-qa	FIBERQA	QAFIBERSTATUS

FIBERSTATUS Bit Definitions

Bit 3 (RESTRICTED) is informative and doesn’t necessarily mean that the spectrum is bad, i.e. a FIBERSTATUS value of 0 or 8=2**3 is good.

Bit Name	Bit Number	Description
UNASSIGNED	0	Fiber is not assigned to a known target or sky location
STUCKPOSITIONER	1	INFO: Stuck positioner (but could still be on a good sky location)
BROKENFIBER	2	Broken fiber
RESTRICTED	3	INFO: Positioner has restricted reach (but might still be on valid target)
MISSINGPOSITION	8	Fiber location information is missing
BADPOSITION	9	Fiber >100 microns from target location
POORPOSITION	10	Fiber >30 microns from target location
LOWTRANSMISSION	12	Low fiber transmission. Cannot use for sky.
LOWEFFTIME	15	Effective time for this fiber is too low
BADFIBER	16	Unusable fiber
BADTRACE	17	Bad trace solution
BADFLAT	18	Bad fiber flat
BADARC	19	Bad arc solution
MANYBADCOL	20	>10% of pixels are bad columns
MANYREJECTED	21	>10% of pixels rejected in extraction
BADAMPB	22	Issues in the amplifier readouts of camera B make this unusable
BADAMPR	23	Issues in the amplifier readouts of camera R make this unusable
BADAMPZ	24	Issues in the amplifier readouts of camera Z make this unusable
BADPETALPOS	25	Too many fibers with bad positioning in petal
BADPETALSKY	26	Bad sky model across petal
BADPETALSTDSTAR	27	To few standard stars or rms between stars too large in the petal
BADPETALFLUXCAL	28	Unphysical flux calibration for the petal (calib vector too high or too low)
BADPETALSNR	29	TSNR is too low for this petal compared to the others
BADREADNOISE	30	Bad read noise in one of the 3 cameras
RESERVED31	31	Reserved sign bit; do not use

SPECMASK

The SPECMASK is stored as an image HDU in files with spectra, matched to the FLUX HDU, i.e. specmask[i,j] is the mask for fiber i wavelength j with flux value flux[i,j]. All bits in SPECMASK are bad, i.e. non-zero values mean that the corresponding flux should not be used.

The canonical code location defining SPECMASK bits is desispec.maskbits L84.

SPECMASK Mask Locations

Note: the FITS file HDU EXTNAME=MASK or MASK_B/R/Z, not “SPECMASK”.

File	Image HDU
frame	MASK
sframe	MASK
cframe	MASK
spectra	MASK_B/R/Z
coadd	MASK_B/R/Z

SPECMASK Bit Definitions

Bit Name	Bit Number	Description
SOMEBADPIX	0	Some input pixels were masked or ivar=0
ALLBADPIX	1	All input pixels were masked or ivar=0
COSMIC	2	Input pixels included a masked cosmic
LOWFLAT	3	Fiber flat < 0.5
BADFIBERFLAT	4	Bad fiber flat solution
BRIGHTSKY	5	Bright sky level (details TBD)
BADSKY	6	Bad sky model
BAD2DFIT	7	Bad fit of extraction 2D model to pixel data
NODATA	8	No data exists
BADFIBER	9	fibermask has a non-zero bit
BADCOLUMN	10	Bad CCD column biases the flux

CCDMASK

The CCDMASK is used for masking spectrograph CCD images during preprocessing, prior to extracting the spectra. It is stored in the MASK HDU of preproc files.

The canonical code location defining CCDMASK bits is desispec.maskbits L42.

CCDMASK Bit Definitions

Bit Name	Bit Number	Description
BAD	0	Pre-determined bad pixel (any reason)
HOT	1	Hot pixel
DEAD	2	Dead pixel
SATURATED	3	Saturated pixel from object
COSMIC	4	Cosmic ray
PIXFLATZERO	5	pixflat is 0
PIXFLATLOW	6	pixflat < 0.1
HIGHVAR	7	High variability in pixel value
BADREADNOISE	8	Very high CCD amplifier read noise

Target masks

Target masks record the reasons why each target was selected for DESI observations. These are stored in the *_TARGET columns of the TARGETS, FIBERASSIGN, and FIBERMAP tables in data files.

These masks are described in more detail in Section 2 of Myers et al. (2023) and Appendices A and B of the the DESI EDR Overview paper (DESI Collaboration et al. 2023).

The following table lists a subset of the most commonly used bits that maintained the same definition throughout different phases of DESI observations. For the full definition of all bits, see the EDR Overview paper appendices and the code links in the second table below.

Bit Name	Bit Number	Description
LRG	0	Luminous Red Galaxies
ELG	1	Emission Line Galaxies
QSO	2	Quasars
SKY	32	Blank sky locations
(various STD_*)	33-35	Standard stars
BGS_ANY	60	Bright Galaxy Survey galaxies
MWS_ANY	61	Milky Way Survey stars
SCND_ANY	62	Secondary targets

Canonical code locations where targeting bits are defined:

BIT_MASK	URL
CMX_TARGET	CMX
SV1_DESI_TARGET	SV1
SV1_BGS_TARGET	SV1
SV1_MWS_TARGET	SV1
SV2_DESI_TARGET	SV2
SV2_BGS_TARGET	SV2
SV2_MWS_TARGET	SV2
SV2_SCND_TARGET	SV2
SV3_DESI_TARGET	SV3
SV3_BGS_TARGET	SV3
SV3_MWS_TARGET	SV3
SV3_SCND_TARGET	SV3
DESI_TARGET	TARGET
BGS_TARGET	TARGET
MWS_TARGET	TARGET
SCND_TARGET	TARGET
OBSCONDITIONS	TARGET_L188

Imaging masks

These masks were defined or used by the DESI Legacy Imaging Surveys. Please see their information on these masks at the links below.

BIT_MASK	URL
WISEMASK_W1	BITMASKS_LEGACY
WISEMASK_W2	BITMASKS_LEGACY
MASKBITS	BITMASKS_LEGACY