Legacy Survey Files

Throughout this page <region> denotes either north for BASS/MzLS or south for DECaLS.

Directory Structures

For Web Access

Top level directory:
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/

Top level directory for DECaLS data:
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/south/

Top level directory for MzLS/BASS data:
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/north/
Top level directories for sweeps catalogs:
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/south/sweep/
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/north/sweep/

At NERSC (for collaborators)

Top level directory:
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/

Top level directory for DECaLS data:
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/south/

Top level directory for MzLS/BASS data:
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/north/
Top level directories for sweeps catalogs:
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/south/sweep/
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/north/sweep/

FITS binary table with the RA, Dec bounds of each geometrical "brick" on the sky. This includes all bricks on the sky, not just the ones in our footprint or with coverage in DR8. For that information, see the next file description.

Column	Type	Description
`BRICKNAME`	char[8]	Name of the brick.
`BRICKID`	int32	A unique integer with 1-to-1 mapping to `brickname`.
`BRICKQ`	int16	A "priority" factor used for processing.
`BRICKROW`	int32	Dec row number.
`BRICKCOL`	int32	Number of the brick within a Dec row.
`RA`	float64	RA of the center of the brick.
`DEC`	float64	Dec of the center of the brick.
`RA1`	float64	Lower RA boundary.
`RA2`	float64	Upper RA boundary.
`DEC1`	float64	Lower Dec boundary.
`DEC2`	float64	Upper Dec boundary.

<region>/survey-bricks-dr8-<region>.fits.gz

A FITS binary table with information that summarizes the contents of each brick for a region of DR8.

Column	Type	Description
`brickname`	char[8]	Name of the brick
`ra`	float64	RA of the center of the brick
`dec`	float64	Dec of the center of the brick
`nexp_g`	int16	Median number of exposures in the unique area (i.e. `BRICK_PRIMARY` area) of the brick in g-band
`nexp_r`	int16	Median number of exposures in the unique area of the brick in r-band
`nexp_z`	int16	Median number of exposures in the unique area of the brick in z-band
`nexphist_g`	int32[6]	Histogram of number of pixels in the unique brick area with 0, 1, 2, 3, 4, or > 5 exposures in g
`nexphist_r`	int32[6]	Histogram of number of pixels in the unique brick area with 0, 1, 2, 3, 4, or > 5 exposures in r
`nexphist_z`	int32[6]	Histogram of number of pixels in the unique brick area with 0, 1, 2, 3, 4, or > 5 exposures in z
`nobjs`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of all types
`npsf`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `PSF`
`nsimp`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `SIMP` (there should be 0 such objects)
`nrex`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `REX`
`nexp`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `EXP`
`ndev`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `DEV`
`ncomp`	int16	Total number of `BRICK_PRIMARY` objects in this brick, of type `COMP`
`psfsize_g`	float32	Median PSF size, in arcsec, evaluated at the `BRICK_PRIMARY` objects in this brick in g-band
`psfsize_r`	float32	Median PSF size, in arcsec, evaluated at the `BRICK_PRIMARY` objects in this brick in r-band
`psfsize_z`	float32	Median PSF size, in arcsec, evaluated at the `BRICK_PRIMARY` objects in this brick in z-band
`psfdepth_g`	float32	5-sigma PSF detection depth in \(g\)-band (AB mag), using PsfEx PSF model
`psfdepth_r`	float32	5-sigma PSF detection depth in \(r\)-band (AB mag), using PsfEx PSF model
`psfdepth_z`	float32	5-sigma PSF detection depth in \(z\)-band (AB mag), using PsfEx PSF model
`galdepth_g`	float32	5-sigma galaxy (0.45" round exp) detection depth in \(g\)-band (AB) mag
`galdepth_r`	float32	5-sigma galaxy (0.45" round exp) detection depth in \(r\)-band (AB) mag
`galdepth_z`	float32	5-sigma galaxy (0.45" round exp) detection depth in \(z\)-band (AB) mag
`ebv`	float32	Median SFD98 dust map E(B-V) extinction, in magnitudes, evaluated at `BRICK_PRIMARY` objects in this brick
`trans_g`	float32	Median Milky Way dust transparency in \(g\)-band, based on `ebv`. See also `MW_TRANSMISSION_G`
`trans_r`	float32	Median Milky Way dust transparency in \(g\)-band, based on `ebv`. See also `MW_TRANSMISSION_R`
`trans_z`	float32	Median Milky Way dust transparency in \(z\)-band, based on `ebv`. See also `MW_TRANSMISSION_Z`
`ext_g`	float32	Extinction in \(g\)-band
`ext_r`	float32	Extinction in \(r\)-band
`ext_z`	float32	Extinction in \(z\)-band
`wise_nobs`	int16[4]	Number of images that contributed to WISE calculations in each filter (not profile-weighted)
`trans_wise`	float32[4]	Median Milky Way dust transparency in WISE bands, based on `ebv`. See also, e.g., `MW_TRANSMISSION_W1`
`ext_w1`	float32	Extinction in \(W1\)-band
`ext_w2`	float32	Extinction in \(W2\)-band
`ext_w3`	float32	Extinction in \(W3\)-band
`ext_w4`	float32	Extinction in \(W4\)-band

Note that, for the nexphist rows, pixels that are masked by the NOIRLab Community Pipeline as, e.g., cosmic rays or saturation (see, e.g. the ALLMASK/ANYMASK information on the DR8 bitmasks page), do not count toward the number of exposures. More information about the morphological types and MW_TRANSMISSION can be found on the catalogs page.

survey-ccds-<camera>-dr8.fits.gz

A FITS binary table with almanac information about each individual CCD image for each camera (where <camera> is one of 90prime for BASS, decam for DECaLS or mosaic for MzLS).

This file contains information regarding the photometric and astrometric zero points for each CCD of every image that is part of the DR8 data release. Photometric zero points for each CCD are computed by identifying stars and comparing their instrumental magnitudes to color-selected stars in the PanSTARRS "qz" catalog.

The photometric zeropoints (zpt, ccdzpt, etc) are magnitude-like numbers (e.g. 25.04), and indicate the magnitude of a source that would contribute one count per second to the image. For example, in an image with zeropoint of 25.04 and exposure time of 30 seconds, a source of magnitude 22.5 would contribute \(30 \times 10^{((25.04 - 22.5) / 2.5)} = 311.3\) counts.

Column	Type	Description
`image_filename`	char[120]	Path to FITS image, e.g. "north/DECam_CP/CP20170729/c4d_170730_045351_ooi_g_v1.fits.fz"
`image_hdu`	int16	FITS HDU number in the `image_filename` file where this image can be found
`camera`	char[9]	The camera that took this image e.g. "90prime"
`expnum`	int64	Exposure number, eg 348224
`plver`	char[8]	Community Pipeline (CP) version number
`procdate`	char[19]	CP processing date
`plprocid`	char[7]	Unique, time-based, CP processing hash - see the plprocid page for how to convert this to a date
`ccdname`	char[5]	CCD name, e.g. "N10", "S7" for DECam
`object`	char[35]	Name listed in the object tag from the CCD header
`propid`	char[10]	Proposal ID of the program that took this image, eg "2014B-0404"
`filter`	char[1]	Filter used for observation, eg "\(g\)", "\(r\)", "\(z\)"
`exptime`	float32	Exposure time in seconds, eg 30
`mjd_obs`	float64	Date of observation in MJD (in UTC system), eg 56884.99373389
`airmass`	float32	Airmass of observation (measured at the telescope bore-sight)
`fwhm`	float32	FWHM (in pixels) measured by the CP
`width`	int16	Width in pixels of this image, eg 2046
`height`	int16	Height in pixels of this image, eg 4096
`ra_bore`	float64	Telescope boresight RA of this exposure (deg)
`dec_bore`	float64	Telescope boresight Dec of this exposure (deg)
`crpix1`	float32	Astrometric header value: X reference pixel
`crpix2`	float32	Astrometric header value: Y reference pixel
`crval1`	float64	Astrometric header value: RA of reference pixel
`crval2`	float64	Astrometric header value: Dec of reference pixel
`cd1_1`	float32	Astrometric header value: transformation matrix
`cd1_2`	float32	Astrometric header value: transformation matrix
`cd2_1`	float32	Astrometric header value: transformation matrix
`cd2_2`	float32	Astrometric header value: transformation matrix
`yshift`	boolean	(ignore; it's always `False`)
`ra`	float64	Approximate RA center of this CCD (deg)
`dec`	float64	Approximate Dec center of this CCD (deg)
`skyrms`	float32	Sky rms for the entire image (in counts/second)
`sig1`	float32	Median per-pixel error standard deviation, in nanomaggies
`ccdzpt`	float32	Zeropoint for the CCD (AB mag)
`zpt`	float32	Median zero point for the entire image (median of all CCDs of the image), eg 25.0927
`ccdraoff`	float32	Median astrometric offset for the CCD <GAIA-Legacy Survey> in arcsec
`ccddecoff`	float32	Median astrometric offset for the CCD <GAIA-Legacy Survey> in arcsec
`ccdskycounts`	float32	Mean sky counts level per second per pixel (AVSKY divided by EXPTIME) in the CP-processed frames measured (with iterative rejection) for each CCD in the image section [500:1500,1500:2500]
`ccdskysb`	float32	Sky surface brightness (in AB mag/arcsec2)
`ccdrarms`	float32	rms in astrometric offset for the CCD <Gaia-Legacy Survey> in arcsec
`ccddecrms`	float32	rms in astrometric offset for the CCD <Gaia-Legacy Survey> in arcsec
`ccdphrms`	float32	Photometric rms for the CCD (in mag)
`ccdnastrom`	int16	Number of stars (after sigma-clipping) used to compute astrometric correction
`ccdnphotom`	int16	Number of Gaia+PS1 stars detected with signal-to-noise ratio greater than five
`ccd_cuts`	int32	Bit mask describing CCD image quality (see, e.g., the DR9 bitmasks page)

survey-ccds-<camera>-dr8.kd.fits

As for the survey-ccds-<camera>-dr8.fits.gz files but limited by the depth of each observation. These files contain the CCDs actually used for the DR8 reductions. Columns are the same as for the survey-ccds-<camera>-dr8.fits.gz files.

ccds-annotated-<camera>-dr8.fits.gz

Versions of the survey-ccds-<camera>-dr8.fits.gz files with additional information gathered during calibration pre-processing before running the Tractor reductions.

Includes all of the columns in the survey-ccds-<camera>-dr8.fits.gz files plus the columns listed below. Note that string columns can have different lengths in the survey-ccds-<camera>-dr8.fits.gz and ccds-annotated-<camera>-dr8.fits.gz files. For example the camera column can change from char[9] to char[7] (see, e.g. legacypipe issue #379).

Column	Type	Description
`annotated`	boolean	`True` unless there is an error when computing the "annotated" quantities in this row of the file
`good_region`	int16[4]	If only a subset of the CCD images was used, this array of x0,x1,y0,y1 values gives the coordinates that were used, [x0,x1), [y0,y1). -1 for no cut (most CCDs)
`ra0`	float64	RA coordinate of pixel (1,1)...Note that the ordering of the CCD corners is detailed here
`dec0`	float64	Dec coordinate of pixel (1,1)
`ra1`	float64	RA coordinate of pixel (1,H)
`dec1`	float64	Dec coordinate of pixel (1,H)
`ra2`	float64	RA coordinate of pixel (W,H)
`dec2`	float64	Dec coordinate of pixel (W,H)
`ra3`	float64	RA coordinate of pixel (W,1)
`dec3`	float64	Dec coordinate of pixel (W,1)
`dra`	float32	Maximum distance from RA,Dec center to the edge midpoints, in RA
`ddec`	float32	Maximum distance from RA,Dec center to the edge midpoints, in Dec
`ra_center`	float64	RA coordinate of CCD center
`dec_center`	float64	Dec coordinate of CCD center
`meansky`	float32	Our pipeline (not the CP) estimate of the sky level, average over the image, in nanomaggies
`stdsky`	float32	Standard deviation of our sky level, in nanomaggies
`maxsky`	float32	Max of our sky level, in nanomaggies
`minsky`	float32	Min of our sky level, in nanomaggies
`pixscale_mean`	float32	Pixel scale (via sqrt of area of a 10x10 pixel patch evaluated in a 5x5 grid across the image), in arcsec/pixel.
`pixscale_std`	float32	Standard deviation of pixel scale
`pixscale_max`	float32	Max of pixel scale
`pixscale_min`	float32	Min of pixel scale
`psfnorm_mean`	float32	PSF norm = 1/sqrt of N_eff = sqrt(sum(psf_i^2)) for normalized PSF pixels i; mean of the PSF model evaluated on a 5x5 grid of points across the image. Point-source detection standard deviation is `sig1 / psfnorm`.
`psfnorm_std`	float32	Standard deviation of PSF norm
`galnorm_mean`	float32	Norm of the PSF model convolved by a 0.45" exponential galaxy.
`galnorm_std`	float32	Standard deviation of galaxy norm.
`psf_mx2`	float32	PSF model second moment in x (pixels^2)
`psf_my2`	float32	PSF model second moment in y (pixels^2)
`psf_mxy`	float32	PSF model second moment in x-y (pixels^2)
`psf_a`	float32	PSF model major axis (pixels)
`psf_b`	float32	PSF model minor axis (pixels)
`psf_theta`	float32	PSF position angle (deg)
`psf_ell`	float32	PSF ellipticity 1 - minor/major
`humidity`	float32	Percent humidity outside
`outtemp`	float32	Outside temperate (deg C).
`tileid`	int32	tile number, 0 for data from programs other than MzLS or DECaLS
`tilepass`	uint8	tile pass number, 1, 2 or 3, if this was an MzLS or DECaLS observation, or 0 for data from other programs. Set by the observers (the meaning of `tilepass` is on the status page)
`tileebv`	float32	Mean SFD98 E(B-V) extinction in the tile, 0 for data from programs other than BASS, MzLS or DECaLS
`ebv`	float32	SFD98 E(B-V) extinction for CCD center
`decam_extinction`	float32[6]	Extinction for optical filters \(ugrizY\)
`wise_extinction`	float32[4]	Extinction for WISE bands W1,W2,W3,W4
`psfdepth`	float32	5-sigma PSF detection depth in AB mag, using PsfEx PSF model
`galdepth`	float32	5-sigma galaxy (0.45" round exp) detection depth in AB mag
`gausspsfdepth`	float32	5-sigma PSF detection depth in AB mag, using Gaussian PSF approximation (using `seeing` value)
`gaussgaldepth`	float32	5-sigma galaxy detection depth in AB mag, using Gaussian PSF approximation

<region>/dr8-<region>-depth.fits.gz

A concatenation of the depth histograms for each brick, for each region, from the coadd/*/*/*-depth.fits tables. HDU1 contains histograms that describe the number of pixels in each brick with a 5-sigma AB depth in the given magnitude bin. HDU2 contains the bin edges of the histograms.

HDU1

Column	Type	Description
`counts_ptsrc_g`	int32[50]	Histogram of pixels for point source depth in \(g\) band
`counts_gal_g`	int32[50]	Histogram of pixels for canonical galaxy depth in \(g\) band
`counts_ptsrc_r`	int32[50]	Histogram of pixels for point source depth in \(r\) band
`counts_gal_r`	int32[50]	Histogram of pixels for canonical galaxy depth in \(r\) band
`counts_ptsrc_z`	int32[50]	Histogram of pixels for point source depth in \(z\) band
`counts_gal_z`	int32[50]	Histogram of pixels for canonical galaxy depth in \(z\) band
`brickname`	char[8]	Name of the brick

HDU2

Column	Type	Description
`depthlo`	float32	Lower bin edge for each histogram in HDU1 (5-sigma AB depth)
`depthhi`	float32	Upper bin edge for each histogram in HDU1 (5-sigma AB depth)

<region>/dr8-<region>-depth-summary.fits.gz

A summary of the depth histogram for a region of DR8. FITS table with the following columns:

Column	Type	Description
`depthlo`	float32	Lower limit of the depth bin
`depthhi`	float32	Upper limit of the depth bin
`counts_ptsrc_g`	int64	Number of pixels in histogram for point source depth in \(g\) band
`counts_gal_g`	int64	Number of pixels in histogram for canonical galaxy depth in \(g\) band
`counts_ptsrc_r`	int64	Number of pixels in histogram for point source depth in \(r\) band
`counts_gal_r`	int64	Number of pixels in histogram for canonical galaxy depth in \(r\) band
`counts_ptsrc_z`	int64	Number of pixels in histogram for point source depth in \(z\) band
`counts_gal_z`	int64	Number of pixels in histogram for canonical galaxy depth in \(z\) band

The depth histogram runs from magnitude of 20.1 to 24.9 in steps of 0.1 mag. The first and last bins are "catch-all" bins: 0 to 20.1 and 24.9 to 100, respectively. The histograms count the number of pixels in each brick's unique area with the given depth. These numbers can be turned into values in square degrees using the brick pixel area of 0.262 arcseconds square. These depth estimates take into account the small-scale masking (cosmic rays, edges, saturated pixels) and detailed PSF model.

Random Catalogs

randoms/randoms-inside-dr8-0.31.0-*.fits

Ten files of random points sampled across the CCDs that comprise the geometry of DR8. Random locations were generated across the footprint at a density of 5,000 per square degree and meta-information about the survey was extracted from pixels at each random location from files in the coadd directory (see below, e.g. coadd/*/*/*-depth-<filter>.fits.fz, coadd/*/*/*-galdepth-<filter>.fits.fz, coadd/*/*/*-nexp-<filter>.fits.fz, coadd/*/*/*-maskbits.fits.fz, coadd/*/*/*-invvar-<filter>.fits.fz). The order of the points within each file is also random (meaning that randomness is retained if just the first N rows of the file are read). Contains the following columns:

Column	Type	Description
`RA`	float64	Right ascension at equinox J2000
`DEC`	float64	Declination at equinox J2000
`BRICKNAME`	char[8]	Name of the brick
`NOBS_G`	int16	Number of images that contribute to the central pixel in the \(g\) filter for this location (not profile-weighted)
`NOBS_R`	int16	Number of images that contribute to the central pixel in the \(r\) filter for this location (not profile-weighted)
`NOBS_Z`	int16	Number of images that contribute to the central pixel in the \(z\) filter for this location (not profile-weighted)
`PSFDEPTH_G`	float32	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_G})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_G})) - 9]\) gives corresponding magnitude
`PSFDEPTH_R`	float32	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_R})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_R})) - 9]\) gives corresponding magnitude
`PSFDEPTH_Z`	float32	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_Z})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_Z})) - 9]\) gives corresponding magnitude
`GALDEPTH_G`	float32	As for `PSFDEPTH_G` but for a galaxy (0.45" exp, round) detection sensitivity
`GALDEPTH_R`	float32	As for `PSFDEPTH_R` but for a galaxy (0.45" exp, round) detection sensitivity
`GALDEPTH_Z`	float32	As for `PSFDEPTH_Z` but for a galaxy (0.45" exp, round) detection sensitivity
`PSFDEPTH_W1`	float32	As for `PSFDEPTH_G` (and also on the AB system) but for WISE W1
`PSFDEPTH_W2`	float32	As for `PSFDEPTH_G` (and also on the AB system) but for WISE W2
`PSFSIZE_G`	float32	Weighted average PSF FWHM in arcsec in the \(g\) band
`PSFSIZE_R`	float32	Weighted average PSF FWHM in arcsec in the \(r\) band
`PSFSIZE_Z`	float32	Weighted average PSF FWHM in arcsec in the \(z\) band
`APFLUX_G`	float32	Total flux in nanomaggies extracted in a 0.75 arcsec radius in the \(g\) band at this location
`APFLUX_R`	float32	Total flux in nanomaggies extracted in a 0.75 arcsec radius in the \(r\) band at this location
`APFLUX_Z`	float32	Total flux in nanomaggies extracted in a 0.75 arcsec radius in the \(z\) band at this location
`APFLUX_IVAR_G`	float32	Inverse variance of `APFLUX_G`
`APFLUX_IVAR_R`	float32	Inverse variance of `APFLUX_R`
`APFLUX_IVAR_Z`	float32	Inverse variance of `APFLUX_Z`
`MASKBITS`	int16	Bitwise mask for optical data in the `coadd///maskbits` maps (see the DR8 bitmasks page)
`WISEMASK_W1`	uint8	Bitwise mask for WISE W1 data in the `coadd///maskbits` maps (see the DR8 bitmasks page)
`WISEMASK_W2`	uint8	Bitwise mask for WISE W2 data in the `coadd///maskbits` maps (see the DR8 bitmasks page)
`EBV`	float32	Galactic extinction E(B-V) reddening from SFD98
`PHOTSYS`	char[1]	'N' for an MzLS/BASS location, 'S' for a DECaLS location
`HPXPIXEL`	int64	HEALPixel containing this location at NSIDE=64 in the NESTED scheme

The 0.31.0 in the file names refers to the version of the desitarget code used to generate the random catalogs. The code is available on GitHub (see also here). The northern and southern imaging footprints overlap, so, randoms are resolved at a Declination of 32.375° and by the Galactic plane, such that locations at Dec > 32.375° that are north of the Galactic Plane have PHOTSYS set to "N".

randoms/randoms-outside-dr8-0.31.0-*.fits

Ten files of random points in bricks that do not contain an observation in DR8 (that are "outside" of the DR8 footprint). The columns in this file are simplified compared to the other random catalogs as most of the entries in the additional columns would be zeros. As with the other random catalogs, points were generated at a density of 5,000 per square degree and the order of the points within the file is also randomized. Contains the following columns:

Column	Type	Description
`RA`	float64	Right ascension at equinox J2000
`DEC`	float64	Declination at equinox J2000
`BRICKNAME`	char[8]	Name of the brick
`NOBS_G`	int16	Always zero in this file.
`NOBS_R`	int16	Always zero in this file.
`NOBS_Z`	int16	Always zero in this file.
`EBV`	float32	Galactic extinction E(B-V) reddening from SFD98

randoms/randoms-allsky-dr8-0.31.0.fits

The (randomly shuffled) combination of each of the randoms-inside-dr8-0.31.0-X.fits and randoms-outside-dr8-0.31.0-X.fits files (where X = 1, 2, 3 etc.). This creates ten "all-sky" random catalogs (at a density of 5,000 locations per square degree) where each brick is either populated with observations from the Legacy Surveys, or zeros. Contains the same columns as the randoms-inside-dr8-0.31.0-\*.fits files.

randoms/survey-bricks-dr8-randoms-0.31.0.fits

A similar file to the survey-bricks.fits.gz file, but with extra columns to help interpret the random catalogs. Contains the same columns as the survey-bricks.fits.gz file, plus the additional columns:

Column	Type	Description
`PHOTSYS`	char[1]	`"N"`, `"S"` or `" "` for bricks resolved to be in the north, south, or outside of the footprint, respectively.
`AREA_PER_BRICK`	float64	The area of the brick in square degrees.

External Files (`<region>/external/*`)

The Legacy Survey photometric catalogs have been matched to a range of external spectroscopic files from the SDSS. These external spectroscopic files can be accessed on the NERSC computers (for collaborators) at:

/global/cfs/cdirs/sdss/data/sdss

The resulting catalogs, which are listed below, can be accessed through the web at:
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/north/external/
https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr8/south/external/

Or on the NERSC computers at:
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/north/external/
/global/cfs/cdirs/cosmo/data/legacysurvey/dr8/south/external/

Each row of each external-match file contains the full record of the nearest object in our Tractored survey imaging catalogs, matched at a radius of 1.5 arcsec. The structure of the imaging catalog files is documented on the catalogs page. If no match is found, then OBJID is set to -1.

In addition to the columns from the Tractor catalogs, we have added columns from the SDSS files that can be used to track objects uniquely. These are typically some combination of PLATE, FIBER, MJD (or SMJD) and, in some cases, RERUN.

survey-dr8-<region>-specObj-dr14.fits

HDU1 (the only HDU) contains Tractored survey photometry that is row-by-row-matched to the SDSS DR14 spectrosopic pipeline file such that the photometric parameters in row "N" of survey-dr8-specObj-dr14.fits matches the spectroscopic parameters in row "N" of specObj-dr14.fits. The spectroscopic file is documented in the SDSS DR14 data model for specObj-dr14.fits.

survey-dr8-<region>-dr12Q.fits

HDU1 (the only HDU) contains Tractored survey photometry that is row-by-row-matched to the SDSS DR12 visually inspected quasar catalog (Paris et al. 2017) such that the photometric parameters in row "N" of survey-dr8-dr12Q.fits matches the spectroscopic parameters in row "N" of DR12Q.fits. The spectroscopic file is documented in the SDSS DR12 data model for DR12Q.fits.

survey-dr8-<region>-dr14Q_v4_4.fits

HDU1 (the only HDU) contains Tractored survey photometry that is row-by-row-matched to the SDSS DR14 visually inspected quasar catalog (Paris et al. 2018) such that the photometric parameters in row "N" of survey-dr8-dr14Q_v4_4.fits matches the spectroscopic parameters in row "N" of DR14Q_v4_4.fits. The spectroscopic file is documented in the SDSS DR14 data model for DR14Q_v4_4.fits.

survey-dr8-<region>-superset-dr12Q.fits

HDU1 (the only HDU) contains Tractored survey photometry that is row-by-row-matched to the superset of all SDSS DR12 spectroscopically confirmed objects that were visually inspected as possible quasars (Paris et al. 2017) such that the photometric parameters in row "N" of survey-dr8-Superset_dr12Q.fits matches the spectroscopic parameters in row "N" of Superset_DR12Q.fits. The spectroscopic file is documented in the SDSS DR12 data model for Superset_DR12Q.fits.

survey-dr8-<region>-dr7Q.fits

HDU1 (the only HDU) contains Tractored survey photometry that is row-by-row-matched to the SDSS DR7 visually inspected quasar catalog (Schneider et al. 2010) such that the photometric parameters in row "N" of survey-dr8-dr7Q.fits matches the spectroscopic parameters in row "N" of DR7qso.fit. The spectroscopic file is documented on the DR7 quasar catalog description page.

Tractor Catalogs (`<region>/tractor/*`)

In the file listings outlined below:

brick names (<brick>) have the format <AAAa>c<BBB> where A, a and B are digits and c is either the letter m or p (e.g. 1126p222). The names are derived from the (RA, Dec) center of the brick. The first four digits are \(int(RA \times 10)\), followed by p to denote positive Dec or m to denote negative Dec ("plus"/"minus"), followed by three digits of \(int(Dec \times 10)\). For example the case 1126p222 corresponds to (RA, Dec) = (112.6°, +22.2°).
<brickmin> and <brickmax> denote the corners of a rectangle in (RA, Dec). Explicitly, <brickmin> has the format <AAA>c<BBB> where <AAA> denotes three digits of the minimum \(int(RA)\) in degrees, <BBB> denotes three digits of the minimum \(int(Dec)\) in degrees, and c uses the p/m ("plus"/"minus") format outlined in the previous bullet point. The convention is similar for <brickmax> and the maximum RA and Dec. For example 000m010-010m005 would correspond to a survey region limited by \(0^\circ \leq RA < 10^\circ\) and \(-10^\circ \leq Dec < -5^\circ\).
sub-directories are listed by the RA of the brick center, and sub-directory names (<AAA>) correspond to RA. For example 002 corresponds to brick centers between an RA of 2° and an RA of 3°.
<filter> denotes the \(g\), \(r\) or \(z\) band, using the corresponding letter.

Note that it is not possible to go from a brick name back to an exact (RA, Dec) center (the bricks are not on 0.1° grid lines). The exact brick center for a given brick name can be derived from columns in the survey-bricks.fits.gz file (i.e. brickname, ra, dec).

<AAA>/tractor-<brick>.fits

FITS binary table containing Tractor photometry, documented on the catalogs page.

Users interested in database access to the Tractor catalogs can contact the Astro Data Lab [1] at datalab@noirlab.edu.

Sweep Catalogs (`<region>/sweep/*`)

8.0/sweep-<brickmin>-<brickmax>.fits

The sweeps are light-weight FITS binary tables (containing a subset of the most commonly used Tractor measurements) of all the Tractor catalogs for which BRICK_PRIMARY==T in rectangles of RA, Dec.

Name	Type	Units	Description
`RELEASE`	int16		Unique integer denoting the camera and filter set used (RELEASE is documented here)
`BRICKID`	int32		A unique Brick ID (in the range [1, 662174])
`BRICKNAME`	char[8]		Name of brick, encoding the brick sky position, eg "1126p222" near RA=112.6, Dec=+22.2
`OBJID`	int32		Catalog object number within this brick; a unique identifier hash is `RELEASE,BRICKID,OBJID`; `OBJID` spans [0,N-1] and is contiguously enumerated within each blob
`TYPE`	char[4]		Morphological model: "PSF"=stellar, "REX"="round exponential galaxy" = round EXP galaxy with a variable radius, "EXP"=exponential, "DEV"=deVauc, "COMP"=composite, "DUP"==Gaia source fit by different model. Note that in some FITS readers, a trailing space may be appended for "PSF ", "EXP " and "DEV " since the column data type is a 4-character string
`RA`	float64	deg	Right ascension at equinox J2000
`DEC`	float64	deg	Declination at equinox J2000
`RA_IVAR`	float32	1/deg²	Inverse variance of `RA` (no cosine term!), excluding astrometric calibration errors
`DEC_IVAR`	float32	1/deg²	Inverse variance of `DEC`, excluding astrometric calibration errors
`DCHISQ`	float32[5]		Difference in χ² between successively more-complex model fits: PSF, REX, DEV, EXP, COMP. The difference is versus no source.
`EBV`	float32	mag	Galactic extinction E(B-V) reddening from SFD98, used to compute `MW_TRANSMISSION`
`FLUX_G`	float32	nanomaggies	model flux in \(g\)
`FLUX_R`	float32	nanomaggies	model flux in \(r\)
`FLUX_Z`	float32	nanomaggies	model flux in \(z\)
`FLUX_W1`	float32	nanomaggies	WISE model flux in \(W1\) (AB system)
`FLUX_W2`	float32	nanomaggies	WISE model flux in \(W2\) (AB)
`FLUX_W3`	float32	nanomaggies	WISE model flux in \(W3\) (AB)
`FLUX_W4`	float32	nanomaggies	WISE model flux in \(W4\) (AB)
`FLUX_IVAR_G`	float32	1/nanomaggies²	Inverse variance of `FLUX_G`
`FLUX_IVAR_R`	float32	1/nanomaggies²	Inverse variance of `FLUX_R`
`FLUX_IVAR_Z`	float32	1/nanomaggies²	Inverse variance of `FLUX_Z`
`FLUX_IVAR_W1`	float32	1/nanomaggies²	Inverse variance of `FLUX_W1` (AB system)
`FLUX_IVAR_W2`	float32	1/nanomaggies²	Inverse variance of `FLUX_W2` (AB)
`FLUX_IVAR_W3`	float32	1/nanomaggies²	Inverse variance of `FLUX_W3` (AB)
`FLUX_IVAR_W4`	float32	1/nanomaggies²	Inverse variance of `FLUX_W4` (AB)
`MW_TRANSMISSION_G`	float32		Galactic transmission in \(g\) filter in linear units [0,1]
`MW_TRANSMISSION_R`	float32		Galactic transmission in \(r\) filter in linear units [0,1]
`MW_TRANSMISSION_Z`	float32		Galactic transmission in \(z\) filter in linear units [0,1]
`MW_TRANSMISSION_W1`	float32		Galactic transmission in \(W1\) filter in linear units [0,1]
`MW_TRANSMISSION_W2`	float32		Galactic transmission in \(W2\) filter in linear units [0,1]
`MW_TRANSMISSION_W3`	float32		Galactic transmission in \(W3\) filter in linear units [0,1]
`MW_TRANSMISSION_W4`	float32		Galactic transmission in \(W4\) filter in linear units [0,1]
`NOBS_G`	int16		Number of images that contribute to the central pixel in \(g\): filter for this object (not profile-weighted)
`NOBS_R`	int16		Number of images that contribute to the central pixel in \(r\): filter for this object (not profile-weighted)
`NOBS_Z`	int16		Number of images that contribute to the central pixel in \(z\): filter for this object (not profile-weighted)
`NOBS_W1`	int16		Number of images that contribute to the central pixel in \(W1\): filter for this object (not profile-weighted)
`NOBS_W2`	int16		Number of images that contribute to the central pixel in \(W2\): filter for this object (not profile-weighted)
`NOBS_W3`	int16		Number of images that contribute to the central pixel in \(W3\): filter for this object (not profile-weighted)
`NOBS_W4`	int16		Number of images that contribute to the central pixel in \(W4\): filter for this object (not profile-weighted)
`RCHISQ_G`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(g\)
`RCHISQ_R`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(r\)
`RCHISQ_Z`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(z\)
`RCHISQ_W1`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(W1\)
`RCHISQ_W2`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(W2\)
`RCHISQ_W3`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(W3\)
`RCHISQ_W4`	float32		Profile-weighted χ² of model fit normalized by the number of pixels in \(W4\)
`FRACFLUX_G`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(g\) (typically [0,1])
`FRACFLUX_R`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(r\) (typically [0,1])
`FRACFLUX_Z`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(z\) (typically [0,1])
`FRACFLUX_W1`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(W1\) (typically [0,1])
`FRACFLUX_W2`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(W2\) (typically [0,1])
`FRACFLUX_W3`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(W3\) (typically [0,1])
`FRACFLUX_W4`	float32		Profile-weighted fraction of the flux from other sources divided by the total flux in \(W4\) (typically [0,1])
`FRACMASKED_G`	float32		Profile-weighted fraction of pixels masked from all observations of this object in \(g\), strictly between [0,1]
`FRACMASKED_R`	float32		Profile-weighted fraction of pixels masked from all observations of this object in \(r\), strictly between [0,1]
`FRACMASKED_Z`	float32		Profile-weighted fraction of pixels masked from all observations of this object in \(z\), strictly between [0,1]
`FRACIN_G`	float32		Fraction of a source's flux within the blob in \(g\), near unity for real sources
`FRACIN_R`	float32		Fraction of a source's flux within the blob in \(r\), near unity for real sources
`FRACIN_Z`	float32		Fraction of a source's flux within the blob in \(z\), near unity for real sources
`ANYMASK_G`	int16		Bitwise mask set if the central pixel from any image satisfies each condition in \(g\) (see the DR8 bitmasks page)
`ANYMASK_R`	int16		Bitwise mask set if the central pixel from any image satisfies each condition in \(r\) (see the DR8 bitmasks page)
`ANYMASK_Z`	int16		Bitwise mask set if the central pixel from any image satisfies each condition in \(z\) (see the DR8 bitmasks page)
`ALLMASK_G`	int16		Bitwise mask set if the central pixel from all images satisfy each condition in \(g\) (see the DR8 bitmasks page)
`ALLMASK_R`	int16		Bitwise mask set if the central pixel from all images satisfy each condition in \(r\) (see the DR8 bitmasks page)
`ALLMASK_Z`	int16		Bitwise mask set if the central pixel from all images satisfy each condition in \(z\) (see the DR8 bitmasks page)
`WISEMASK_W1`	uint8		W1 bitmask as cataloged on the DR8 bitmasks page
`WISEMASK_W2`	uint8		W2 bitmask as cataloged on the DR8 bitmasks page
`PSFSIZE_G`	float32	arcsec	Weighted average PSF FWHM in the \(g\) band
`PSFSIZE_R`	float32	arcsec	Weighted average PSF FWHM in the \(r\) band
`PSFSIZE_Z`	float32	arcsec	Weighted average PSF FWHM in the \(z\) band
`PSFDEPTH_G`	float32	1/nanomaggies²	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_G})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_G})) - 9]\) gives corresponding magnitude
`PSFDEPTH_R`	float32	1/nanomaggies²	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_R})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_R})) - 9]\) gives corresponding magnitude
`PSFDEPTH_Z`	float32	1/nanomaggies²	For a \(5\sigma\) point source detection limit in \(g\), \(5/\sqrt(\mathrm{PSFDEPTH\_Z})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{PSFDEPTH\_Z})) - 9]\) gives corresponding magnitude
`GALDEPTH_G`	float32	1/nanomaggies²	As for `PSFDEPTH_G` but for a galaxy (0.45" exp, round) detection sensitivity
`GALDEPTH_R`	float32	1/nanomaggies²	As for `PSFDEPTH_R` but for a galaxy (0.45" exp, round) detection sensitivity
`GALDEPTH_Z`	float32	1/nanomaggies²	As for `PSFDEPTH_Z` but for a galaxy (0.45" exp, round) detection sensitivity
`PSFDEPTH_W1`	float32	1/nanomaggies²	As for `PSFDEPTH_G` (and also on the AB system) but for WISE W1
`PSFDEPTH_W2`	float32	1/nanomaggies²	As for `PSFDEPTH_G` (and also on the AB system) but for WISE W2
`WISE_COADD_ID`	char[8]		unWISE coadd file name for the center of each object
`FRACDEV`	float32		Fraction of model in deVauc [0,1]
`FRACDEV_IVAR`	float32		Inverse variance of `FRACDEV`
`SHAPEDEV_R`	float32	arcsec	Half-light radius of deVaucouleurs model (>0)
`SHAPEDEV_R_IVAR`	float32	1/arcsec	Inverse variance of `SHAPEDEV_R`
`SHAPEDEV_E1`	float32		Ellipticity component 1
`SHAPEDEV_E1_IVAR`	float32		Inverse variance of `SHAPEDEV_E1`
`SHAPEDEV_E2`	float32		Ellipticity component 2
`SHAPEDEV_E2_IVAR`	float32		Inverse variance of `SHAPEDEV_E2`
`SHAPEEXP_R`	float32	arcsec	Half-light radius of exponential model (>0)
`SHAPEEXP_R_IVAR`	float32	1/arcsec2	Inverse variance of `SHAPEEXP_R`
`SHAPEEXP_E1`	float32		Ellipticity component 1
`SHAPEEXP_E1_IVAR`	float32		Inverse variance of `SHAPEEXP_E1`
`SHAPEEXP_E2`	float32		Ellipticity component 2
`SHAPEEXP_E2_IVAR`	float32		Inverse variance of `SHAPEEXP_E2`
`FIBERFLUX_G`	float32	nanomaggies	Predicted \(g\)-band flux within a fiber of diameter 1.5 arcsec from this object in 1 arcsec Gaussian seeing
`FIBERFLUX_R`	float32	nanomaggies	Predicted \(r\)-band flux within a fiber of diameter 1.5 arcsec from this object in 1 arcsec Gaussian seeing
`FIBERFLUX_Z`	float32	nanomaggies	Predicted \(z\)-band flux within a fiber of diameter 1.5 arcsec from this object in 1 arcsec Gaussian seeing
`FIBERTOTFLUX_G`	float32	nanomaggies	Predicted \(g\)-band flux within a fiber of diameter 1.5 arcsec from all sources at this location in 1 arcsec Gaussian seeing
`FIBERTOTFLUX_R`	float32	nanomaggies	Predicted \(r\)-band flux within a fiber of diameter 1.5 arcsec from all sources at this location in 1 arcsec Gaussian seeing
`FIBERTOTFLUX_Z`	float32	nanomaggies	Predicted \(z\)-band flux within a fiber of diameter 1.5 arcsec from all sources at this location in 1 arcsec Gaussian seeing
`REF_CAT`	char[2]		Reference catalog source for this star: "T2" for Tycho-2, "G2" for Gaia DR2, "L2" for the SGA, empty otherwise
`REF_ID`	int64		Reference catalog identifier for this star; Tyc11,000,000+Tyc210+Tyc3 for Tycho2; "sourceid" for Gaia-DR2 and SGA
`REF_EPOCH`	float32	yr	Reference catalog reference epoch (eg, 2015.5 for Gaia DR2)
`GAIA_PHOT_G_MEAN_MAG`	float32	mag	Gaia G band magnitude
`GAIA_PHOT_G_MEAN_FLUX_OVER_ERROR`	float32		Gaia G band signal-to-noise
`GAIA_PHOT_BP_MEAN_MAG`	float32	mag	Gaia BP magnitude
`GAIA_PHOT_BP_MEAN_FLUX_OVER_ERROR`	float32		Gaia BP signal-to-noise
`GAIA_PHOT_RP_MEAN_MAG`	float32	mag	Gaia RP magnitude
`GAIA_PHOT_RP_MEAN_FLUX_OVER_ERROR`	float32		Gaia RP signal-to-noise
`GAIA_ASTROMETRIC_EXCESS_NOISE`	float32		Gaia astrometric excess noise
`GAIA_DUPLICATED_SOURCE`	boolean		Gaia duplicated source flag (1/0 for True/False)
`GAIA_PHOT_BP_RP_EXCESS_FACTOR`	float32		Gaia BP/RP excess factor
`GAIA_ASTROMETRIC_SIGMA5D_MAX`	float32	mas	Gaia longest semi-major axis of the 5-d error ellipsoid
`GAIA_ASTROMETRIC_PARAMS_SOLVED`	uint8		Which astrometric parameters were estimated for a Gaia source
`PARALLAX`	float32	mas	Reference catalog parallax
`PARALLAX_IVAR`	float32	1/(mas)²	Reference catalog inverse-variance on `PARALLAX`
`PMRA`	float32	mas/yr	Reference catalog proper motion in RA direction (\(\mu_\alpha^*\equiv\mu_\alpha\cos\delta\)) in the ICRS at `REF_EPOCH`
`PMRA_IVAR`	float32	1/(mas/yr)²	Reference catalog inverse-variance on `PMRA`
`PMDEC`	float32	mas/yr	Reference catalog proper motion in Dec direction (\(\mu_\delta\)) in the ICRS at `REF_EPOCH`
`PMDEC_IVAR`	float32	1/(mas/yr)²	Reference catalog inverse-variance on `PMDEC`
`MASKBITS`	int16		Bitwise mask indicating that an object touches a pixel in the `coadd///maskbits` maps (see the DR8 bitmasks page)

Photometric Redshift files (8.0-photo-z/sweep-<brickmin>-<brickmax>-pz.fits)

The Photometric Redshifts for the Legacy Surveys (PRLS, Zhou et al. 2021) catalog is line-matched to the DR8 sweep catalogs as described above.

The photometric redshifts are computed using the random forest algorithm. Details of the photo-z training and performance can be found in Zhou et al. (2021). For computing the photo-z's, we require at least one exposure in \(g\), \(r\) and \(z\) bands (NOBS_G,R,Z>1). For objects that do not meet the NOBS cut, the photo-z values are filled with -99. Although we provide photo-z's for all objects that meet the NOBS cut, only relatively bright objects have reliable photo-z's. As a rule of thumb, objects brighter than \(z\)-band magnitude of 21 are mostly reliable, whereas fainter objects are increasingly unreliable with large systematic offsets.

The photo-z catalogs do not provide information on star-galaxy separation. Stars are excluded from the photo-z training data, and we do not attempt to identify stars. To perform star-galaxy separation, one can use the morphological "TYPE" and/or the photometry (e.g., the optical-WISE color cut, as applied in Zhou et al. 2021, can be very effective for selecting redshift ≳ 0.3 galaxies) in the sweep catalogs.

Name	Type	Description
`z_phot_mean`	float32	photo-z derived from the mean of the photo-z PDF
`z_phot_median`	float32	photo-z derived from the median of the photo-z PDF
`z_phot_std`	float32	standard deviation of the photo-z's derived from the photo-z PDF
`z_phot_l68`	float32	lower bound of the 68% confidence region, derived from the photo-z PDF
`z_phot_u68`	float32	upper bound of the 68% confidence region, derived from the photo-z PDF
`z_phot_l95`	float32	lower bound of the 95% confidence region, derived from the photo-z PDF
`z_phot_u95`	float32	upper bound of the 68% confidence region, derived from the photo-z PDF
`z_spec`	float32	spectroscopic redshift, if available
`survey`	char[10]	source of the spectroscopic redshift
`training`	boolean	whether or not the spectroscopic redshift is used in photometric redshift training

Work which uses this photometric redshift catalog should cite Zhou et al. (2021) and include the additional acknowledgment for photometric redshifts.

Image Stacks (`<region>/coadd/*`)

Image stacks are on tangent-plane (WCS TAN) projections, 3600 × 3600 pixels, at 0.262 arcseconds per pixel.

<AAA>/<brick>/legacysurvey-<brick>-ccds.fits

FITS binary table with the list of CCD images that were used in this brick. Contains the same columns as survey-ccds-<camera>-dr8.fits.gz, and also contains the additional columns listed below. Note that string columns can have different lengths in the survey-ccds-<camera>-dr8.fits.gz and legacysurvey-<brick>-ccds.fits files and can differ for <region> equal to <north> and <south>. For example the camera column can change from char[9] to char[7] (see, e.g. legacypipe issue #379).

Column	Type	Description
`ccd_x0`	int16	Minimum x image coordinate overlapping this brick
`ccd_y0`	int16	Minimum y image coordinate overlapping this brick
`ccd_x1`	int16	Maximum x image coordinate overlapping this brick
`ccd_y1`	int16	Maximum y image coordinate overlapping this brick
`brick_x0`	int16	Minimum x brick image coordinate overlapped by this image
`brick_x1`	int16	Maximum x brick image coordinate overlapped by this image
`brick_y0`	int16	Minimum y brick image coordinate overlapped by this image
`brick_y1`	int16	Maximum y brick image coordinate overlapped by this image
`psfnorm`	float32	Same as `psfnorm` in the ccds-annotated- file
`galnorm`	float32	Same as `galnorm` in the ccds-annotated- file
`skyver`	char[8]	Git version of the sky calibration code
`wcsver`	char[1]	Git version of the WCS calibration code
`psfver`	char[7]	Git version of the PSF calibration code
`skyplver`	char[8]	Community Pipeline (CP) version of the input to sky calibration
`wcsplver`	char[5]	CP version of the input to WCS calibration
`psfplver`	char[8]	CP version of the input to PSF calibration

<AAA>/<brick>/legacysurvey-<brick>-chi2-<filter>.fits.fz

Stacked χ² image, which is approximately the summed χ² values from the single-epoch images.
<AAA>/<brick>/legacysurvey-<brick>-depth-<filter>.fits.fz
Stacked depth map in units of the point-source flux inverse-variance at each pixel.
- The 5σ point-source depth can be computed as \(5 / \sqrt(\mathrm{depth\_ivar})\) .
<AAA>/<brick>/legacysurvey-<brick>-galdepth-<filter>.fits.fz
Stacked depth map in units of the canonical galaxy flux inverse-variance at each pixel. The canonical galaxy is an exponential profile with effective radius 0.45" and round shape.
- The 5σ galaxy depth can be computed as \(5 / \sqrt(\mathrm{galdepth\_ivar})\) .
<AAA>/<brick>/legacysurvey-<brick>-image-<filter>.fits.fz
Stacked image centered on a brick location covering 0.25° × 0.25°. The primary HDU contains the coadded image (inverse-variance weighted coadd), in units of nanomaggies per pixel.
- NOTE: These are not the images used by Tractor, which operates on the single-epoch images.
- NOTE: These images are resampled using Lanczos-3 resampling.
- NOTE: Images in WISE bands are on the Vega system, all other flux-related quantities in DR8 are reported on the AB system. The description page lists the Vega-to-AB conversions recommended by the WISE team.

<AAA>/<brick>/legacysurvey-<brick>-invvar-<filter>.fits.fz
Corresponding stacked inverse variance image based on the sum of the inverse-variances of the individual input images in units of 1/(nanomaggies)² per pixel.
- NOTE: These are not the inverse variance maps used by Tractor, which operates on the single-epoch images.
- NOTE: Images in WISE bands are on the Vega system, all other flux-related quantities in DR8 are reported on the AB system. The description page lists the Vega-to-AB conversions recommended by the WISE team.
<AAA>/<brick>/legacysurvey-<brick>-maskbits.fits.fz
Bitmask of possible problems with pixels in this brick.
- HDU1: The optical bitmasks, corresponding to MASKBITS on the DR8 bitmasks page.
- HDU2: The WISE W1 bitmasks, corresponding to WISEMASK_W1 on the DR8 bitmasks page.
- HDU3: The WISE W2 bitmasks, corresponding to WISEMASK_W2 on the DR8 bitmasks page.
<AAA>/<brick>/legacysurvey-<brick>-model-<filter>.fits.fz
Stacked model image centered on a brick location covering 0.25° × 0.25°.
- The Tractor's idea of what the coadded images should look like; the Tractor's model prediction.
- NOTE: Images in WISE bands are on the Vega system, all other flux-related quantities in DR8 are reported on the AB system. The description page lists the Vega-to-AB conversions recommended by the WISE team.
<AAA>/<brick>/legacysurvey-<brick>-nexp-<filter>.fits.fz

Number of exposures contributing to each pixel of the stacked images.
<AAA>/<brick>/legacysurvey-<brick>-psfsize-<filter>.fits.fz

Weighted average PSF FWHM in arcsec at each pixel of the stacked images.
<AAA>/<brick>/legacysurvey-<brick>-image.jpg

JPEG image of the calibrated image using the \(g,r,z\) filters as the colors.
<AAA>/<brick>/legacysurvey-<brick>-model.jpg

JPEG image of the Tractor's model image using the \(g,r,z\) filters as the colors.
<AAA>/<brick>/legacysurvey-<brick>-resid.jpg

JPEG image of the residual image (data minus model) using the \(g,r,z\) filters as the colors.
<AAA>/<brick>/legacysurvey-<brick>-wise.jpg

JPEG image of the calibrated image using the WISE filters as the colors.
<AAA>/<brick>/legacysurvey-<brick>-wisemodel.jpg

JPEG image of the model image using the WISE filters as the colors.

Forced Photometry Files (`forced/*`)

These files contain forced photometry results, for all CCDs that were included in the DR8 processing.

That is, after we produce the catalogs based on fitting to all images simultaneously, we go back to the individual CCDs, select the catalog objects that overlap, and ask what fluxes those objects should have to best match what is observed in the CCD. When selecting objects from the catalog, we resolve the north and south components using the same cut as in the sweep files and randoms.

We perform two fits. The first is regular forced photometry, where the position and profile of the sources are fixed, and all we are fitting is the flux. In the second fit, we compute the source-centered spatial derivatives and fit the amplitudes of those derivatives as well. For sources moving less than a pixel or two, this produces an approximate estimate of the motion of the source (dra and ddec). Note that for Gaia sources, this is relative to the Gaia measured proper motion!

forced/<camera>/<EXPOS>/forced-<camera>-<EXPOSURE>.fits

Where <camera> is one of 90prime for BASS, decam for DECaLS or mosaic for MzLS, <EXPOSURE> is the exposure number (not necessarily as an 8-character string, unlike some other data products), and <EXPOS> is the first 5 characters of the exposure number printed as an 8-character string.

This file contains a single FITS binary table for all the CCDs in this exposure, contatenated into one long table.

For the columns pertaining to the catalog objects, see the catalog description page.

Column	Type	Description
`release`	int16	Unique integer denoting the camera and filter set used (RELEASE is documented here) for the catalog object
`brickid`	int32	Unique Brick ID (in the range [1, 662174]) that the catalog object came from
`brickname`	char[8]	Name of brick, encoding the brick sky position, eg "1126p222" near RA=112.6, Dec=+22.2, of the catalog object
`objid`	int32	Catalog object number within this brick; a unique identifier hash is `release,brickid,objid`
`camera`	char[7]	The camera for the CCD being measured, eg "decam"
`expnum`	int64	The exposure number of the CCD being measured, eg 574299
`ccdname`	char[4]	The name of the CCD being measured, eg "N10" or "CCD4"
`filter`	char[1]	The filter of the CCD being measured ("g", "r" or "z")
`mjd`	float64	The Modified Julian Date when the exposure was taken, in UTC, eg 57644.31537588
`exptime`	float32	The exposure time in seconds, eg 90.0
`psfsize`	float32	PSF FWHM in this exposure, in arcsec
`ccd_cuts`	int64	Bit mask describing CCD image quality (see, e.g., the DR9 bitmasks page)
`airmass`	float32	Airmass of this observation
`sky`	float32	Sky background surface brightness, in nanomaggies/arcsec²
`psfdepth`	float32	Inverse-variance for the flux measured from a point source; for a \(5\sigma\) point source detection limit use \(5/\sqrt(\mathrm{psfdepth})\) for the flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{psfdepth})) - 9]\) for the corresponding AB magnitude
`galdepth`	float32	Inverse-variance for the flux measured from a nominal galaxy source (0.45" round exponential galaxy)
`ra`	float64	Right Ascension in degrees
`dec`	float64	Declination in degrees
`flux`	float32	Measured flux for this catalog object in this CCD, in nanomaggies
`flux_ivar`	float32	Inverse-variance of the flux measurement, in 1/nanomaggies²
`fracflux`	float32	Profile-weighted fraction of the flux from other sources over total flux
`rchisq`	float32	Profile-weighted χ² residual chi-squared per pixel
`fracmasked`	float32	Profile-weighted fraction of pixels masked
`apflux`	float32[8]	Aperture fluxes in this CCD, in nanomaggies, for aperture radii [0.5, 0.75, 1.0, 1.5, 2.0, 3.5, 5.0, 7.0] arcsec
`apflux_ivar`	float32[8]	Inverse-variance on apflux, in 1/nanomaggies²
`x`	float32	Horizontal pixel position of the catalog source in this CCD, in zero-indexed pixels
`y`	float32	Vertical pixel position of the catalog source in this CCD, in zero-indexed pixels
`dqmask`	int16	Data Quality mask from the CP pipeline for the center pixel (defined as for `ALLMASK/ANYMASK` on the DR8 bitmasks page)
`dra`	float32	When fitting for spatial derivatives, the motion of the source in the RA direction, in arcsec
`ddec`	float32	Motion of the source in the Dec direction, in arcsec
`dra_ivar`	float32	Inverse-variance on dra, in 1/arcsec\|sup2\|
`ddec_ivar`	float32	Inverse-variance on ddec, in 1/arcsec\|sup2\|

Splinesky Files (`calib/<camera>/splinesky-*`)

splinesky-merged/<EXPOS>/<camera>-<EXPOSURE>.fits

Where <camera> is one of 90prime, decam or mosaic, <EXPOSURE> is the exposure number as an 8-character string and <EXPOS> is the first 5 characters of <EXPOSURE>.

This file contains all of the sky models for a given exposure number, as a single FITS binary table with 60 rows, one per CCD. Each row in this table contains the sky model for a single CCD. The splinesky files describe a smooth 2-dimensional function, implemented using the scipy RectBivariateSpline function. This is defined by a number of grid cell locations and function values at those locations, interpolated with a cubic spline. The spline grid cells for DR8 are ~256 pixels in size, and extend from edge to edge, so, for example DECam images (~2048 x 4096 pixels) have 9 x 17 cells.

For MzLS (mosaic) files, some early exposures lack an EXPNUM in the headers; these have a fake exposure number like 160125082555 corresponding to the date and time the image was taken (2016-01-25T08:25:55). For BASS (90prime) files, the exposure number comes from the DTACQNAM header card; for example, 20160710/d7580.0144.fits becomes exposure number 75800144.

Column	Type	Description
`gridw`	int64	The number of grid cells in the horizontal direction
`gridh`	int64	The number of grid cells in the vertical direction
`gridvals`	float32	The spline values (an array of size `gridh` \(\times\) `gridw`)
`xgrid`	int32	The horizontal locations of the grid cells (an array of length `gridw`)
`ygrid`	int32	The vertical locations of the grid cells (an array of length `gridh`)
`order`	uint8	The order of the spline (i.e. 3 = cubic)
`x0`	int32	Pixel offset of the model in the x direction (always 0 for these files)
`y0`	int32	Pixel offset of the model in the y direction (always 0 for these files)
`skyclass`	char[27]	Always set to `tractor.splinesky.SplineSky` (the name of a Python class that is used to read the model)
`legpipev`	char[19]	Version of legacypipe used for this reduction
`plver`	char[4]	Community Pipeline (CP) version number
`plprocid`	char[7]	Unique, time-based, CP processing hash - see the plprocid page for how to convert this to a date
`imgdsum`	int64	The DATASUM value from the image header (a checksum)
`procdate`	char[19]	CP processing date
`sig1`	float32	Estimated per-pixel noise in CP image units, from \(1/\sqrt(\mathrm{median}(wt[good]))\) where \(wt\) is the weight map and \(good\) are un-masked pixels
`sky_mode`	float32	Scalar mode of the image, estimated by fitting a quadratic to the histogram of unmasked pixels
`sky_med`	float32	Scalar median of the image, based on unmasked pixels
`sky_cmed`	float32	Median of the \(2\sigma\)-clipped image pixel values, based on unmasked pixels
`sky_john`	float32	Starting from a 5-pixel boxcar average over the `sky_cmed`-subtracted pixels, find and mask \(3\sigma\) sources (dilated by 3 pixels), then take the median of \(2\sigma\)-clipped pixels
`sky_fmasked`	float32	Total fraction of pixels masked by the source mask, the reference-source mask, and where the weightmap is 0
`sky_fine`	float32	RMS difference between a splinesky model at normal and at twice the resolution, to characterize the splinesky model had it more freedom
`sky_p0`	float32	Identical to `sky_fine`
`sky_p10`	float32	0th percentile of unmasked image pixels minus the splinesky model
`sky_p20`	float32	10th percentile of unmasked image pixels minus the splinesky model
`sky_p30`	float32	20th percentile of unmasked image pixels minus the splinesky model
`sky_p40`	float32	30th percentile of unmasked image pixels minus the splinesky model
`sky_p50`	float32	40th percentile of unmasked image pixels minus the splinesky model
`sky_p60`	float32	50th percentile of unmasked image pixels minus the splinesky model
`sky_p70`	float32	60th percentile of unmasked image pixels minus the splinesky model
`sky_p80`	float32	70th percentile of unmasked image pixels minus the splinesky model
`sky_p90`	float32	80th percentile of unmasked image pixels minus the splinesky model
`sky_p100`	float32	90th percentile of unmasked image pixels minus the splinesky model
`expnum`	int64	Exposure number, eg 348224
`ccdname`	char[4]	CCD name, e.g. "N10", "S7" for DECam

Other Files

Much additional information is available as part of the DESI Legacy Imaging Surveys Data Releases, including, in separate directories, statistics of the Tractor fits (<region>/metrics), code outputs from the fitting processes (<region>/logs) and additional files detailing the calibrations (calib). We don't expect that most users will need a description of these files, but contact us if you require more information.

Raw Data

See the raw data page.

Footnotes