Legacy Survey Files

Top level directory for web access:: https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr2/
Top level directory local to NERSC computers (for collaborators):: /global/cfs/cdirs/cosmo/data/legacysurvey/dr2/

Summary Files

decals-bricks.fits

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 DR2. For that information, see the next file description.

HDU1 (only HDU) - tags in the decals-bricks.fits file

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`	double	RA of the center of the brick.
`dec`	double	Dec of the center of the brick.
`ra1`	double	Lower RA boundary.
`ra2`	double	Upper RA boundary.
`dec1`	double	Lower Dec boundary.
`dec2`	double	Upper Dec boundary.

decals-bricks-dr2.fits

A FITS binary table with information about what is included in DR2.

This table contains only rows (bricks) for which we have some imaging coverage.

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`	double	RA of the center of the brick.
`dec`	double	Dec of the center of the brick.
`ra1`	double	Lower RA boundary.
`ra2`	double	Upper RA boundary.
`dec1`	double	Lower Dec boundary.
`dec2`	double	Upper Dec boundary.
`nobs_med_g`	uint8	Median number of observations in g
`nobs_med_r`	uint8	Median number of observations in r
`nobs_med_z`	uint8	Median number of observations in z
`nobs_max_g`	uint8	Maximum number of observations in g
`nobs_max_r`	uint8	Maximum number of observations in r
`nobs_max_z`	uint8	Maximum number of observations in z

decals-ccds.fits.gz

A FITS binary table with almanac information (e.g. seeing, etc.) about each individual CCD image. Note that this is the only file in the top-level directory that is gzipped (it is slightly larger than other such files and is gzipped for compliance with the legacysurvey github repository).

This file contains information regarding the photometric and astrometric zero points for each CCD of every DECam image that is part of the DECaLS DR2 data release. Photometric zero points for each CCD are computed by identifying stars and comparing their instrumental magnitudes (measured in an approximately 7 arcsec diameter aperture) to color-selected stars in the PanSTARRS "qy" 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 \(30 * 10^{((25.04 - 22.5) / 2.5)} = 311.3\) counts.

Column	Type	Description
`expnum`	int32	Unique DECam exposure number, eg 348224.
`exptime`	float	Exposure time in seconds, eg 30.
`filter`	char[1]	Filter used for observation, eg "g", "r", "z".
`seeing`	float	Seeing in arcseconds determined by fitting a 2-dimensional gaussian to the median PSF of stars on the CCD, eg 1.1019.
`date_obs`	char[10]	Date of observation start, eg "2014-08-15". Can be combined with `ut`, or use `mjd_obs` instead.
`mjd_obs`	double	Date of observation in MJD (in UTC system), eg 56884.99373389.
`ut`	char[15]	Time of observation start, eg "23:50:58.608241".
`airmass`	float	Airmass, eg 1.35.
`propid`	char[10]	NOAO Proposal ID that took this image, eg "2014B-0404".
`zpt`	float	Median zero point for the entire image (median of all CCDs of the image), eg 25.0927.
`avsky`	float	Average sky level in this image, in ADU, eg 36.9324. `avsky` is detailed more here
`arawgain`	float	Average gain for this CCD, eg 4.34.
`fwhm`	float	[use "seeing" instead]
`crpix1`	float	Astrometric header value: X reference pixel.
`crpix2`	float	Astrometric header value: Y reference pixel.
`crval1`	double	Astrometric header value: RA of reference pixel.
`crval2`	double	Astrometric header value: Dec of reference pixel.
`cd1_1`	float	Astrometric header value: transformation matrix.
`cd1_2`	float	Astrometric header value: transformation matrix.
`cd2_1`	float	Astrometric header value: transformation matrix.
`cd2_2`	float	Astrometric header value: transformation matrix.
`ccdnum`	int16	CCD number (see DECam layout), eg 1.
`ccdname`	char[3]	CCD name (see DECam layout), eg "N10", "S7".
`ccdzpt`	float	Zeropoint for the CCD (AB mag).
`ccdzpta`	float	Zeropoint for amp A (AB mag).
`ccdzptb`	float	Zeropoint for amp B (AB mag).
`ccdphoff`	float	(ignore)
`ccdphrms`	float	Photometric rms for the CCD (in mag).
`ccdskyrms`	float	Sky rms (in counts).
`ccdraoff`	float	Median astrometric offset for the CCD <PS1-DECaLS> in arcsec.
`ccddecoff`	float	Median astrometric offset for the CCD <PS1-DECaLS> in arcsec
`ccdtransp`	float	(ignore)
`ccdnstar`	int16	Number of stars found on the CCD.
`ccdnmatch`	int16	Number of stars matched to PS1 (and used to compute the photometric zero points and astrometric offsets).
`ccdnmatcha`	int16	Number of stars in amp A matched.
`ccdnmatchb`	int16	Number of stars in amp B matched.
`ccdmdncol`	float	Median (g-i) color from the PS1 catalog of the matched stars.
`camera`	char[5]	The camera that took this image; "decam".
`expid`	char[12]	Exposure ID string, eg "00348224-S29" (from `expnum` and `ccdname`)
`image_hdu`	int16	FITS HDU number in the `image_filename` file where this image can be found.
`image_filename`	char[61]	Path to FITS image, eg "decam/CP20140810_g_v2/c4d_140815_235218_ooi_g_v2.fits.fz".
`width`	int16	Width in pixels of this image, eg 2046.
`height`	int16	Height in pixels of this image, eg 4096.
`ra_bore`	double	Telescope boresight RA of this exposure (deg).
`dec_bore`	double	Telescope boresight Dec of this exposure (deg).
`ra`	double	Approximate RA center of this CCD (deg).
`dec`	double	Approximate Dec center of this CCD (deg).

decals-ccds-annotated.fits

A version of the decals-ccds.fits file with additional information gathered during calibration pre-processing before running the Tractor reductions.

Includes everything listed in the decals-ccds.fits file plus the following:

Column	Type	Description
`photometric`	boolean	True if this CCD was considered photometric and used in the DR2 reductions
`blacklist_ok`	boolean	We blacklisted certain programs (Proposal IDs) from other PIs where there were a large number of images covering a single patch of sky, because our pipeline code didn't handle the extreme depth very well. True if this CCD was not blacklisted, ie, was used.
`good_region`	int[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`	double	RA coordinate of pixel (1,1)
`dec0`	double	Dec coordinate of pixel (1,1)
`ra1`	double	RA coordinate of pixel (1,H)
`dec1`	double	Dec coordinate of pixel (1,H)
`ra2`	double	RA coordinate of pixel (W,H)
`dec2`	double	Dec coordinate of pixel (W,H)
`ra3`	double	RA coordinate of pixel (W,1)
`dec3`	double	Dec coordinate of pixel (W,1)
`dra`	float	Maximum distance from RA,Dec center to the edge midpoints, in RA
`ddec`	float	Maximum distance from RA,Dec center to the edge midpoints, in Dec
`ra_center`	double	RA coordinate of CCD center
`dec_center`	double	RA coordinate of CCD center
`sig1`	float	Median per-pixel error standard deviation, in nanomaggies.
`meansky`	float	Our pipeline (not the CP) estimate of the sky level, average over the image, in ADU.
`stdsky`	float	Standard deviation of our sky level
`minsky`	float	Min of our sky level
`maxsky`	float	Max of our sky level
`pixscale_mean`	float	Pixel scale (via sqrt of area of a 10x10 pixel patch evaluated in a 5x5 grid across the image), in arcsec/pixel.
`pixscale_std`	float	Standard deviation of pixel scale
`pixscale_min`	float	Min of pixel scale
`pixscale_max`	float	Max of pixel scale
`psfnorm_mean`	float	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`	float	Standard deviation of PSF norm
`galnorm_mean`	float	Norm of the PSF model convolved by a 0.45" exponential galaxy.
`galnorm_std`	float	Standard deviation of galaxy norm.
`psf_mx2`	float	PSF model second moment in x (pixels^2)
`psf_my2`	float	PSF model second moment in y (pixels^2)
`psf_mxy`	float	PSF model second moment in x-y (pixels^2)
`psf_a`	float	PSF model major axis (pixels)
`psf_b`	float	PSF model minor axis (pixels)
`psf_theta`	float	PSF position angle (deg)
`psf_ell`	float	PSF ellipticity 1 - minor/major
`humidity`	float	Percent humidity outside
`outtemp`	float	Outside temperate (deg C).
`tileid`	int32	DECaLS tile number, if this was a DECaLS observation; or 0 for data from other programs.
`tilepass`	uint8	DECaLS tile pass number, 1, 2 or 3, if this was a DECaLS observation, or 0 for data from other programs. Set by the observers; pass 1 is supposed to be photometric with good seeing, pass 3 unphotometric or bad seeing, and pass 2 in between.
`tileebv`	float	Mean SFD E(B-V) extinction in the DECaLS tile, or 0 for non-DECaLS data.
`plver`	char[6]	Community Pipeline (CP) PLVER version string
`ebv`	float	SFD E(B-V) extinction for CCD center
`decam_extinction`	float[6]	Extinction for DECam filters ugrizY
`wise_extinction`	float[4]	Extinction for WISE bands W1,W2,W3,W4
`psfdepth`	float	5-sigma PSF detection depth in AB mag, using PsfEx PSF model
`galdepth`	float	5-sigma galaxy (0.45" round exp) detection depth in AB mag
`gausspsfdepth`	float	5-sigma PSF detection depth in AB mag, using Gaussian PSF approximation (using `seeing` value)
`gaussgaldepth`	float	5-sigma galaxy detection depth in AB mag, using Gaussian PSF approximation

External Files

The DECaLS photometric catalogs have been matched to the following three external spectroscopic files from the SDSS, which can be accessed through the web at:: https://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr2/external/
Or on the NERSC computers (for collaborators) at:: /global/cfs/cdirs/cosmo/data/legacysurvey/dr2/external/

decals-dr2-specObj-dr12.fits

HDU1 (the only HDU) contains Tractored DECaLS photometry that is row-by-row-matched to the SDSS DR12 spectrosopic pipeline file such that the photometric parameters in row "N" of decals-dr2-specObj-dr12.fits matches the spectroscopic parameters in row "N" of specObj-dr12.fits. The structure of the DECaLS photometric catalog files is documented on the catalogs page and the spectroscopic file is documented in the SDSS DR12 data model for specObj-dr12.fits.

decals-dr2-DR12Q.fits

HDU1 (the only HDU) contains Tractored DECaLS photometry that is row-by-row-matched to the SDSS DR12 visually inspected quasar catalog (Paris et al. 2016, in preparation, see also Paris et al. 2014) such that the photometric parameters in row "N" of decals-dr2-DR12Q.fits matches the spectroscopic parameters in row "N" of DR12Q.fits. The structure of the DECaLS photometric catalog files is documented on the catalogs page and the spectroscopic file is documented in the SDSS DR12 data model for DR12Q.fits.

decals-dr2-Superset_DR12Q.fits

HDU1 (the only HDU) contains Tractored DECaLS photometry catalog 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. 2016, in preparation, see also Paris et al. 2014) such that the photometric parameters in row "N" of decals-dr2-Superset_DR12Q.fits matches the spectroscopic parameters in row "N" of Superset_DR12Q.fits. The structure of the DECaLS photometric catalog files is documented on the catalogs page and the spectroscopic file is documented in the SDSS DR12 data model for Superset_DR12Q.fits.

Tractor Catalogs

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 decals-bricks.fits file (i.e. brickname, ra, dec).

tractor/<AAA>/tractor-<brick>.fits

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

Sweep Catalogs

sweep/2.0/sweep-<brickmin>-<brickmax>.fits

Light-weight FITS binary tables (containing a subset of the most commonly used Tractor measurements) of all the Tractor catalogs in rectangles of RA,Dec. Includes:

Name	Type	Units	Description
`BRICK_PRIMARY`	boolean		True if the object is within the brick boundary
`BRICKID`	int32		Brick ID [1,662174]
`BRICKNAME`	char		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 `BRICKID,OBJID`; `OBJID` spans [0,N-1] and is contiguously enumerated within each blob
`TYPE`	char[4]		Morphological model: "PSF"=stellar, "SIMP"="simple galaxy" = 0.45" round EXP galaxy, "EXP"=exponential, "DEV"=deVauc, "COMP"=composite. 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
`RA_IVAR`	float32	1/deg²	Inverse variance of RA (no cosine term!), excluding astrometric calibration errors
`DEC`	float64	deg	Declination at equinox J2000
`DEC_IVAR`	float32	1/deg²	Inverse variance of DEC, excluding astrometric calibration errors
`DECAM_FLUX`	float32[6]	nanomaggies	DECam model flux in ugrizY
`DECAM_FLUX_IVAR`	float32[6]	1/nanomaggies²	Inverse variance oF DECAM_FLUX
`DECAM_MW_TRANSMISSION`	float32[6]		Galactic transmission in ugrizY filters in linear units [0,1]
`DECAM_NOBS`	uint8[6]		Number of images that contribute to the central pixel in each filter for this object (not profile-weighted)
`DECAM_RCHI2`	float32[6]		Profile-weighted χ² of model fit normalized by the number of pixels
`DECAM_PSFSIZE`	float32[6]	arcsec	Weighted average PSF FWHM per band
`DECAM_FRACFLUX`	float32[6]		Profile-weight fraction of the flux from other sources divided by the total flux (typically [0,1])
`DECAM_FRACMASKED`	float32[6]		Profile-weighted fraction of pixels masked from all observations of this object, strictly between [0,1]
`DECAM_FRACIN`	float32[6]		Fraction of a source's flux within the blob, near unity for real sources
`DECAM_DEPTH`	float32[6]	1/nanomaggies²	For a \(5\sigma\) point source detection limit, \(5/\sqrt(\mathrm{DECAM\_DEPTH})\) gives flux in nanomaggies and \(-2.5[\log_{10}(5 / \sqrt(\mathrm{DECAM\_DEPTH})) - 9]\) gives corresponding magnitude
`DECAM_GALDEPTH`	float32[6]	1/nanomaggies²	As for DECAM_DEPTH but for a galaxy (0.45" exp, round) detection sensitivity
`OUT_OF_BOUNDS`	boolean		True for objects whose center is on the brick; less strong of a cut than BRICK_PRIMARY
`DECAM_ANYMASK`	int16[6]		Bitwise mask set if the central pixel from any image satisfy each condition
`DECAM_ALLMASK`	int16[6]		Bitwise mask set if the central pixel from all images satisfy each condition
`WISE_FLUX`	float32[4]	nanomaggies	WISE model flux in W1,W2,W3,W4
`WISE_FLUX_IVAR`	float32[4]	1/nanomaggies²	Inverse variance of WISE_FLUX
`WISE_MW_TRANSMISSION`	float32[4]		Galactic transmission in W1,W2,W3,W4 filters in linear units [0,1]
`WISE_NOBS`	int16[4]		Number of images that contribute to the central pixel in each filter for this object (not profile-weighted)
`WISE_FRACFLUX`	float32[4]		Profile-weight fraction of the flux from other sources divided by the total flux (typically [0,1])
`WISE_RCHI2`	float32[4]		Profile-weighted χ² of model fit normalized by the number of pixels
`DCHISQ`	float32[5]		Difference in χ² between successively more-complex model fits: PSF, SIMPle, DEV, EXP, COMP. The difference is versus no source.
`FRACDEV`	float32		Fraction of model in deVauc [0,1]
`TYCHO2INBLOB`	boolean		Is there a Tycho-2 (very bright) star in this blob?
`SHAPEDEV_R`	float32	arcsec	Half-light radius of deVaucouleurs model (>0)
`SHAPEEXP_R`	float32	arcsec	Half-light radius of exponential model (>0)
`EBV`	float32	mag	Galactic extinction E(B-V) reddening from SFD98, used to compute DECAM_MW_TRANSMISSION and WISE_MW_TRANSMISSION

Image Stacks

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

coadd/<AAA>/<brick>/decals-<brick>-ccds.fits

FITS binary table with the list of CCD images that were used in this brick. Same columns as decals-ccds.fits, plus:

Column	Type	Description
`ccd_x0`	int16	Minimum x image coordinate overlapping this brick
`ccd_x1`	int16	Maximum x image coordinate overlapping this brick
`ccd_y0`	int16	Minimum y 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`	float	Same as `psfnorm` in decals-ccds-annotated.fits
`galnorm`	float	Same as `galnorm` in decals-ccds-annotated.fits
`plver`	char[6]	Community Pipeline (CP) version
`skyver`	char[16]	Git version of the sky calibration code
`wcsver`	char[16]	Git version of the WCS calibration code
`psfver`	char[16]	Git version of the PSF calibration code
`skyplver`	char[16]	CP version of the input to sky calibration
`wcsplver`	char[16]	CP version of the input to WCS calibration
`psfplver`	char[16]	CP version of the input to PSF calibration

coadd/<AAA>/<brick>/decals-<brick>-image-<filter>.fits

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: that these images are resampled using nearest-neighbor resampling, so should not be used for numerical purposes (eg, photometry)

coadd/<AAA>/<brick>/decals-<brick>-invvar-<filter>.fits

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.

coadd/<AAA>/<brick>/decals-<brick>-model-<filter>.fits.gz

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.

coadd/<AAA>/<brick>/decals-<brick>-chi2-<filter>.fits

Stacked χ² image, which is approximately the summed χ² values from the single-epoch images.

coadd/<AAA>/<brick>/decals-<brick>-depth-<filter>.fits.gz

Stacked depth map in units of the point-source inverse-variance at each pixel.

The 5σ point-source depth can be computed as 5 / sqrt(depth_ivar) .

coadd/<AAA>/<brick>/decals-<brick>-nexp-<filter>.fits.gz

Number of exposures contributing to each pixel of the stacked images.

coadd/<AAA>/<brick>/decals-<brick>-image.jpg

JPEG image of calibrated image using the g,r,z filters as the colors.

coadd/<AAA>/<brick>/decals-<brick>-model.jpg

JPEG image of the Tractor's model image using the g,r,z filters as the colors.

coadd/<AAA>/<brick>/decals-<brick>-resid.jpg

JPEG image of the residual image (data minus model) using the g,r,z filters as the colors.

Raw Data

See the raw data page.