Legacy Survey Files

Top level directory for web access:
http://portal.nersc.gov/project/cosmo/data/legacysurvey/dr2/
Top level directory local to NERSC computers (for collaborators):
/global/project/projectdirs/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:
http://portal.nersc.gov/project/cosmo/data/legacysurvey/dr2/external/
Or on the NERSC computers (for collaborators) at:
/global/project/projectdirs/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 \(int(RA * 10)\), followed by p to denote positive Dec or m to denote negative Dec ("plus"/"minus"), followed by three digits of \(int(Dec * 10)\). 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 using the format outlined in the previous bullet point. 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 epoch J2000
RA_IVAR float32 1/deg² Inverse variance of RA, excluding astrometric calibration errors
DEC float64 deg Declination at epoch J2000
DEC_IVAR float32 1/deg² Inverse variance of DEC (no cos term!), 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 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 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, EXP, DEV, 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

Raw Legacy Survey images are available through the NOAO Science Archive. The input data used to create the stacked images, Tractor catalogs, etc. comprises images taken by the dedicated DECam Legacy Survey project, as well as other DECam images, and images from other surveys. These instructions are for obtaining raw images from the DECam Legacy Survey only.

  1. Visit the NOAO Science Archive.
  2. Click on General Search for NOAO data (all users).
  3. From the menu of "Available Collections" at left, select the desired DECaLS data release (e.g. DECaLS-DR2).
  4. Under "Data products - Raw data" check "Object".
  5. Optionally, you may select data from specific DECam filters, or restrict the search by other parameters such as sky coordinates, observing date, or exposure time.
  6. Click Search.
  7. The Results page offers several different ways to download the detail. See the Tutorials page for details.