Legacy Survey Files

Top level directory for web access:
http://portal.nersc.gov/project/cosmo/data/legacysurvey/dr1/
Top level directory local to NERSC computers (for collaborators):
/global/project/projectdirs/cosmo/data/legacysurvey/dr1/

Summary Files

decals-bricks.fits

FITS binary table with the RA, DEC bounds of each geometrical "brick" on the sky. The subset of bricks which have g-, r-, or z-band imaging in this data release can be identified using the has_image_g, has_image_r, and has_image_z tags, respectively. The bricks with a corresponding Tractor photometric catalog in this release can be identified using the has_catalog tag.

  • 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.
has_image_g byte This brick has imaging data in this band.
has_image_r byte This brick has imaging data in this band.
has_image_z byte This brick has imaging data in this band.
has_catalog byte This brick has Tractor photometry.

decals-ccds.fits

Column Type Description
filter char[1]  
ra double  
dec double  
airmass float  
date_obs char[26]  
g_seeing float  
exptime float  
expnum int32  
mjd_obs float  
propid char[10]  
guider int32  
object char[31]  
avsky float  
arawgain float  
fwhm float  
crpix1 double  
crpix2 double  
crval1 double  
crval2 double  
cd1_1 float  
cd1_2 float  
cd2_1 float  
cd2_2 float  
extname char[8]  
ccdnum int32  
cpimage char[82]  
cpimage_hdu int32  
calname float[38]  
height int32  
width int32  
ra_bore double  
dec_bore double  
dr1 byte  

FITS binary table with almanac information (seeing, sky-brighness, etc.) about each individual CCD image. The subset of CCDs included in this release can be identified using the DR1 tag.

decals-specz.fits

Merged photometric and spectroscopic catalog for 751,070 sources with spectroscopic redshifts from SDSS/DR12. This file is a binary FITS table with 4 extensions (HDUs). Each extension contains the following row-matched catalogs:

  • HDU1 - DECaLS photometry and related Tractor outputs, WISE forced photometry (http://unwise.me)
Column Type Description
brickid int32 Name of the brick.
brickname char[8] A unique integer with 1-to-1 mapping to brickname.
objid int32  
brick_primary char[1]  
blob int32  
type char[4]  
ra double  
ra_ivar float  
dec double  
dec_ivar float  
bx float  
by float  
bx0 float  
by0 float  
left_blob char[1]  
decam_flux fltarr[6]  
decam_flux_ivar fltarr[6]  
decam_apflux fltarr[48]  
decam_apflux_resid fltarr[48]  
decam_apflux_ivar fltarr[48]  
decam_mw_transmission fltarr[6]  
decam_nobs bytarr[6]  
decam_rchi2 fltarr[6]  
decam_fracflux fltarr[6]  
decam_fracmasked fltarr[6]  
decam_fracin fltarr[6]  
out_of_bounds char[1]  
decam_anymask intarr[6]  
decam_allmask intarr[6]  
wise_flux fltarr[4]  
wise_flux_ivar fltarr[4]  
wise_mw_transmission fltarr[4]  
wise_nobs intarr[4]  
wise_fracflux fltarr[4]  
wise_rchi2 fltarr[4]  
dchisq fltarr[4]  
fracdev float  
fracdev_ivar float  
shapeexp_r float  
shapeexp_r_ivar float  
shapeexp_e1 float  
shapeexp_e1_ivar float  
shapeexp_e2 float  
shapeexp_e2_ivar float  
shapedev_r float  
shapedev_r_ivar float  
shapedev_e1 float  
shapedev_e1_ivar float  
shapedev_e2 float  
shapedev_e2_ivar float  
ebv float  

ZeroPoints-DR1.fits

The ZeroPoints-DR1.fits file contains information regarding the photometric and astrometric zero points for each CCD of every DECam image that is part of the DECaLS DR1 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.

  • HDU1 (only HDU) - tags in the ZeroPoints-DR1.fits file
Column Type Description
filename char[78] Name of the NOAO CP-reduced image FITS file.
expnum char[12] Unique DECam exposure number.
exptime float Exposure time in seconds.
filter char[32] Filter used for observation.
seeing float Seeing in arcseconds determined by fitting a 2-dimensional gaussian to the median PSF of stars on the CCD.
ra double Right Ascension in decimal degrees.
dec double Declination in decimal degrees.
zpt float Median zero point for the entire image (median of all CCDs of the image).
ccdnum int32 CCD number (see DECam layout).
ccdname char[3] CCD name (see DECam layout).
ccdra double Central RA for the CCD.
ccddec double Central Dec for the CCD.
ccdzpt double Zeropoint for the CCD (AB mag).
ccdphrms double Photometric rms for the CCD (in mag).
ccdskyrms double Sky rms (in counts).
ccdraoff double Median astrometric offset for the CCD <PS1-DECaLS> in arcsec.
ccddecoff double Median astrometric offset for the CCD <PS1-DECaLS> in arcsec
ccdnstar int32 Number of stars found on the CCD.
ccdnmatch int32 Number of stars matched to PS1 (and used to compute the photometric zero points and astrometric offsets).
ccdmdncol float Median (g-i) color from the PS1 catalog of the matched stars.
dr1 byte Included in the DR1 data set? 1=Yes, 0=No. Images that are non-photometric or on malfunctioning CCDs are excluded from DR1.

Tractor Catalogs

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

FITS binary table containing Tractor photometry, documented here: http://legacysurvey.org/dr1/catalogs/.

Sweep Catalogs

tractor-sweep-<AAA>.fits

Light-weight FITS binary tables (containing a subset of the most commonly used Tractor measurements) of all the Tractor catalogs in a given degree of right ascension <AAA>.

Image Stacks

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

FITS binary table with the list of CCD images that were used in this brick. EXPNUM indicates the DECam exposure ID and FILTER indicates the filter (u,g,r).

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 (simple unweighted coadd), in units of nano-maggies per pixel.

  • NOTE: These are not the images used by Tractor, which operates on the single-epoch images.

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/(nano-maggies)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. In the Simple Query Form, enter "2014B-0404" in the Program number box.
  4. Check "Raw" under All instruments.
  5. Click Search
  6. Once the query finishes, you can "Categorize by" "Observation type". The "object" images are actual on-sky data. Other Observation types are flats, darks, etc.
  7. The Results page offers several different ways to download the detail. See the Tutorials page for details.