# Legacy Survey Files

Top level directory for web access:

http://portal.nersc.gov/cfs/cosmo/data/legacysurvey/dr1/

Top level directory local to NERSC computers (for collaborators):

/global/cfs/cdirs/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

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).

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.

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.

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.

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

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) .

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

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