DECaPS provides two types of catalogs: individual-image catalogs and band-merged catalogs. The individual catalogs are the outputs of crowdsource on the individual DECam CCDs. The band-merged catalogs are derived from the individual-image catalogs, merged so that different detections of the same source are collected and averaged together in the same catalog entry. Most users will want the band-merged catalogs.

• image database

The image database contains the headers from all images composing the DECaPS survey. This is useful to see exactly which images contribute to different parts of the footprint, and their properties. Documentation on DECam header fields is available in the NOAO Data Handbook.

• calibration file

The calibration file contains the zero points and flat fields from the photometric calibration, as well as some quality assurance information. The absolute calibration is tied to Tonry et al. (2012), which may be off AB by up to 0.03 mag.

• individual epoch catalogs

The individual epoch catalogs are available for every DECaPS image. The tabulated fields are described below. Missing CCDs in the pipeline output files occur when no sources were detected in that CCD, which occasionally occurs during periods of thick cloud cover.

• band-merged catalogs

The band-merged catalogs have been sliced into 1-by-1 degree chunks for manageable downloading, and are described below.

crowdsource analyzes each DECaPS CCD individually to determine the position and fluxes of all sources in the image. The resulting output catalog has the following fields:

 x f8 x coordinate (pix) y f8 y coordinate (pix) flux f4 flux (ADU) dx f4 uncertainty in x dy f4 uncertainty in y dflux f4 uncertainty in flux (statistical only) qf f4 "quality factor" rchi2 f4 average $$\chi^2$$ per pixel, weighted by PSF fracflux f4 fraction of flux in this object's PSF that comes from this object fluxlbs f4 local-background-subtracted flux (ADU) dfluxlbs f4 uncertainty in local-background-subtracted flux fwhm f4 full-width at half-maximum of PSF (pixels) flags i4 Community Pipeline flags at central pixel sky f4 sky (ADU) psf i4 octant of CCD where model was instantiated ra f8 right ascension (degree) dec f8 declination (degree) decapsid i8 unique ID gain f4 gain (e-/ADU) mjd_obs f8 MJD exptime f4 exposure time (sec) filterid |S1 filter (one of grizY) airmass f8 airmass chip_id i4 CCDNUM zp f4 Community Pipeline zero point (not for precision use)

The "quality factor" is a concept inherited from PS1. It tabulates the PSF-weighted fraction of pixels contributing to this detection. Good detections will have quality factors close to one. Heavily saturated detections will have quality factors close to zero.

The DECaPS band-merged catalogs tabulate a number of quantities for each source.

Field Name Data Type Description
obj_id u8 Unique DECaPS object ID
ra f8 Right ascension (degrees)
dec f8 Declination (degrees)
posstdev f4 Standard deviation in position of object between different detections (arcsec)
ra_ok f8 Right ascension, good detections only (degrees)
dec_ok f8 Declination, good detections only (degrees)
posstdev_ok f4 Standard deviation in position of object between different detections (arcsec)
ndet i2 Total number of detections
ndet_ok i2 Number of good detections
nmag 6i2 Number of detections in each band
nmag_ok 6i2 Number of good detections in each band
mean 6f4 Mean flux, good detections only (3631 Jy)
stdev 6f4 Standard deviation in flux; may be NaN in rare cases of objects with 1 measurement
err 6f4 Uncertainty in mean flux (statistical only)
median 6f4 Median flux (3631 Jy)
q25 6f4 25th percentile flux (3631 Jy)
q75 6f4 75th percentile flux (3631 Jy)
ndet_lbs i2 Number of detections
ndet_lbs_ok i2 Number of good detections
nmag_lbs 6i2 Number of detections in each band
nmag_lbs_ok 6i2 Number of good detections in each band
mean_lbs 6f4 Mean flux, using a local background subtraction (3631 Jy)
stdev_lbs 6f4 Standard deviation in local background subtracted fluxes; may be NaN in rare cases of objects with 1 measurement
err_lbs 6f4 Uncertainty in mean local background subtracted flux (statistical only)
median_lbs 6f4 Median local background subtracted flux (3631 Jy)
q25_lbs 6f4 25th percentile local background subtracted flux (3631 Jy)
q75_lbs 6f4 75th percentile local background subtracted flux (3631 Jy)
maglimit 6f4 5 sigma magnitude limit for deepest detection of this object (AB mag)
epochrange f8 MJD difference between first and last detection
epochrange_ok f8 MJD difference between first and last good detection
epochmean f8 Mean MJD
epochmean_ok f8 Mean good MJD
fracflux 6f4 PSF-weighted fraction of flux coming from this object (i.e., one minus the the fraction of flux in this object's PSF that comes from neighboring objects?)

A "good" detection in these catalogs means that the detection's central pixel had no "bad" flags set in the Community Pipeline data quality image, and that at least 85% of the source's flux (according to the model PSF and model location of the source) lands in a pixel with non-zero weight, according to the Community Pipeline inverse variance image.

All fluxes are in units of "3631 Jy," so that the AB magnitudes of the stars are given by $$-2.5\log\mathrm{flux}$$. The absolute calibration is based on the Tonry et al. (2012) absolute calibration of PS1. Subsequent analyses (Scolnic et al., 2014, 2015) of the PS1 survey found offsets relative to AB. To shift the DECaPS magnitudes to the Scolnic et al. (2015) absolute calibration, offsets of 0.020, 0.033, 0.024, 0.028, and 0.011 mag must be added to the DECaPS grizY magnitudes, respectively.

Many fields are arrays of 6 elements. These 6 elements correspond to the ugrizY filters. No observations were made in the u filter, however, so this field is always empty.

Most DECaPS flags are inherited from the Community Pipeline. In order to allow a single source to have multiple problem flags, we have converted the Community Pipeline integer flags into a bitmask. DECaPS flags are two to the power of the Community Pipeline flags. We also add a few additional flags.

Bit Description Exclude?