Catalogs

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.

All catalogs are available for download.

• 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:

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.

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.

Excluded flags mean that detections with that flag set are not considered "ok" in the computation of the band-merged catalogs. Flags with bits greater than or equal to 20 are not part of the Community Pipeline, and were added in DECaPS.