Catalogs

The full suite of DECaPS2 data products are document in a datamodel that parallels the release file structure. Some of the main products are highlighted below.

DECaPS provides two types of catalogs: individual-image catalogs and band-merged catalogs. The individual catalogs are the outputs of crowdsource and CloudCovErr.jl 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. The raw and DECam Community Pipeline (CP) processed images are formally available through the NOIRLab Astro Data Archive, but since some images were processed by the CP multiple times, we also provide the exact set of images used to create the catalog here.

• 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 and in the ooi datamodel, which describes the image inputs to the DECaPS2 pipelines, which are the outputs of the DECam Community Pipeline.

• calibration file

  The calibration file contains the zero points and flat fields from the photometric calibration, as well as some quality assurance information which is detailed in the datamodel. 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. The individual epoch catalog files include additional information such as background and PSF models, as described by their datamodel.

• band-merged catalogs

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

The band-merged catalogs can also be accessed remotely using any TAP-aware client such as TOPCAT or astroquery via astropy. They are hosted by NOIRLab's AstroDataLab at the TAP address https://datalab.noirlab.edu/tap. This is not a default end-point for TOPCAT and needs to be added manually.

crowdsource analyzes each DECaPS CCD individually to determine the position and fluxes of all sources in the image. CloudCovErr.jl then processes the residuals to correct the flux and flux uncertainties in the presence of nebulosity by interpolating the unmodeled (sharp) background behind the star. The resulting output catalog has the following fields:

The DECaPS band-merged catalogs have undergone photometric calibration and tabulate a number of quantities for each source.

For quantities with an additional "_OK" field, the first is an average over all detections and the "_OK" field is limited to only "OK" detections. Detections are "OK" if no bad flag bits were set for the center pixel, the quality factor and RCHI2 pass a cut, and the flux uncertainty is nonzero. See the DECaPS2 release paper for these cuts and bad flag lists. The fields with a "_AVG", "_ANY", and "_ALL" were computed only over "OK" detections. The weights used for "_AVG" differ slightly from those used to compute the flux mean, using 0.1 instead of 0.0921 to regularize multiplicative errors.

Fields that are arrays of 6 elements in one dimension are "per band" with the band ordering ugrizY. However, no u-band observations were taken during DECaPS and thus the first element in that dimension should always be zero.

All fluxes are reported in "maggies" (Mgy) which are equivalent to 3631 Jy and are a convenient unit so that the AB magnitudes of the stars are given by \(-2.5\log_{10}\mathrm{flux}\). 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.

The crowdsource flags are combined with those from the DECam Community Pipeline into a single bitmask. Bits 0 - 8 are inherited from the CP, 20 - 24 indicate a special region on the CCD or sky, and 30 - 31 indicate a change in the crowdsource source identification parameters.

Excluded flags mean that detections with that flag set are not considered "ok" in the computation of the band-merged catalogs or for determining the distribution of sources to use in synthetic injection tests.

The flags from CloudCovErr.jl are saved as a separate bitmask field known as dnt.

Bits beyond 1 should not appear in the band-merged catalog because we explicitly excluded cases with PSF models that had even infinitesimally negative pixels during the photometric calibration. Such cases are extrordinarily rare and occur when the PSF model cannot be well estimated because of clouds obscuring the image or unmasked saturated sources distorting the PSF. Only order 10 CCDs (out of 1.3 million) that were otherwise considered photoemtric were excluded as a result of sources with dnt bits larger than 1.