# External catalogs used in processing

## External Catalogs used for Astrometry and Photometry

Several external catalogs are used for astrometric and photometric calibration of DR9. These catalogs are currently availble in the indicated directories at NERSC, although we will soon make them available to the wider public.

### Pan-STARRS-1 (PS1)

/global/cfs/cdirs/cosmo/work/ps1/cats/chunks-qz-star-v3
These files were produced by Doug Finkbeiner from proprietary Pan-STARRS1 (PS1) data. The data in the files should be public as of Data Release 1 of PS1. The format of the file names is ps1-<hhhhh>.fits where <hhhhh> corresponds to HEALPixel numbers in the nside=32, NESTED scheme. Each file contains a FITS table of PS1 sources.

### Gaia DR2

/global/cfs/cdirs/cosmo/work/gaia/chunks-gaia-dr2-astrom-2
These files contain all sources from the Gaia DR2 public data release, with columns corresponding to the Gaia DR2 data model. The format of the the file names is chunk-<hhhhh>.fits where <hhhhh> corresponds to HEALPixel numbers in the nside=32, NESTED scheme. The provenance of these files is documented at /global/cfs/cdirs/cosmo/work/gaia/chunks-gaia-dr2-astrom-2/README and /global/cfs/cdirs/cosmo/staging/gaia/dr2/README.

### Tycho-2

/global/cfs/cdirs/cosmo/staging/tycho2/tycho2.kd.fits
This file corresponds to the Tycho-2 star catalog, produced from the original binary .dat files, and augmented with an isgalaxy column, plus $J$, $H$, $K$ and estimated $z$ magnitudes. How these extra columns were derived is detailed in /global/cfs/cdirs/cosmo/staging/tycho2/README.

## External Catalogs used for Masking

External catalogs are used for masking regions near foreground sources in DR9 (e.g. to construct MASKBITS on the bitmasks page). These catalogs are available in the indicated directories at NERSC and at the listed urls.

### "BRIGHT" stars

Bright stars are defined from a starting sample of all sources in the Tycho-2 catalog that have MAG_VT < 13. The BRIGHT bit is set for all such Tycho-2 stars. In addition, Gaia DR2 sources with phot_g_mean_mag < 13 have the BRIGHT bit set, provided they do not already match a Tycho-2 source. Gaia and Tycho-2 sources are matched after accounting for proper motion. In the gaia-mask-dr9.fits.gz file, bright stars have the isbright column set to True. In the legacypipe code, the radius of the BRIGHT star mask is set by pixels within half of the locus of a hardcoded radius-magnitude relation that is set for all Tycho-2 stars and for Gaia DR2 stars to G < 13. But, note that the specific radii in the gaia-mask-dr9.fits.gz do not include the factor-of-two correction (i.e. they are the appropriate radii for the MEDIUM masks, as described below).