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GCAT
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Data Products, Data Access, and Catalog Column Descriptions

Summary of Available Data Products

Users can browse all of the GASC, GMSC, and Kepler GCAT files through their web browser by going directly to http://archive.stsci.edu/pub/hlsp/gcat/. Many users will find that accessing the source catalogs is best accomplished via SQL on our CasJobs server. Visit the CasJobs link above for more details and examples.

Directory Structure

In this section we describe the organization of the data files and describe the contents of the various files that we are releasing as a part of the GASC and GMSC data sets. We have divided the sky into 192 "chunks," numbered from 0 to 191 although some chunks do not have any observations in them. Each chunk corresponds to a healpix pixel at the Nside=4 level in the nested scheme in Galactic coordinates. Each chunk is therefore 215^2 deg in area and there is no overlap between adjacent chunks. THE GASC consists of 189 chunks while the GMSC consists of 174 chunks. Examples showing the GMSC footprint for the chunk 111 are plotted in Figures 3-6. Each chunk is given its own directory labeled with the chunk ID number. Within that directory are sub-directories for each survey labeled with "asc" or "msc." Within each of those sub-directories are all of the catalog and footprint files for the data in that chunk and survey.

Some users may not be interested in all the products that have been produced at the chunk level. With that in mind we have produced a simpler distribution with as few files as possible. These are located in the "fullsky" directories for each survey. This version of the catalogs concatenates the chunk catalogs into only 24 files. In addition the HealPix chunk maps have been merged into fullsky maps at lower resolution Nside=1024 (3.442 arcmin2) and nside=4096 (51.532 arcsec2). The pixels in these maps will no longer correspond to the nside = 16384 HealPix_Pixel column in the catalogs but are useful in other ways.

Finally, we provide some ancillary products: images of the healpix maps in either PNG or PDF format as well as FITS image projections of the HealPix maps for those that may not be familiar with HealPix. The FITS projections have limited functionality as the resulting pixels are not equal area.

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File Descriptions

The files available in each chunk/ and fullsky/ subdirectory are listed in the tables below.

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Description of Catalog Columns

Users should be aware that many of the column names and units for the GASC and GMSC are different from the standard GALEX MCAT pipeline products. All units are consistent physical quantities. Fluxes are micro-Janskys (never counts-per-second), magnitudes are AB (never instrumental), sizes are either arcseconds or degrees (never pixels). To avoid confusion with MCAT values the "band designation" has been appended to the label/name instead of prepended as with MCAT columns. For example, the GASC/GMSC catalogs use the column name "MAG_FUV" to represent the FUV magnitude, compared to the MCAT name "FUV_MAG".

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