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Description
This catalog comprises the Faint Images of the Radio Sky at Twenty cm (FIRST) Survey. The FIRST survey began in 1993, and covers the north and south Galactic caps. The present 14Dec17 version is derived from the 1993 through 2011 observations. The catalog covers a total of about 10,575 square degrees of sky (8,444 square degrees in the north Galactic cap and 2,131 square degrees in the south Galactic cap). See the coverage maps at <a href="http://sundog.stsci.edu/first/catalogs/readme_14dec17.html#coverage">http://sundog.stsci.edu/first/catalogs/readme_14dec17.html#coverage</a> for more details of the area covered. Both the northern and southern areas were chosen to coincide approximately with the area covered by the Sloan Digital Sky Survey (SDSS). The catalog is identical to the previous version of the catalog (14Mar04) except that it has more accurate data on which sources are not covered by the SDSS DR10 catalog. Approximately 1000 sources that were indicated as covered by DR10 in the previous version are now correctly marked as not covered. The source list, radio fluxes, etc., are all the same as the 14Mar04 version. In this version of the catalog, images taken in the the new EVLA configuration have been re-reduced using shallower CLEAN thresholds in order to reduce the "CLEAN bias" in those images. Also, the EVLA images are not co-added with older VLA images to avoid problems resulting from the different frequencies and noise properties of the configurations. That leads to small gaps in the sky coverage at boundaries between the EVLA and VLA regions. As a result, the area covered by this release of the catalog is about 60 square degrees smaller than the earlier release of the catalog (13Jun05), and the total number of sources is reduced by nearly 25,000. The previous version of the catalog does have sources in the overlap regions, but their flux densities are considered unreliable due to calibration errors. The flux densities should be more accurate in this catalog, biases are smaller, and the incidence of spurious sources is also reduced. Over most of the survey area, the detection limit is 1 mJy. A region along the equatorial strip (RA = 21.3 to 3.3 hrs, Dec = -1 to 1 deg) has a deeper detection threshold because two epochs of observation were combined. The typical detection threshold in this region is 0.75 mJy. There are approximately 4,500 sources below the 1 mJy threshold used for most previous versions of the catalog. The format of this catalog is the same as releases since 13Jun05 but differs from earlier versions of the catalog. It contains two parameters which give information on the epoch of observation for each source (called mean_epoch and rms_epoch in this HEASARC version) which are described below. The P(S) parameter (called sidelobe_prob herein), which indicates the probability that the source is a sidelobe, replaces the previous binary sidelobe flag column. The parameters sdss_matches, sdss_first_offset, sdss_imag, sdss_class, twomass_matches, twomass_first_offset and twomass_kmag give information on counterparts to the FIRST source in the SDSS DR10 catalog and the 2MASS catalog, respectively. Other catalog parameters are common with FIRST catalog releases extending back over the past decade. The co-added images are available online: see the FIRST page at <a href="http://sundog.stsci.edu/first/images.html">http://sundog.stsci.edu/first/images.html</a> for details. The source catalog presented here is derived from the images. Data for the FIRST survey were collected in all VLA B-configurations from Spring 1993 through Spring 2004. For all data collected for the FIRST project, the raw u-v visibility data are placed in the VLA public archive on the day they are taken, and are available for use without restriction. Additional data in the southern Galactic cap were acquired in Spring 2009 and Spring 2011. The VLA was in a hybrid condition in 2009, with some new EVLA receivers and some old VLA receivers. The characteristics of those images are slightly different from the older data, but for most purposes they should be equivalent. In 2011 the EVLA receivers were available with an early version of the new EVLA data system, so there are a number of differences from the old data: <pre> Date Frequencies Bandpass Integration Before 2011 1365, 1435 MHz 2x7 3-MHz channels 180 seconds 2011 1335, 1730 MHz 2x64 2-MHz channels 60 seconds </pre> Note particularly the frequency difference between the new and older data. The new data are in co-added fields with names ending with 'S' (and later letters in the alphabet) and are found entirely in the south Galactic cap. This table was last updated by the HEASARC in May 2015 based on the file: <a href="http://sundog.stsci.edu/first/catalogs/catalog_14dec17.bin.gz">http://sundog.stsci.edu/first/catalogs/catalog_14dec17.bin.gz</a> which contains the 17 December 2014 version of the FIRST Source Catalog. Some of the values for the name parameter in the HEASARC's implementation of this table were corrected in April 2018. This is a service provided by NASA HEASARC .
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