Abstract

The FIRST survey to produce Faint Images of the Radio Sky at Twenty-cm is now underway using the NRAO Very Large Array. We describe here the scientific motivation for a large-area sky survey at radio frequencies which has a sensitivity and angular resolution comparable to the Palomar Observatory Sky Survey, and recount the history that led to the current survey project. The technical design of the survey is covered in detail, including a description and justification of the grid pattern chosen, the rationale behind the integration time and angular resolution selected, and a summary of the other considerations which informed our planning for the project. A comprehensive description of the automated data analysis pipeline we have developed is presented.

We also report here the results of the first year of FIRST observations. A total of 144 hours of time in April and May of 1993 were used to cover a strip extending between and in a 2.8 degree-wide declination zone passing through the local zenith (). A total of 2153 individual pointings yielded an image database containing 1039 coadded images in extent with pixels and a typical rms of 0.13 mJy. A catalog derived from this 300 region contains 28,000 radio sources. We have performed extensive tests on the images and source list in order to establish the photometric and astrometric accuracy of these data products. We find systematic astrometric errors of ; individual sources down to the 1 mJy survey flux density threshold have 90%confidence error circles with radii of . CLEAN bias introduces a systematic underestimate of point source flux densities of mJy; the bias is more severe for extended sources. Nonetheless, a comparison with a published deep survey field demonstrates that we successfully detect 39/49 sources with integrated flux densities mJy, including 19 of 20 sources above 2.0 mJy; the sources not detected are known to be very extended and so have surface brightnesses well below our threshold.

With the 480 hours of observing time committed for each of the next three B-configuration periods, FIRST will complete one-half of its goal of covering the 10,000 of the north Galactic cap scheduled for inclusion in the Sloan Digital Sky Survey. All of the FIRST data - raw visibilities, self-calibrated UV datasets, individual pointing maps, final coadded images, source catalogs, and individual source images - are being placed in the public domain as soon as they are verified; all of the 1993 data are now available through the NRAO and/or the STScI archive. We conclude with a brief summary of the scientific significance of FIRST, which represents an improvement by a factor of 50 in both angular resolution and sensitivity over the best available large area radio surveys.