In its initial five years, the FIRST project surveyed 4670 deg of the high Galactic latitude sky to a point-source flux density limit of 1.0 mJy at a wavelength of 20 cm. Using the B-configuration of the Very Large Array (VLA) in bandwidth-synthesis mode, we have produced nearly uniform-sensitivity images of the sky with a median rms of 0.14 mJy and an angular resolution of 5"; the positions for each of the half million sources in our current catalog are accurate to better than 1" (90% confidence). Details of the observing strategy and the pipeline processing system that creates the final images from the raw uv data are presented in Becker, White, and Helfand (1995; hereafter, BWH).
For the program described here, we have utilized the 98Feb04 version of the FIRST catalog, which is available from our Web homepage (http://sundog.stsci.edu). The catalog contains positions, peak and integrated flux densities, source morphological parameters, and information on the field from which each entry was derived. A detailed description of the catalog's construction can be found in White et al. (1997; hereafter WBHG); an exhaustive discussion of the astrometric, photometric, and morphological uncertainties is included in that paper and in BWH, and is summarized on our Web homepage.
The catalog has a mean surface density of 90 ``sources'' per square degree. However, it is important to note that a significant fraction of radio sources at these flux densities are extended. In particular, classical double-lobed radio sources in which the components are separated by more than 3" are typically represented by two catalog entries; for complex sources extended on scales of 1', several components can be required to fit adequately the source surface brightness distribution (see WBHG). We have not yet completed a definitive study of multi-component sources; even when this is done, it will not be possible to decide unambiguously whether or not two nearby catalog entries are parts of the same source hosted by a single optical counterpart, or whether they are chance alignments of unrelated objects. We briefly investigate these issues in § 5.4; prior to that discussion, we use the term ``sources'' synonymously with ``radio catalog entries''.
For clarity, we present in this paper only the results of the APM identification program for the 4150 deg in the north Galactic cap. This region has approximate boundaries of and and includes 382,892 radio sources. Subsequent extensions to this work for the two southern cap strips ( and in the range for a total of 610 deg) as well as the remainder of the north cap region will be added to the Web site as they become available.