Why might you want to obtain FIRST images rather than rely on the FIRST catalog for source information? The catalog is only complete to 5 times the map rms noise level (typically 0.75 mJy), so even in cataloged parts of the sky, faint uncataloged sources may be found. Also, while the elliptical Gaussian source models used in the catalog do a reasonably good job of capturing the structure of extended sources, complex sources have structure than can only be fully appreciated by examining the original image data. Finally, faint cataloged sources are sometimes sidelobes from nearby bright sources. While we attempt to flag such sources in the catalog, the human eye is still better at recognizing spurious sources.
Currently the FIRST survey covers two regions:
For expert users that are writing scripts to access the FIRST search engine, the combined RA+Dec field is passed in a parameter named "RA". If the RA and Dec are available as separate strings then they can be passed in the separate CGI parameters "RA" and "Dec".
A square image of the specified size (in arcminutes) is extracted from
a FIRST image. A typical 5 arcmin image will be 167x167 pixels in
size. The corresponding GIF image will be about 18 kbytes; a FITS
image will be about 121 kbytes. (See
below for information about image formats.)
The largest cutout that can be extracted is 1024x1024 pixels (about 30x30
arcmin). Large images are likely to extend past the edge of the enclosing
FIRST map, so that part of the image is blank. This does not necessarily
mean that the blank area has not been observed; try extracting another
image at a nearby position to see the adjacent field. Note that, as is
typical in astronomical images, the Right Ascension increases from right
to left (opposite the usual direction for the x-axis). The
Declination increases from bottom to top.
There are 3 choices for the format of the returned image:
