Help for FIRST Cutout Extractions

The FIRST cutout page allows you to extract a section of an image from the FIRST 1400 MHz sky survey. The images have 1.8 arcsec pixels with a FWHM resolution of 5.4 arcsec.

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.

The FIRST survey covers two regions:

North: RA = ~8^h to ~17^h, Dec = -8° to +58°
South: RA = 21^h20^m to 3^h20^m, Dec = -2.5° to +1.6°

The edges of the covered areas are a little ragged, but the coverage is now continuous with no significant embedded holes. For more details of the sky coverage, see the FIRST coverage maps. There are 34000 images comprising about 250 Gbytes of data. All of these images are available on our image server.

Cutout Extraction Form Parameters

The form allows you to specify the position and size of the extracted image, the type of the returned image (FITS or GIF), and, for GIF images, the intensity scaling used to display the image.

RA, Dec, Equinox
Specify the central position of the image in either J2000 or B1950 coordinates. The positions can be in a variety of formats; for example, acceptable formats include
- hh mm ss.s dd mm ss.s
- hh.hhh +dd.ddd
- hh mm.mm -dd mm.mm
- hh:mm:ss.s dd:mm
and variations on these, where hh, mm, and ss indicate the hours, minutes and seconds of RA and dd indicates degrees of Declination. If the RA and Dec are not fully specified to the seconds in the combined RA/Dec field then either a sign must be given for the Declination (even if it is "+") or colons must be used as field separators. For fully specified RA and Dec values or separate RA/Dec fields, the sign is optional.
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".
Image Size
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.
Image Type
There are 3 choices for the format of the returned image:
- GIF -- The image will be displayed within your Web page. This is useful for visual examination, but the resulting image cannot be used for scientific analysis. Some useful information about the image is also included.
- FITS Image -- The image is returned to the browser with a MIME type of 'image/x-fits'. You will need to configure your browser so that it can display a FITS images. Note that xv is not a very useful FITS viewer for these images because they have a high dynamic range. Saoimage or FITSview are better choices.
- FITS File -- This option also produces a FITS file as output, but the file is written to your disk (in a location you select) rather than being displayed by some image display helper. This is the best way to get an archival copy of your cutout.
Maximum Intensity for Scaling
If the image is returned as a GIF image, this parameter determines the contrast enhancement. FIRST images have a large dynamic range, typically a factor of 1000 or greater. A linear mapping of color to pixel value often allows only the brightest objects to be visible. This parameter sets the saturation level (above which all pixels appear white) in milliJanskys. If the objects of interest and the sky background appear too faint, reduce this value to a smaller number. For very high contrast, try 2 mJy. If this field is blank, the image will be displayed with a scale set by the brightest pixel in the image.