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
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:
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
There are 30279 images comprising about 210 Gbytes of data.
All of these images are available on our image server.
North: RA = ~8h
to ~17h, Dec =
-8° to +58°
South: RA = 21h20m
to 3h20m, Dec =
-2.5° to +1.6°
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
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".
- hh mm ss.s dd mm ss.s
- hh.hhh +dd.ddd
- hh mm.mm -dd mm.mm
- hh:mm:ss.s dd:mm
- 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.
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
- 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