[AstroPy] Past Mention of Source Extractor Command in IRAF?
erwin at mpe.mpg.de
Wed Apr 29 00:58:41 EDT 2009
On Apr 29, 2009, at 2:25 AM, Wayne Watson wrote:
> Probably not, but is what you mentioned source extraction? The idea is
> that you take a picture of some part of the night sky, and then turn
> over to the source extractor. It twists, rescales, and turns your
> every which way until it matches 10 or more corresponding objects
> (stars, etc.) in a photographic atlas of some sort. If it looks right,
> you can then extract information about those objects and maybe others.
> It's an astrometric tool.
This sounds like a combination of two or three things: source
catalog matching, and generating astrometric solutions. I can't quite
from your description how much of each you're interested in. But here's
1. Source extraction: this refers to a program which analyzes a FITS
finds "sources" -- places that are brighter than the background, such
and galaxies (but hopefully not random noise spikes). It generates a
lists each source, its position, and various measurements derived
directly from the image (brightness, size, shape, orientation, etc.).
It knows nothing
about other atlases or catalogs; it just reports what it can measure
on that particular
Note that the position that the source extraction program reports are
on the image in pixel coordinates (x, y). If the FITS image happens
astrometric information in the header already (see below), then it
will also calculate and
report the sky coordinates (RA, Declination) for each source. The
coordinates will be as good or bad as the astrometric information in
header. A source extraction program does not do astrometry by itself
at all; what
it does do is generate lists of sources which can then be used in the
As others have pointed out, there are some tasks within IRAF that will
There is also the standalone program SExtractor, which is designed to
from the command line (Unix/MacOS X, or Cygwin).
2. Catalog matching: this means taking each source in the extracted
identifying it with a known astronomical object, such as a star in a
Generally, this requires having good sky coordinates for the image.
If you *don't* have sky coordinates for the image, then you may need
to match a few
objects (e.g. three or four bright stars) by hand (e.g., compare your
image with a sky
atlas to figure out which star is which, then copy the RA,Dec
coordinates by hand). This
will give you a starting list for making an initial astometry
solution. From there,
you can repeat the process (using the image with updated headers) and
get a better
astrometry solution using the full list of sources from the source
3. Generating an astrometry solution: this means taking a matched list
coordinates in your image -- that is, a list of objects for which you
on the image (pixel coordinates x, y) *and* their correct positions on
(e.g., RA, Declination from a catalog) -- and then finding a general
maps pixel coordinates (x,y) to sky coordinates (RA, Dec). The
is then stored in the FITS header, so that other programs can read it
pixel coordinates (x,y) to sky coordinates (RA,Dec) automatically.
There are tasks within IRAF which do this; there is also a standalone
called SCAMP (designed to work with catalogs produced by SExtractor).
The astrometry.net project is intended to be a sort of all-in-one
solution for Step 3,
doing Steps 1 and 2 automatically along the way as needed. If it
works out as
planned (and it sounds like they're making progress), it will probably
end up being
all you (or even most of us!) ever need. I'm impressed as hell with
But at the moment it's in an alpha stage; I've never tried it, the web
registering as an "alpha tester", and it looks like installing it on
your own machine
would require several gigabytes of disk space.
In the meantime, you might also look at the programs here:
Peter Erwin Max-Planck-Insitute for Extraterrestrial
erwin at mpe.mpg.de Physics, Giessenbachstrasse
tel. +49 (0)89 30000 3695 85748 Garching, Germany
fax +49 (0)89 30000 3495 http://www.mpe.mpg.de/~erwin
More information about the AstroPy