This recipe shows how to use GAIA to automatically detect objects in an image. A catalogue of the objects detected is assembled, tabulating various properties for each object: position, flux, ellipticity etc. The catalogue can both be examined within GAIA and saved as a file for further analysis.
The object detection facilities of GAIA are provided by invoking the EXTRACTOR package (see SUN/226[3]) `behind the scenes', though you will just see seamless interaction via the GAIA windows. EXTRACTOR is a version of the source-extraction program by Bertin and Arnouts[2]. It is particularly well-suited to the analysis of large extragalactic survey data, but also performs well on other sorts of astronomical images.
The image used in this recipe is ngc1275jkt.sdf, a reduced V band
CCD image of NGC 1275 obtained with the JKT (see the recipe in
Section for its provenance). The image contains a mixture
of stars and galaxies. Proceed as follows.
The object detection dialogue box (Figure ) has
numerous options, but you do not need to adjust any of them; the
default settings will generate a reasonable catalogue.
After a few moments, ellipses corresponding to the objects found will
be drawn over the image in the main window and a dialogue box similar
to Figure will be created, showing the catalogue of
objects detected.
The catalogue created is automatically written as a file in the
EXTRACTOR `ASCII Header' format. It is also possible to save a copy
in one of the other catalogue formats described in Section .
Click on the File button on the left of the the menu-bar along
the top of the catalogue window and choose the Save as...
option. A window allowing you to specify the file-name required will
appear. The format in which the catalogue is saved depends on the
file-type specified at the end of the file-name (again see
Section
). Catalogues saved in the FITS tables, TST or STL
formats can subsequently be imported into CURSA (see
SUN/190[5]) which provides additional
catalogue manipulation facilities.
The GAIA Cookbook