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) ‘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. 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 8 for its provenance). The image contains a mixture of stars and
galaxies. Proceed as follows.
The object detection dialogue box (Figure 14) 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 15 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 4.2. 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 4.2). Catalogues saved in the FITS tables, TST or STL formats can subsequently be imported into CURSA (see SUN/190) which provides additional catalogue manipulation facilities.