13 Automatic Object Detection

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 8 for its provenance). The image contains a mixture of stars and galaxies. Proceed as follows.

(1)

Start GAIA and load image ngc1275jkt.sdf.

(2)

You may wish to adjust the appearance of the displayed image. The most likely items to change are the colour table (click on the Color Map: button in the lower right of the control panel in the centre top of the window) and the magnification (click on the Scale: button in the bottom left of the control panel).

(3)

Click on the Image-Analysis button on the menu-bar along the top of the main dialogue box and choose the Object detection… option. A dialogue box similar to Figure 14 should appear.

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Figure 14: The object detection dialogue box

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(4)

To generate a catalogue of the objects in the image simply click on the Detect objects button.

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.

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Figure 15: Window showing a catalogue of automatically detected objects

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(5)

The catalogue dialogue box (Figure 15) has various options for manipulating the catalogue, such as sorting and selecting objects; the on-line help gives the details.

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[5]) which provides additional catalogue manipulation facilities.