Automatic Object Detection Toolbox

Detecting the objects on your image

Just press the "Detect objects" button. This will by default find all the objects, create a catalogue of their positions, intensity and shape parameters, display the catalogue and finally get the catalogue window to draw some detection ellipses.

You can now use the catalogue window to investigate the results (see the help there for making selections).

Changing the detection limits, etc. etc. etc.

The parameters shown in the various "pages" of the toolbox, control the values used in the detection and analysis of the objects on your image. These are directly related to the parameters used by the SExtractor program, the main and associated documentation (principally SUN/226 and MUD/165) should be consulted for their exact meaning.

Usually the meaning is obvious from the description in the short help (displayed at the bottom of the window), so you can proceed, at least initially, from just these.

Configuring the default parameters

When the toolbox starts up it configures itself by reading files called "" and "default.param". To locate these files it searches the following directories:

  1. The current directory.
  2. The config/ directory of the SExtractor program installation.
  3. The directory of the SExtractor program.
If they are not found then built-in defaults are used. So you may configure the default setup by changing these two files and placing them in the current directory. Note, however, that the interface itself will update these two files (unless you've changed the names to be used) whenever a detection pass is made (i.e. you press the "Detect objects" button), so they always reflect the current state of the interface of the last detection pass, except for the case of when the toolbox is first created. To read in a set of default parameters after the toolbox has been created use the options found in the file menu.

Running SExtractor/EXTRACTOR directly using the values in the toolbox

When it runs the toolbox updates the "" and "default.param" files (unless you've changed in names used). These can then be used to run SExtractor or EXTRACTOR. You can also use the File menu items to save the program and catalogue parameters to other files. So you can use the GAIA interface to fine tune the detection parameters for an image and then use these to do the detections on other images.

Making detections on one image and measurements on another

The toolbox always makes its measurements on the displayed image. If you want to use another image to do the initial detections then just enter its name in the "Detection image:" entry field.

Stopping the plotting of detection ellipses

If you want to stop plotting detection ellipses, then look under the "Options" menu and de-select the currently highlighted option.

Getting the same ellipses as SExtractor draws

The CHECK_APERTURES option (identified as the Aperture limits (fixed & Kron) option) produces ellipses which are the same as GAIA's Kron ellipses. Select these in the Options menu. Note that the default isophotal detection ellipses drawn by GAIA are based on the ISOAREA_IMAGE, ELONGATION and THETA_WORLD parameters, the ellipses have area ISOAREA_IMAGE.

Getting a printout

To get a hardcopy of your image plus detections use the "Print..." item under the "File" menu in the main image window. Remember you only get a copy of what is visible, so if you have a large image unzoom it until it is all visible.

If you want to draw axes around your image then see the "Overlay axes grid..." item under the "Image-Analysis" menu in the main window.

Using a different version of SExtractor

To use your own version of SExtractor you need to define the environment variable EXTRACTOR_DIR to point to the directory that contains your "sex" program. Note this directory, or a sub-directory of it called "config" should contain the default SExtractor configuration files.

The toolbox was developed using a Starlink modified SExtractor version 2.2.1. Later versions may not be supported.

Using EXTRACTOR on my FITS image

By default the toolbox will use the native version of SExtractor on FITS files. To stop this just open the "Options" menu and de-select the "Use Native SExtractor" item. You may need to do this if you have an image with a DSS astrometric solution and you want to measure sky coordinates.

Viewing stars and galaxies in different colours

To view your catalogue using a cut in the star/galaxy measurement parameter you need to use the catalogue window controls.

One way to do this is to sort by the "CLASS_STAR" column and just drag out all the objects you want to see (these will then be highlighted in the main window). The other way to do this is to copy the catalogue and open it twice in local catalogue windows. You can then use the catalogue window methods for selecting rows with values between given ranges (see the help in the catalogue window for how to do this).

Changing the output catalogue file type

The file types output by the toolbox are restricted to the following:

for FITS-tables (SExtractor FITS_1.0)
for plain ASCII tables (SExtractor ASCII_HEAD)
for tab-tables, the "native" format.

Reading back and re-displaying a catalogue

To read back a catalogue and display it as a series of detection ellipses you need to use the "Read old catalogue..." item in the "File" menu.

You can also read back catalogues as using the "Data-Servers" - "Local Catalogues" item on the main window. However it is unlikely that you'll get suitable ellipses drawn (this depends on whether your "skycat.cfg" file has been updated for this catalogue).

Changing the colour of the ellipses

The "Options" - "Set Plot Symbols..." item in the catalogue window creates a dialog window in which you can define many things about the properties of any shapes that are drawn (these are Tcl expressions of the column names). To change the colour just choose a new one and apply it.