Image analysis capabilities

GAIA provides the following features, which are specifically aimed at astronomical work with images:

• Aperture & optimal photometry. A highly interactive environment for controlling the positions, sizes and orientations of circular and elliptical apertures. The sky estimates can be made from annuli of these apertures or from related sky apertures. The measurements can be in either instrumental magnitudes or mean counts (based on the PHOTOM - SUN/45 - program AUTOPHOTOM)

• Automatic source detection. This feature provides the ability to automatically detect and parameterise all the objects on an image. These objects are then identified by displaying ellipses over the image and are available for interactive inspection. The resultant measurements are displayed in a catalogue window which can be used to inspect the individual values, select data on the bases of range limits and change the appearance of the ellipses. This is based on the EXTRACTOR (SUN/226) package, a derivative of SExtractor.

• Extended surface photometry. This allows the interactive identification of extended objects (galaxies) and profile measurements using ellipse fitting. The resultant fits are shown over the images and the measurements can be inspected (based on the ESP - SUN/180 - programs ELLPRO and ELLFOU).

• Image patching. This provides the ability to select arbitrary shaped regions on an image and replace them with a surface fit to other regions, together with an artificial noise component. An ideal way to remove unwanted defects from an image for cosmetic reasons.

• Interactive ARD (SUN/183) regions. This allows the calculation of statistics, the masking out and extraction of arbitrary shaped parts of your images as well defining ARD regions for other programs to use.

• Contouring. This allows the contouring of the displayed image, or the overlay of contours derived from other images. The images are aligned using sky coordinates, if available. Control over individual line colours and thicknesses is provided.

• Polarization vector plotting and manipulation. This provides the ability to display vector maps produced by the POLPACK (SUN/223) package. It also allows you to select sub-sets of vectors, either by algebraic expressions or by rectangular regions. Selected parts can be removed or saved to new catalogues. New vector maps can be created by binning vectors and reports of the properties of selections can be displayed.

• Blink comparison. All the displayed images can be animated (as quickly as your hardware/CPU combination allow), or you can cycle through them by hand.

• Interactive position marking: Mark and label positions on your images and print or record them for future reference. This also allows positions to be read in from text files and input sky coordinates to be transformed to different celestial coordinate systems. Mean seeing and shape parameters of any marked stars are also available as part of this analysis.

• Celestial coordinates readout. If your data have suitable astrometric calibration data available (in recognised headers i.e. FITS/IRAF or NDF WCS) then a continuous readout of the current RA and Dec is provided.

• Astrometric calibration. You can either fit a new astrometric calibration to your image (using semi-automated matching between detected images and on-line catalogues or you can define your own reference positions), set one using known information (such the image scale and a reference point), copy one from another image and finally tweak one to make it fit better.

• Astrometric grid overlay. If you can read out sky coordinates then you can also plot a grid overlay with labelled axes. The grid facility is provided with an enormous range of customization options, such as being able to change the celestial coordinate system (from say FK5 to Galactic etc.).

• Celestial coordinate system control. You can change the celestial coordinate system used by your image to a new one (to/from FK5, FK4, Ecliptic, Geocentric apparent, Galactic and SuperGalactic).

• Sky coordinate offsets. This feature also relies on being able to read celestial coordinates, if so then you can determine the distance and offsets between any two points on your image (in arc minutes).

• Real time profiling. Two separate toolboxes allow you to either move a line around on the image and see the image data values displayed as a profile change (this profile can also be saved as a spectrum), or do the same with a rectangle, in which case the mean profiles along the X and Y directions are shown.

• Object parameterization. This allows you to select a single object on the image. Details such as its full width half maximum, peak intensity etc. will be shown. These values are based on a 2D gaussian fit.