### 3 Facilities Available

This section outlines the facilities available in GAIA. The hypertext version of the cookbook contains links to ‘outline recipes’ which summarise how to use these facilities. These outline recipes complement the smaller number of detailed recipes in Part II of the cookbook. The facilities provided by GAIA divide naturally into three areas:

• image display,
• image analysis,
• remote access to catalogues and image databases via the Internet.

The facilities available in each of these three areas are briefly outlined below.

#### 3.1 Image Display

Displaying images
Opening files, displaying more than one image, maximising the display area.
Controlling the view
Changing the zoom, position, detail and colours.
Inspecting image values
Panel readouts, interactive slices, pixel table.
Overlay graphics
Creating, resizing, moving, deleting, saving and restoring.
Printing
Image and overlay graphics, colour ramp.

#### 3.2 Image Analysis

Astrometry
applying and displaying celestial coordinates to an image.
Photometry
interactive measurements of selected stars using a circular or elliptical aperture.
Automatic object detection
automatic detection of the objects in an image. A set of parameters (positions, brightness, ellipticity etc) are computed for all the objects detected.
Surface photometry
Surface photometry of galaxies, including determining radial profiles and fitting elliptical isophotes.
Image patching
interactive removal of image blemishes and contaminating objects from an image. For example, foreground stars might be removed from a galaxy image.
Properties of image regions
statistics, extraction and removal of shaped regions of an image.
Image comparison
two images can be compared by either plotting one as a set of contours superimposed on the other or by blinking between the two images (cf the ‘blink comparator’ used to compare two photographic plates).
Object position and statistics
two-dimensional Gaussian fits to single or multiple objects.
Polarisation plotting
polarisation maps can be plotted, regions selected from them etc. These facilities are intended to be used in conjunction with the POLPACK package for polarimetry and spectropolarimetry (see SUN/223[1]).
Profiles of rectangular regions
Interactive averaged profiles along the $x$ and $y$ axes of a rectangle.

#### 3.3 Catalogues and Online Resources

Catalogues and image stores
Access to remote on-line catalogues, image stores and archives via the Internet. Local catalogues can also be read and written.