\subsection{\xlabel{image_analysis_capabilities}Image analysis capabilities}
GAIA provides the following features, which are specifically aimed at
astronomical work with images:
\begin{itemize}
\item 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 - \xref{SUN/45}{sun45}{} -
  program \xref{AUTOPHOTOM}{sun45}{AUTOPHOTOM})

\item 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 (\xref{SUN/226}{sun226}{}) package, a derivative of
  SExtractor.

\item 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 - \xref{SUN/180}{sun180}{} - programs
  \xref{ELLPRO}{sun180}{ELLPRO} and \xref{ELLFOU}{sun180}{ELLFOU}).

\item 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.

\item Interactive ARD (\xref{SUN/183}{sun183}{}) 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.

\item 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.

\item Polarization vector plotting and manipulation. This provides the
 ability to display vector maps produced by the POLPACK
 (\xref{SUN/223}{sun223}{}) 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.

\item 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.

\item 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.

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

\item 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.

\item 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 {\em etc.}).

\item 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).

\item 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).

\item 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.

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

