### 5 Starting GAIA

If you are working at a Starlink site then GAIA should automatically be available to you, provided that your account is set up to access Starlink software, which will usually be the case. No special quotas or privileges are required to run GAIA. However, it must be run on a workstation console or X-display capable of receiving X-output. In practice GAIA requires a colour display (strictly speaking it will run on a black and white one, but any image displayed is not visible).

GAIA is usually run from the Unix shell. Simply type:

%  gaia &

(The ‘&’ merely makes gaia run as a detached process so that you can, if you wish, continue to issue Unix commands from the command line.) After a few moments the main GAIA window will appear.

Now click on the File menu, which is the leftmost item in the menu-bar along the top of the window. Click on the Open… item. A file-picker window appears which allows you to choose the image file to be displayed. The final appearance should be similar to Figure 1. See the recipe in Section 8 for further details.

Alternatively, the required file can be specified on the command line when GAIA is started:

% gaia file-name  &

for example:

%  gaia ngc1275jkt.sdf &
%  gaia ngc1275hri.fits &

The first example is a Starlink NDF file, the second a FITS file (see Section 4 for the data formats accessible to GAIA).

If you have already started GAIA and want to display a different image from the Unix command line then type:

% gaiadisp file-name

It is also possible to plot an image in a given window by specifying its ‘clone number’:

% gaiadisp file-name  clone-number

for example:

The plot will be displayed in a window titled ‘GAIA::Skycat:file-name(2)’. If this window does not exist then it will be created.

#### 5.1 Using GAIA from IRAF

GAIA can be run from the IRAF command language, cl (see SG/12[15] for an introduction to IRAF on Starlink systems). However, before this option is available the GAIA package must have been included in the version of IRAF that you are using. This customisation will usually already have been performed if you are using IRAF at a Starlink site.

Assuming that the IRAF GAIA package is available, your first step must be to initialise it. From the IRAF cl type:

cl>  gaia

(Alternatively, if you think that you will often want to use GAIA from within IRAF, then you could include this step in your IRAF login.cl file.)

To display an image type:

cl> gaiadispimage  plane-number

image is the image to be displayed and plane-number is the number of the IRAF display plane in which the image is to appear (which is identical to the GAIA clone number). For example:

will display the default IRAF image in display plane 1. The image will either be displayed into an existing GAIA window, or a new window will be created, as required. Note that because GAIA is not a native IRAF image display task it does not support any of the IRAF cursor commands.

#### 5.2 Multiple image files and image sections

Some data formats, notably FITS and NDF, can include more than one image in a single file. If you encounter such a file then you need to specify which image is to be displayed. In a FITS file each image occurs in a separate ‘extension’, which is identified by a sequential integer number. To display a FITS extension image, either open the disk file and choose the extension from the ‘HDU’ selector window that appears (HDU or ‘Header and Data Unit’ is FITS jargon for an image and its associated auxiliary information), or add the extension number to the disk-file name:

% gaia mef_file.fits’[2]’

(the quotation marks embedded in the file-name in this example are to prevent the square brackets from being interpreted by the Unix shell). The first image in the FITS file is called the ‘primary array’ and is numbered 1.

A similar mechanism exists for NDFs stored in container files at other than the top-level (‘container file’ is NDF jargon for a file which contains one or more NDFs):

% gaia hdscontainer.ndf_1

In this case any other NDFs stored at the same level in the container file will also be shown in a selector window. It is also possible to access an ‘NDF slice’, that is, a portion of an image rather than the whole thing (this facility can be useful for very large images). For example type:

% gaia hdscontainer.ndf_1’(200:500,100:700)’

See SUN/33[19] for further details of specifying NDF slices. This notation can also be applied to FITS files and other ‘foreign’ formats:

% gaia file.fits’(300:700,300:700)’

Note, however, that here the FITS file will now be accessed as a foreign format; that is, it will automatically be converted to an NDF prior to being read.

#### 5.3 Demonstration mode

GAIA has an automatic ‘demonstration mode’ which shows many of its facilities. To run this demonstration: move to an empty directory and start GAIA as described above. Then click on the Image-Analysis menu in the menu-bar at the top of the window and choose the Demonstration mode… item. A window will appear asking if you wish to copy the files for the demonstration into your current directory. Click Yes. A further window with preliminary information about the demonstration will appear. Click on the Start button. The demonstration will now run completely automatically, without further intervention. Annotation describing the facilities being demonstrated will appear in the information window as the demonstration proceeds.

Note that the demonstration contains flashing images: abort it if these might affect you.