This section gives a very brief and simple outline of how to add an RA/DEC
calibration to a 2-dimensional image using GAIA .
Start up GAIA, including the name of the NDF containing the image
on the command line. For instance,
% gaia fred
will start up GAIA, displaying the image in file fred.sdf. If you
are using non-NDF images, include the file type (e.g. ``fred.fit'').
Press the Image-Analysis menu button. Select the
Astrometry calibration item from the displayed menu, then select
Fits to star positions from the next menu. This should display the
dialogue box shown in the next figure:
The main GAIA dialogue box used to create an RA/DEC calibration.
Indicate the co-ordinate system in which your feature positions are
expressed. Do this by specifying suitable values using the co-ordinate
system:, Equinox: and Epoch: buttons. You will usually have
either (FK4, equinox B1950) or (FK5, equinox J2000) positions. The epoch
defaults to B1950 for FK4 positions, and J2000 for FK5 positions.
Press the New button, to indicate that you want to add some
new positions to the (currently empty) list of reference positions. This
should produce the dialogue box shown in
the next figure:
The GAIA dialogue box used to give a new reference position.
Now press the Pick object... button. This will create the
dialogue box shown in the next figure.
The upper black box displays a section of the main image centred on the position
of the cursor. Position the cursor in the main GAIA window over a feature
(usually a star) for which you know the RA/DEC. This star should appear
in the box in the ``object picker'' dialogue box (Figure ).
If there are any other significant features visible in the box, you should
reduce the size of the box using the zoom-out button below the displayed
image section. Once you are happy with the
sample size, re-position the cursor over the star and press the left
mouse button. The pixel co-ordinates at the centre of the feature are
displayed in the Image X: and Image Y: fields within the
object picker dialogue box
The GAIA object picker dialogue box.
Raise the dialogue box shown in Figure . You should
find that the pixel co-ordinates of the star have been copied into the
X and Y fields. Click in the ra field, and enter the RA of the
star (e.g. "12:23:34.1"). Then do the same with the dec field. Then
click in the id field, and enter an integer value which can be
used to identify the star (for instance, use 1 for your first star, 2 for
your second, etc). The star is now fully specified, so press the
Enter button. After confirmation, this will add the object into
the list of reference objects in the dialogue box shown in
All the windows will remain open, so you can continue to enter new
reference stars in the same way. First click on the Pick Object
button in the dialogue box shown in Figure . Then position
the cursor over a star in the main window, click with the left button, enter
the RA, DEC and ID values for the star, and press Enter.
Once you have entered all your reference positions, close the
dialogue boxes shown in Figures and by
pressing the Close button in each one.
Press the Fit/Test button in the dialogue box shown in
Figure . This will calculate the best RA/DEC calibration
given your reference positions. It then uses this calibration to work out
the pixel co-ordinates corresponding to each of your reference positions,
and displays markers in the main image at these pixel co-ordinates. You
should find that they fall on top of the corresponding stars. If they do
not, you have probably entered a bad RA or DEC value. You should correct
the reference positions, and then press the Fit/Test button again. To
correct the reference positions, position the pointer over the erroneous
line in the list of reference positions, and click the left button (the
selected line will be highlighted). Then click the Edit button, the
dialogue box shown in Figure will re-appear, loaded with
the details of the selected feature. Correct the values, and then press
Enter, followed by Close. Now you can press Fit/Test
again to re-calculate the RA/DEC calibration.
Once you are happy with the calibration, press the Accept
button. You can now save the RA/DEC calibration by pressing the File
menu button in the main window, and then the Save as menu item. Use the
resulting dialog box to save the image (with the RA/DEC calibration) in a
new file. This is most simply done by entering the new file name in the
Selection box, and pressing OK. Do not worry if a message is
displayed saying that the WCS could only be saved as an AST native