User manual

Calibration by WCS header
The third board displays FITS keywords
giving the parameters for the calibration in
the WCS standard. If you know this standard,
you can directly modify these parameters. It
is for example possible to use the
astrometrical solution from another image
directly by copy/paste of its WCS
parameters.
Tip: It is also possible to modify a value in a
WCS keyword via the script command: set
PlanID FITS:keyword=value (see 7.1
– Aladin by script)
Calibration by matching stars
The second board displays a table used to write a list of XY image coordinates and their
corresponding celestial coordinates. It is a powerful and flexible method to quickly
obtain excellent calibrations. We will briefly describe this method here. For more details,
please refer to the online tutorial dedicated to this manipulation (http://aladin.u-
strasbg.fr/tutorials/aladin.gml).
Let’s assume that we loaded in the left view, a JPEG image that is not calibrated, and a
DSS image of the same field, with an astrometrical catalogue like UCAC2 overlaid in the
right view. The method to calibrate the JPEG image is to click successively on a star in
the non-calibrated image, and then on the corresponding UCAC2 source. For each click
with the mouse, Aladin fills the table in the calibration board. Four corresponding
measures are usually enough to get a good calibration.
Note: When one needs to get the coordinates for a start by clicking with the mouse,
Aladin uses an algorithm with a barycentre centroid. If it is not possible to determine the
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