Using SETSKY to Add a Celestial Co-ordinate Frame to an NDF

As mentioned in the previous section, the SETSKY application stores astrometry information within an NDF in the form of either a WCS component or an IRAS90 astrometry structure.

To use SETSKY, you need to know the celestial co-ordinates at a set of points within the image. You may be able to find these by comparing your image with other images, such as those available from the Digitised Sky Survey, which already have astrometry information associated with them. You create a text file holding the pixel and celestial co-ordinates at a single position on each line. For instance, if you have five known RA/DEC (B1950) positions in your image, the file may look like:

 0 49 05.9,   42 25 30,    32,    266
 0 48 31.7,   40 03 36,    39,    29
 0 37 03.0,   40 04 48,    258,   31
 0 36 54.6,   42 26 47,    257,   268
 0 45 47.7,   41 54 03,    93,    213

The first column gives the RA values (hours, minutes and seconds), the second gives the DEC values (degrees, arcminutes and arcseconds), the third gives the pixel X co-ordinates, and the fourth gives the pixel Y co-ordinates.

If this file is called pos.dat, then the following command can be used to create a WCS component:

   % setsky m31 ^pos.dat coords='equ(b1950)' epoch=1998.0 projtype=gno

     Trying GNOMONIC projection...

     These parameter values give an RMS positional error of 0.3647723 pixels ...
       Projection type                      : GNOMONIC
       Sky co-ordinates of reference point  : 0h 40m 31.29s, 42d 37m 53.15s
       Image co-ordinates of reference point: (190.3287,285.795)
       Pixel dimensions                     : (36.04451,36.00686) arcsecs
       Position angle of image Y axis       : 359d 38m 35.77s
       Tilt of celestial sphere             : 0d 0m 0.00s

Note the up-arrow character ("^") before the file name (pos.dat). This tells SETSKY that the string is a file name. A gnomonic (or tangent plane) projection was requested using the PROJTYPE parameter. If no projection type is specified then SETSKY will try four different projections (gnomonic, Aitoff, Lambert equivalent cylindrical, and orthographic) in turn, and choose the one that gives the smallest RMS position error.

An alternative way to add a celestial co-ordinate Frame  to an NDF is to use the facilities of GAIA (see SUN/214). This provides much more sophisticated facilities.