Example AST file

Finally, we give an example of an AST file which has been constructed for a real instrument. The instrument is the Wide Field Camera on the Isaac Newton Telescope at La Palma, and consists of four CCDs each 2048 x 4096 pixels. Because the area occupied by detectors is large, optical distortion effects become quite important near the edges. After registering four images sb1,sb2,sb3,sb4, which included taking account of a pincushion distortion, the file was constructed by the following command:

% astexp astfile=INT-WFC.ast in='"sb[1234]"' outdomain=INT-WFC
         outtitle='"INT wide field camera undistorted"'
         idtype=fitsid fitsid=chipname fitsrot=rotskypa accept

The OUTDOMAIN and OUTTITLE parameters write labels indicating the source of the coordinate information. The IDTYPE=FITSID and FITSID=CHIPNAME parameters indicate that the chip which each one comes from is to be distinguished by the value of its ``CHIPNAME'' FITS header card, and the FITSROT=ROTSKYPA parameter indicates that the ``ROTSKYPA'' FITS header card contains an angle in degrees through which the telescope has been rotated. Applying this to a set of files from the same instrument has the following effect:

% astimp 'o1059??' astfile=INT-WFC.ast accept

    ASTIMP
    ======
  Framesets read from file INT-WFC.ast:
    FITS header "ROTSKYPA" used for rotation

     N    Base domain         Current domain      Frameset ID
     --   -----------         --------------      -----------
     1    PIXEL               INT-WFC             FITSID CHIPNAME 'A5506-4'
     2    PIXEL               INT-WFC             FITSID CHIPNAME 'A5383-17-7'
     3    PIXEL               INT-WFC             FITSID CHIPNAME 'A5530-3'
     4    PIXEL               INT-WFC             FITSID CHIPNAME 'A5382-1-7'
  4 NDFs accessed using parameter IN

  Processing NDF /local2/data/register/intastgen/o105952
    Matched with frameset ID "FITSID CHIPNAME 'A5506-4'"
    Rotating additional 180 degrees
    New frame in domain "INT-WFC" added

  Processing NDF /local2/data/register/intastgen/o105953
    Matched with frameset ID "FITSID CHIPNAME 'A5383-17-7'"
    Rotating additional 180 degrees
    New frame in domain "INT-WFC" added

  Processing NDF /local2/data/register/intastgen/o105954
    Matched with frameset ID "FITSID CHIPNAME 'A5530-3'"
    Rotating additional 180 degrees
    New frame in domain "INT-WFC" added

  Processing NDF /local2/data/register/intastgen/o105955
    Matched with frameset ID "FITSID CHIPNAME 'A5382-1-7'"
    Rotating additional 180 degrees
    New frame in domain "INT-WFC" added

Since they now share a coordinate system they can now be, for instance, aligned with other images from the same instrument or resampled directly.

The AST file can be found in $CCDPACK_DIR/INT-WFC.ast. It is thought to be correct to an accuracy of 1 or 2 pixels, which is around half an arcsecond, over the whole focal plane.