Plotting the GSC II Catalog on a Telescope Image
Using SAOimage and imgsc2

This SAOimage display shows the alignment of GSC-ACT stars with a test image after a world coordinate system has been fit. The coordinates plotted are based on header keywords describing a plane-tangent system fit by matching HST Guide stars and sources found in the image. This image was produced by a one-line Unix shell script for this task, saogsc2: saoimage -reg `imgsc2 -r 10 $1` -wcscom sgsc2_-ha_%s $1 where the argument to the task is the name of the file to be displayed. The immatch program can provide a list of the stars in the image which match catalog stars.

Click on the image for full-screen view

imgsc2 finds all of the GSC-ACT stars in the image based on the world coordinate system parameters in its header and writes them to the region file testx90mw.fits.gsc2, seen on the right. The name of this file is sent to standard output which the backquotes give to SAOimage as a command line argument following the -reg command.

# GSC 2.2 (10" radius) CIRCLE(526,447,16) # gsc2 N1322311422 CIRCLE(738,763,16) # gsc2 N1322311398 CIRCLE(662,661,16) # gsc2 N1322311404 CIRCLE(284,619,16) # gsc2 N1322311410 CIRCLE(529,33,16) # gsc2 N1322311440 . . . CIRCLE(976,418,16) # gsc2 N132231113128 CIRCLE(299,207,16) # gsc2 N132231112796 CIRCLE(312,52,16) # gsc2 N132231112544 CIRCLE(675,783,16) # gsc2 N132231113866 CIRCLE(513,673,16) # gsc2 N132231113622

If the w key is struck when the SAOimage cursor is centered on the galaxy in the middle of this picture, the following appears on the user's terminal:

sgsc2 2.9.4, 29 June 2001, Doug Mink SAO  Find closest star
     gsc2  16:15:56.306 +31:24:24.82 J2000 r= 120.00 at epoch 2000.00
GSC II number  RA2000       Dec2000      MagF  MagJ  MagV  MagN  Arcsec
N1322311422   16:15:56.179 +31:24:24.56 10.70 11.00 99.90 99.90    1.64