Plotting the Guide Star Catalog on a Telescope Image
Using SAOimage and imgsc

This SAOimage display shows the alignment of HST Guide 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, saogsc: saoimage -reg `imgsc -r 15 $1` -wcscom sgsc_-ha_%s $1 where the argument to the task is the name of the file to be displayed. The immatch program will provide a list of the stars in the image which match catalog stars.

Click on the image for full-screen view

imgsc finds all of the HST Guide Stars in the image based on the world coordinate system parameters in its header and writes them to the region file testx90mw.fits.gsc, 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.

# HST Guide Stars (15" radius) CIRCLE(662,661,23) # gsc 2580.1696 CIRCLE(738,763,23) # gsc 2580.1207 CIRCLE(961,978,23) # gsc 2580.1403 CIRCLE(285,620,23) # gsc 2580.2291 CIRCLE(530,33,23) # gsc 2580.1629 CIRCLE(210,866,23) # gsc 2580.1158 CIRCLE(527,446,23) # gsc 2580.1710 CIRCLE(949,475,23) # gsc 2580.1798 CIRCLE(33,920,23) # gsc 2580.2147 CIRCLE(901,395,23) # gsc 2580.1909 CIRCLE(818,653,23) # gsc 2580.1943 CIRCLE(590,526,23) # gsc 2580.1485 CIRCLE(231,195,23) # gsc 2580.1878

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:

sgsc 2.9.4, 29 June 2001, Doug Mink SAO  Find closest star
HST GSC   16:15:56.457 +31:24:24.17 J2000 r= 900.00 at epoch 2000.00
GSC number  RA2000       Dec2000      Mag  Class Band N  Arcsec
2580.1710 16:15:56.083 +31:24:24.44  14.13    3   14  1    4.79