NAME:
  optaxis
 PURPOSE:   (one line only)
  Search for the optical axis in an astronomical image
 DESCRIPTION:
  This routine only makes sense if you need a non-linear solution
    for your astrometry from an image.
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  optaxis,fn,terms,xopt,ypot,raopt,decopt
 INPUTS:
  fn - file name of image to analyze
          This is a valid name that will lead to finding other related
             files based on this name.  The name is typically of the form
             root.NNN where NNN is a three digit number.  In this case,
             the ref file is NNN.ref and is found in the Refstars
             directory relative to the current directory.
  terms - string array with the names of terms to fit (see astterms.pro)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  xopt  - X coordinate of the optical axis in native pixel coordinates.
  yopt  - Y coordinate of the optical axis in native pixel coordinates.
  raopt - (radians) Right Ascension of the position (xopt,yopt)
  decopt - (radians) Declination of the position (xopy,yopt)
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2009/12/08