NAME:
  mkastinfo
 PURPOSE:   (one line only)
  Generate a simple astrometric description of an image
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  mkastinfo,racen,deccen,xcen,ycen,scale,info
 INPUTS:
   racen  - Right ascension of the center of the array, also assumed to
              be the tangent plane point for the xi,eta to ra,dec
              transformation.  Can be provided in radians or as a
              sexigesimal string.
   deccen - Declination of the array center.  Same input options as racen.
   xcen   - X coordinate of the array center
   ycen   - Y coordinate of the array center
   scale  - Scale of the image in arcsec/pixel

 OPTIONAL INPUT PARAMETERS:
   XFLIP  - flag, if set reverses the x axis
   YFLIP  - flag, if set reverses the y axis
   POSANG - Rotation angle between x,y and xi,eta, in degrees, gives
               the position angle of the y axis on the sky measured
               east of north.

   Note: the posang rotation is applied BEFORE either axis is flipped.
           Then, the flip is applied to the rotated system.

   RENORMFAC - Renormalization factor for the x,y coordinates.  The default
                  is 1.0 and rarely needs to be otherwise for the synthetic
                  astrometric solution generated by this routine.

 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
   info   - This is an anonymous structure compatible with the astrometry
              routines (eg., astsn2xy, astsn2rd, and so on).  This structure
              defines a full, strictly linear coordinate transformation
              intended for describing synthetic or linearized images.

 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
   This routine is not intended for fitting images so it does not attempt
      to provide access and control of the full power of the transformation.
      Only linear descriptions are generated and the tangent point is always
      intended to be in the center of the array.
 PROCEDURE:
 MODIFICATION HISTORY:
   Written by Marc W. Buie, Southwest Research Institute, 2009/11/15
   2009/12/02, MWB, changed to consolidate xiterms and etaterms into one.