NAME:
  astsn2xy
 PURPOSE:
  Astrometry conversion from tangent plane ($\xi$,$\eta$) to image (x,y)

 DESCRIPTION:
  This transformation can either be based on a simple linear transformation
    with rotation from the celestial sphere to linear CCD chip coordinates.
    Or, it can use a full astrometric solution (including linear).

 CATEGORY:
  Astrometry

 CALLING SEQUENCE:
  astsn2xy,xi,eta,info,x,y

 INPUTS:
  xi     - Tangent plane coordinates (radians)
  eta    - Tangent plane coordinates (radians)
  info   - Transformation information held in an anonymous structure.  There
              are two different groups of tags that can appear.  The original
              simple linear transformation needs the following tags:
                 pscale - Plate scale (arcsec/pixel).
                 rang   - Rotation angle of image (radians).
                 xflip  - -1 if image flipped in X, 1 if not.
                 yflip  - -1 if image flipped in Y, 1 if not.

              The full-up transformation requires a different set of tags:
                 renormfac - normalization factor
                 cxi - xi transformations coefficients (x,y -> xi)
                 ceta - eta transformations coefficients (x,y -> eta)
                 terms - xi terms flag array

              Both types need the following.
                 xcref  - X center of image.
                 ycref  - Y center of image.
                 raref  - Right ascension of center of image (tangent plane).
                 decref - Declination of center of image (tangent plane).

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

  FULL - Flag, if set indicates the full transformation should be used.
            Only the tags needed must be provided.
  ARCSEC - Flag, if set indicates that the input xi,eta values are in
            arcseconds.  Otherwise they are treated as radians.

 OUTPUTS:
  x      - X coordinate in image
  y      - Y coordinate in image

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  2009/11/02 - Written by Marc W. Buie, Southwest Research Institute
  2009/11/12, MWB, added ARCSEC keyword
  2011/03/21, MWB, drastic improvement to starting guess.  Works on anything
                    now.  Didn't work before if x,y was rotated by 45 degrees.
  2012/05/24, MWB, changed linear case to use optical axis, not reference.
  2015/08/12, MWB, changed back to use xcref,ycref.  Optical axis tags
                      are now removed from all tools.  Use ref instead.