NAME:
  astrd2xy
 PURPOSE:
  Astrometry conversion from ($\alpha$,$\delta$) 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:
  astrd2xy,ra,dec,info,x,y

 INPUTS:
  ra     - Right ascension (radians)
  dec    - Declination (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 - string array with names of terms to fit

              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.

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

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  1997/04/05 - Written by Marc W. Buie, Lowell Observatory
  2009/06/21, MWB added full-up transformation
  2009/11/02, MWB, split off most of code into astsn2xy