NAME:
  astrd2xy
 PURPOSE:
  Astrometry conversion from ($\alpha$,$\delta$) to image (x,y)

 DESCRIPTION:
  This transformation is based on a simple linear transformation with
    rotation from the celestial sphere to linear CCD chip coordinates.
    This is only an approximate treatment and will not work for very large
    fields.

 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 with
              the following tags:
                 raref  - Right ascension of center of tangent plane.
                 decref - Declination of center of tangent plane.
                 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.
                 xcref  - X center of image.
                 ycref  - Y center of image.

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

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

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  97/04/05 - Written by Marc W. Buie, Lowell Observatory