NAME:
  astrd2sn
 PURPOSE:
  Astrometry conversion from ($\alpha$,$\delta$) to ($\xi$,$\eta$)

 DESCRIPTION:
  Standard coordinate conversion (see Smart, p283)

 CATEGORY:
  Astrometry

 CALLING SEQUENCE:
  astrd2sn,ra,dec,racen,deccen,xi,eta

 INPUTS:
  ra     - Right ascension (radians)
  dec    - Declination (radians)
  racen  - Right ascension of tangent point between plane and celestial sphere.
  deccen - Declination of tangent point between plane and celestial sphere.

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
  ARCSEC - Flag, if set indicates that the output xi,eta values are in
            arcseconds.  Otherwise they are returned in radians.

 OUTPUTS:
  xi     - Coordinate in tangent plane.
  eta    - Coordinate in tangent plane.

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  97/04/05 - Written by Marc W. Buie, Lowell Observatory
  2009/12/08, MWB, added ARCSEC keyword