NAME:
  astsn2rd
 PURPOSE:
  Astrometry conversion from ($\xi$,$\eta$) to ($\alpha$,$\delta$)
 DESCRIPTION:
  Standard coordinate conversion (see Smart, p283)

 CATEGORY:
  Astrometry

 CALLING SEQUENCE:
  astsn2rd,xi,eta,racen,deccen,ra,dec

 INPUTS:
  xi     - Coordinate in tangent plane (radians).
  eta    - Coordinate in tangent plane (radians).
  racen  - Right ascension of tangent point between plane and celestial
              sphere (radians).
  deccen - Declination of tangent point between plane and celestial
              sphere (radians).

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
  ARCSEC - Flag, if set indicates that the input xi,eta values are in
            arcseconds.  Otherwise they need to be in radians.  Does not
            affect the output ra,dec.

 OUTPUTS:
  ra     - Right ascension (radians)
  dec    - Declination (radians)

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  1997/04/05 - Written by Marc W. Buie, Lowell Observatory
  2009/06/22, MWB, modified to eliminate numerical singularities
  2010/12/10, MWB, added ARCSEC keyword