.help dtps2c Jun99 "Slalib Package"
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      SUBROUTINE slDPSC (XI, ETA, RA, DEC, RAZ1, DECZ1,
     :                                         RAZ2, DECZ2, N)

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      D P S C
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  From the tangent plane coordinates of a star of known RA,Dec,
  determine the RA,Dec of the tangent point.

  (double precision)

  Given:
     XI,ETA      d    tangent plane rectangular coordinates
     RA,DEC      d    spherical coordinates

  Returned:
     RAZ1,DECZ1  d    spherical coordinates of tangent point, solution 1
     RAZ2,DECZ2  d    spherical coordinates of tangent point, solution 2
     N           i    number of solutions:
                        0 = no solutions returned (note 2)
                        1 = only the first solution is useful (note 3)
                        2 = both solutions are useful (note 3)

  Notes:

  1  The RAZ1 and RAZ2 values are returned in the range 0-2pi.

  2  Cases where there is no solution can only arise near the poles.
     For example, it is clearly impossible for a star at the pole
     itself to have a non-zero XI value, and hence it is
     meaningless to ask where the tangent point would have to be
     to bring about this combination of XI and DEC.

  3  Also near the poles, cases can arise where there are two useful
     solutions.  The argument N indicates whether the second of the
     two solutions returned is useful.  N=1 indicates only one useful
     solution, the usual case;  under these circumstances, the second
     solution corresponds to the "over-the-pole" case, and this is
     reflected in the values of RAZ2 and DECZ2 which are returned.

  4  The DECZ1 and DECZ2 values are returned in the range +/-pi, but
     in the usual, non-pole-crossing, case, the range is +/-pi/2.

  5  This routine is the spherical equivalent of the routine slDPVC.

  Called:  slDA2P

  P.T.Wallace   Starlink   5 June 1995

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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