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+.help dtps2c Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slDPSC (XI, ETA, RA, DEC, RAZ1, DECZ1,
+     :                                         RAZ2, DECZ2, N)
+
+     - - - - - - -
+      D P S C
+     - - - - - - -
+
+  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.
+
+.fi
+.endhelp