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SUBROUTINE sla_DTPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1,
: RAZ2, DECZ2, N)
*+
* - - - - - - -
* D T P S 2 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 sla_DTPV2C.
*
* Called: sla_DRANRM
*
* P.T.Wallace Starlink 5 June 1995
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
DOUBLE PRECISION XI,ETA,RA,DEC,RAZ1,DECZ1,RAZ2,DECZ2
INTEGER N
DOUBLE PRECISION X2,Y2,SD,CD,SDF,R2,R,S,C
DOUBLE PRECISION sla_DRANRM
X2=XI*XI
Y2=ETA*ETA
SD=SIN(DEC)
CD=COS(DEC)
SDF=SD*SQRT(1D0+X2+Y2)
R2=CD*CD*(1D0+Y2)-SD*SD*X2
IF (R2.GE.0D0) THEN
R=SQRT(R2)
S=SDF-ETA*R
C=SDF*ETA+R
IF (XI.EQ.0D0.AND.R.EQ.0D0) R=1D0
RAZ1=sla_DRANRM(RA-ATAN2(XI,R))
DECZ1=ATAN2(S,C)
R=-R
S=SDF-ETA*R
C=SDF*ETA+R
RAZ2=sla_DRANRM(RA-ATAN2(XI,R))
DECZ2=ATAN2(S,C)
IF (ABS(SDF).LT.1D0) THEN
N=1
ELSE
N=2
END IF
ELSE
N=0
END IF
END
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