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Diffstat (limited to 'src/slalib/dtps2c.f')
| -rw-r--r-- | src/slalib/dtps2c.f | 91 |
1 files changed, 91 insertions, 0 deletions
diff --git a/src/slalib/dtps2c.f b/src/slalib/dtps2c.f new file mode 100644 index 0000000..43e3bad --- /dev/null +++ b/src/slalib/dtps2c.f @@ -0,0 +1,91 @@ + 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 |