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109 lines
3.5 KiB
Fortran
109 lines
3.5 KiB
Fortran
SUBROUTINE sla_DTPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1,
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: RAZ2, DECZ2, N)
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*+
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* - - - - - - -
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* D T P S 2 C
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* - - - - - - -
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*
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* From the tangent plane coordinates of a star of known RA,Dec,
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* determine the RA,Dec of the tangent point.
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*
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* (double precision)
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*
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* Given:
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* XI,ETA d tangent plane rectangular coordinates
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* RA,DEC d spherical coordinates
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*
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* Returned:
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* RAZ1,DECZ1 d spherical coordinates of tangent point, solution 1
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* RAZ2,DECZ2 d spherical coordinates of tangent point, solution 2
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* N i number of solutions:
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* 0 = no solutions returned (note 2)
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* 1 = only the first solution is useful (note 3)
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* 2 = both solutions are useful (note 3)
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*
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* Notes:
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*
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* 1 The RAZ1 and RAZ2 values are returned in the range 0-2pi.
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*
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* 2 Cases where there is no solution can only arise near the poles.
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* For example, it is clearly impossible for a star at the pole
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* itself to have a non-zero XI value, and hence it is
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* meaningless to ask where the tangent point would have to be
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* to bring about this combination of XI and DEC.
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*
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* 3 Also near the poles, cases can arise where there are two useful
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* solutions. The argument N indicates whether the second of the
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* two solutions returned is useful. N=1 indicates only one useful
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* solution, the usual case; under these circumstances, the second
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* solution corresponds to the "over-the-pole" case, and this is
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* reflected in the values of RAZ2 and DECZ2 which are returned.
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*
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* 4 The DECZ1 and DECZ2 values are returned in the range +/-pi, but
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* in the usual, non-pole-crossing, case, the range is +/-pi/2.
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*
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* 5 This routine is the spherical equivalent of the routine sla_DTPV2C.
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*
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* Called: sla_DRANRM
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*
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* P.T.Wallace Starlink 5 June 1995
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*
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* Copyright (C) 1995 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION XI,ETA,RA,DEC,RAZ1,DECZ1,RAZ2,DECZ2
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INTEGER N
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DOUBLE PRECISION X2,Y2,SD,CD,SDF,R2,R,S,C
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DOUBLE PRECISION sla_DRANRM
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X2=XI*XI
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Y2=ETA*ETA
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SD=SIN(DEC)
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CD=COS(DEC)
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SDF=SD*SQRT(1D0+X2+Y2)
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R2=CD*CD*(1D0+Y2)-SD*SD*X2
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IF (R2.GE.0D0) THEN
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R=SQRT(R2)
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S=SDF-ETA*R
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C=SDF*ETA+R
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IF (XI.EQ.0D0.AND.R.EQ.0D0) R=1D0
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RAZ1=sla_DRANRM(RA-ATAN2(XI,R))
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DECZ1=ATAN2(S,C)
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R=-R
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S=SDF-ETA*R
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C=SDF*ETA+R
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RAZ2=sla_DRANRM(RA-ATAN2(XI,R))
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DECZ2=ATAN2(S,C)
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IF (ABS(SDF).LT.1D0) THEN
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N=1
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ELSE
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N=2
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END IF
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ELSE
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N=0
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END IF
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END
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