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85 lines
2.4 KiB
Fortran
85 lines
2.4 KiB
Fortran
SUBROUTINE sla_S2TP (RA, DEC, RAZ, DECZ, XI, ETA, J)
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*+
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* - - - - -
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* S 2 T P
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* - - - - -
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*
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* Projection of spherical coordinates onto tangent plane:
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* "gnomonic" projection - "standard coordinates"
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* (single precision)
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*
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* Given:
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* RA,DEC real spherical coordinates of point to be projected
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* RAZ,DECZ real spherical coordinates of tangent point
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*
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* Returned:
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* XI,ETA real rectangular coordinates on tangent plane
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* J int status: 0 = OK, star on tangent plane
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* 1 = error, star too far from axis
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* 2 = error, antistar on tangent plane
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* 3 = error, antistar too far from axis
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*
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* P.T.Wallace Starlink 18 July 1996
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*
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* Copyright (C) 1996 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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REAL RA,DEC,RAZ,DECZ,XI,ETA
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INTEGER J
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REAL SDECZ,SDEC,CDECZ,CDEC,RADIF,SRADIF,CRADIF,DENOM
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REAL TINY
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PARAMETER (TINY=1E-6)
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* Trig functions
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SDECZ=SIN(DECZ)
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SDEC=SIN(DEC)
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CDECZ=COS(DECZ)
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CDEC=COS(DEC)
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RADIF=RA-RAZ
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SRADIF=SIN(RADIF)
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CRADIF=COS(RADIF)
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* Reciprocal of star vector length to tangent plane
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DENOM=SDEC*SDECZ+CDEC*CDECZ*CRADIF
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* Handle vectors too far from axis
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IF (DENOM.GT.TINY) THEN
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J=0
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ELSE IF (DENOM.GE.0.0) THEN
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J=1
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DENOM=TINY
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ELSE IF (DENOM.GT.-TINY) THEN
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J=2
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DENOM=-TINY
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ELSE
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J=3
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END IF
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* Compute tangent plane coordinates (even in dubious cases)
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XI=CDEC*SRADIF/DENOM
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ETA=(SDEC*CDECZ-CDEC*SDECZ*CRADIF)/DENOM
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END
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