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SUBROUTINE slDSTP (RA, DEC, RAZ, DECZ, XI, ETA, J)
*+
* - - - - - -
* D S T P
* - - - - - -
*
* Projection of spherical coordinates onto tangent plane:
* "gnomonic" projection - "standard coordinates" (double precision)
*
* Given:
* RA,DEC dp spherical coordinates of point to be projected
* RAZ,DECZ dp spherical coordinates of tangent point
*
* Returned:
* XI,ETA dp rectangular coordinates on tangent plane
* J int status: 0 = OK, star on tangent plane
* 1 = error, star too far from axis
* 2 = error, antistar on tangent plane
* 3 = error, antistar too far from axis
*
* P.T.Wallace Starlink 18 July 1996
*
* Copyright (C) 1996 Rutherford Appleton Laboratory
*
* License:
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program (see SLA_CONDITIONS); if not, write to the
* Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301 USA
*
* Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
*-
IMPLICIT NONE
DOUBLE PRECISION RA,DEC,RAZ,DECZ,XI,ETA
INTEGER J
DOUBLE PRECISION SDECZ,SDEC,CDECZ,CDEC,
: RADIF,SRADIF,CRADIF,DENOM
DOUBLE PRECISION TINY
PARAMETER (TINY=1D-6)
* Trig functions
SDECZ=SIN(DECZ)
SDEC=SIN(DEC)
CDECZ=COS(DECZ)
CDEC=COS(DEC)
RADIF=RA-RAZ
SRADIF=SIN(RADIF)
CRADIF=COS(RADIF)
* Reciprocal of star vector length to tangent plane
DENOM=SDEC*SDECZ+CDEC*CDECZ*CRADIF
* Handle vectors too far from axis
IF (DENOM.GT.TINY) THEN
J=0
ELSE IF (DENOM.GE.0D0) THEN
J=1
DENOM=TINY
ELSE IF (DENOM.GT.-TINY) THEN
J=2
DENOM=-TINY
ELSE
J=3
END IF
* Compute tangent plane coordinates (even in dubious cases)
XI=CDEC*SRADIF/DENOM
ETA=(SDEC*CDECZ-CDEC*SDECZ*CRADIF)/DENOM
END
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