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SUBROUTINE sla_S2TP (RA, DEC, RAZ, DECZ, XI, ETA, J)
*+
* - - - - -
* S 2 T P
* - - - - -
*
* Projection of spherical coordinates onto tangent plane:
* "gnomonic" projection - "standard coordinates"
* (single precision)
*
* Given:
* RA,DEC real spherical coordinates of point to be projected
* RAZ,DECZ real spherical coordinates of tangent point
*
* Returned:
* XI,ETA real 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
*-
IMPLICIT NONE
REAL RA,DEC,RAZ,DECZ,XI,ETA
INTEGER J
REAL SDECZ,SDEC,CDECZ,CDEC,RADIF,SRADIF,CRADIF,DENOM
REAL TINY
PARAMETER (TINY=1E-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.0.0) 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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