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slalib/tp2v.f

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+      SUBROUTINE sla_TP2V (XI, ETA, V0, V)
+*+
+*     - - - - -
+*      T P 2 V
+*     - - - - -
+*
+*  Given the tangent-plane coordinates of a star and the direction
+*  cosines of the tangent point, determine the direction cosines
+*  of the star.
+*
+*  (single precision)
+*
+*  Given:
+*     XI,ETA    r       tangent plane coordinates of star
+*     V0        r(3)    direction cosines of tangent point
+*
+*  Returned:
+*     V         r(3)    direction cosines of star
+*
+*  Notes:
+*
+*  1  If vector V0 is not of unit length, the returned vector V will
+*     be wrong.
+*
+*  2  If vector V0 points at a pole, the returned vector V will be
+*     based on the arbitrary assumption that the RA of the tangent
+*     point is zero.
+*
+*  3  This routine is the Cartesian equivalent of the routine sla_TP2S.
+*
+*  P.T.Wallace   Starlink   11 February 1995
+*
+*  Copyright (C) 1995 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., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      IMPLICIT NONE
+
+      REAL XI,ETA,V0(3),V(3)
+
+      REAL X,Y,Z,F,R
+
+
+      X=V0(1)
+      Y=V0(2)
+      Z=V0(3)
+      F=SQRT(1.0+XI*XI+ETA*ETA)
+      R=SQRT(X*X+Y*Y)
+      IF (R.EQ.0.0) THEN
+         R=1E-20
+         X=R
+      END IF
+      V(1)=(X-(XI*Y+ETA*X*Z)/R)/F
+      V(2)=(Y+(XI*X-ETA*Y*Z)/R)/F
+      V(3)=(Z+ETA*R)/F
+
+      END