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SUBROUTINE sla_DV2TP (V, V0, XI, ETA, J)
*+
* - - - - - -
* D V 2 T P
* - - - - - -
*
* Given the direction cosines of a star and of the tangent point,
* determine the star's tangent-plane coordinates.
*
* (double precision)
*
* Given:
* V d(3) direction cosines of star
* V0 d(3) direction cosines of tangent point
*
* Returned:
* XI,ETA d tangent plane coordinates of star
* J i status: 0 = OK
* 1 = error, star too far from axis
* 2 = error, antistar on tangent plane
* 3 = error, antistar too far from axis
*
* Notes:
*
* 1 If vector V0 is not of unit length, or if vector V is of zero
* length, the results will be wrong.
*
* 2 If V0 points at a pole, the returned XI,ETA 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_DS2TP.
*
* P.T.Wallace Starlink 27 November 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., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307 USA
*
*-
IMPLICIT NONE
DOUBLE PRECISION V(3),V0(3),XI,ETA
INTEGER J
DOUBLE PRECISION X,Y,Z,X0,Y0,Z0,R2,R,W,D
DOUBLE PRECISION TINY
PARAMETER (TINY=1D-6)
X=V(1)
Y=V(2)
Z=V(3)
X0=V0(1)
Y0=V0(2)
Z0=V0(3)
R2=X0*X0+Y0*Y0
R=SQRT(R2)
IF (R.EQ.0D0) THEN
R=1D-20
X0=R
END IF
W=X*X0+Y*Y0
D=W+Z*Z0
IF (D.GT.TINY) THEN
J=0
ELSE IF (D.GE.0D0) THEN
J=1
D=TINY
ELSE IF (D.GT.-TINY) THEN
J=2
D=-TINY
ELSE
J=3
END IF
D=D*R
XI=(Y*X0-X*Y0)/D
ETA=(Z*R2-Z0*W)/D
END