SUBROUTINE sla_DTPV2C (XI, ETA, V, V01, V02, N) *+ * - - - - - - - * D T P V 2 C * - - - - - - - * * Given the tangent-plane coordinates of a star and its direction * cosines, determine the direction cosines of the tangent-point. * * (double precision) * * Given: * XI,ETA d tangent plane coordinates of star * V d(3) direction cosines of star * * Returned: * V01 d(3) direction cosines of tangent point, solution 1 * V02 d(3) direction cosines of tangent point, solution 2 * N i number of solutions: * 0 = no solutions returned (note 2) * 1 = only the first solution is useful (note 3) * 2 = both solutions are useful (note 3) * * Notes: * * 1 The vector V must be of unit length or the result will be wrong. * * 2 Cases where there is no solution can only arise near the poles. * For example, it is clearly impossible for a star at the pole * itself to have a non-zero XI value, and hence it is meaningless * to ask where the tangent point would have to be. * * 3 Also near the poles, cases can arise where there are two useful * solutions. The argument N indicates whether the second of the * two solutions returned is useful. N=1 indicates only one useful * solution, the usual case; under these circumstances, the second * solution can be regarded as valid if the vector V02 is interpreted * as the "over-the-pole" case. * * 4 This routine is the Cartesian equivalent of the routine sla_DTPS2C. * * P.T.Wallace Starlink 5 June 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 DOUBLE PRECISION XI,ETA,V(3),V01(3),V02(3) INTEGER N DOUBLE PRECISION X,Y,Z,RXY2,XI2,ETA2P1,SDF,R2,R,C X=V(1) Y=V(2) Z=V(3) RXY2=X*X+Y*Y XI2=XI*XI ETA2P1=ETA*ETA+1D0 SDF=Z*SQRT(XI2+ETA2P1) R2=RXY2*ETA2P1-Z*Z*XI2 IF (R2.GT.0D0) THEN R=SQRT(R2) C=(SDF*ETA+R)/(ETA2P1*SQRT(RXY2*(R2+XI2))) V01(1)=C*(X*R+Y*XI) V01(2)=C*(Y*R-X*XI) V01(3)=(SDF-ETA*R)/ETA2P1 R=-R C=(SDF*ETA+R)/(ETA2P1*SQRT(RXY2*(R2+XI2))) V02(1)=C*(X*R+Y*XI) V02(2)=C*(Y*R-X*XI) V02(3)=(SDF-ETA*R)/ETA2P1 IF (ABS(SDF).LT.1D0) THEN N=1 ELSE N=2 END IF ELSE N=0 END IF END