SUBROUTINE sla_DTPS2C (XI, ETA, RA, DEC, RAZ1, DECZ1, : RAZ2, DECZ2, N) *+ * - - - - - - - * D T P S 2 C * - - - - - - - * * From the tangent plane coordinates of a star of known RA,Dec, * determine the RA,Dec of the tangent point. * * (double precision) * * Given: * XI,ETA d tangent plane rectangular coordinates * RA,DEC d spherical coordinates * * Returned: * RAZ1,DECZ1 d spherical coordinates of tangent point, solution 1 * RAZ2,DECZ2 d spherical coordinates 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 RAZ1 and RAZ2 values are returned in the range 0-2pi. * * 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 * to bring about this combination of XI and DEC. * * 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 corresponds to the "over-the-pole" case, and this is * reflected in the values of RAZ2 and DECZ2 which are returned. * * 4 The DECZ1 and DECZ2 values are returned in the range +/-pi, but * in the usual, non-pole-crossing, case, the range is +/-pi/2. * * 5 This routine is the spherical equivalent of the routine sla_DTPV2C. * * Called: sla_DRANRM * * 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,RA,DEC,RAZ1,DECZ1,RAZ2,DECZ2 INTEGER N DOUBLE PRECISION X2,Y2,SD,CD,SDF,R2,R,S,C DOUBLE PRECISION sla_DRANRM X2=XI*XI Y2=ETA*ETA SD=SIN(DEC) CD=COS(DEC) SDF=SD*SQRT(1D0+X2+Y2) R2=CD*CD*(1D0+Y2)-SD*SD*X2 IF (R2.GE.0D0) THEN R=SQRT(R2) S=SDF-ETA*R C=SDF*ETA+R IF (XI.EQ.0D0.AND.R.EQ.0D0) R=1D0 RAZ1=sla_DRANRM(RA-ATAN2(XI,R)) DECZ1=ATAN2(S,C) R=-R S=SDF-ETA*R C=SDF*ETA+R RAZ2=sla_DRANRM(RA-ATAN2(XI,R)) DECZ2=ATAN2(S,C) IF (ABS(SDF).LT.1D0) THEN N=1 ELSE N=2 END IF ELSE N=0 END IF END