| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palDtps2c
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| 5 |
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| 6 | * Purpose:
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| 7 | * Determine RA,Dec of tangent point from coordinates
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| 8 |
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| 9 | * Language:
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| 10 | * Starlink ANSI C
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| 11 |
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| 12 | * Type of Module:
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| 13 | * Library routine
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| 14 |
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| 15 | * Invocation:
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| 16 | * palDtps2c( double xi, double eta, double ra, double dec,
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| 17 | * double * raz1, double decz1,
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| 18 | * double * raz2, double decz2, int *n);
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| 19 |
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| 20 | * Arguments:
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| 21 | * xi = double (Given)
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| 22 | * First rectangular coordinate on tangent plane (radians)
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| 23 | * eta = double (Given)
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| 24 | * Second rectangular coordinate on tangent plane (radians)
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| 25 | * ra = double (Given)
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| 26 | * RA spherical coordinate of star (radians)
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| 27 | * dec = double (Given)
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| 28 | * Dec spherical coordinate of star (radians)
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| 29 | * raz1 = double * (Returned)
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| 30 | * RA spherical coordinate of tangent point, solution 1 (radians)
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| 31 | * decz1 = double * (Returned)
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| 32 | * Dec spherical coordinate of tangent point, solution 1 (radians)
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| 33 | * raz2 = double * (Returned)
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| 34 | * RA spherical coordinate of tangent point, solution 2 (radians)
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| 35 | * decz2 = double * (Returned)
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| 36 | * Dec spherical coordinate of tangent point, solution 2 (radians)
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| 37 | * n = int * (Returned)
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| 38 | * number of solutions: 0 = no solutions returned (note 2)
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| 39 | * 1 = only the first solution is useful (note 3)
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| 40 | * 2 = both solutions are useful (note 3)
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| 41 |
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| 42 |
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| 43 | * Description:
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| 44 | * From the tangent plane coordinates of a star of known RA,Dec,
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| 45 | * determine the RA,Dec of the tangent point.
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| 46 |
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| 47 | * Authors:
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| 48 | * PTW: Pat Wallace (STFC)
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| 49 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 50 | * {enter_new_authors_here}
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| 51 |
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| 52 | * Notes:
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| 53 | * - The RAZ1 and RAZ2 values are returned in the range 0-2pi.
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| 54 | * - Cases where there is no solution can only arise near the poles.
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| 55 | * For example, it is clearly impossible for a star at the pole
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| 56 | * itself to have a non-zero XI value, and hence it is
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| 57 | * meaningless to ask where the tangent point would have to be
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| 58 | * to bring about this combination of XI and DEC.
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| 59 | * - Also near the poles, cases can arise where there are two useful
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| 60 | * solutions. The argument N indicates whether the second of the
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| 61 | * two solutions returned is useful. N=1 indicates only one useful
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| 62 | * solution, the usual case; under these circumstances, the second
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| 63 | * solution corresponds to the "over-the-pole" case, and this is
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| 64 | * reflected in the values of RAZ2 and DECZ2 which are returned.
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| 65 | * - The DECZ1 and DECZ2 values are returned in the range +/-pi, but
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| 66 | * in the usual, non-pole-crossing, case, the range is +/-pi/2.
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| 67 | * - This routine is the spherical equivalent of the routine sla_DTPV2C.
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| 68 |
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| 69 | * History:
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| 70 | * 2012-02-08 (TIMJ):
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| 71 | * Initial version with documentation taken from Fortran SLA
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| 72 | * Adapted with permission from the Fortran SLALIB library.
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| 73 | * {enter_further_changes_here}
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| 74 |
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| 75 | * Copyright:
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| 76 | * Copyright (C) 1995 Rutherford Appleton Laboratory
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| 77 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 78 | * All Rights Reserved.
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| 79 |
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| 80 | * Licence:
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| 81 | * This program is free software: you can redistribute it and/or
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| 82 | * modify it under the terms of the GNU Lesser General Public
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| 83 | * License as published by the Free Software Foundation, either
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| 84 | * version 3 of the License, or (at your option) any later
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| 85 | * version.
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| 86 | *
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| 87 | * This program is distributed in the hope that it will be useful,
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| 88 | * but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 89 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 90 | * GNU Lesser General Public License for more details.
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| 91 | *
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| 92 | * You should have received a copy of the GNU Lesser General
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| 93 | * License along with this program. If not, see
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| 94 | * <http://www.gnu.org/licenses/>.
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| 95 |
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| 96 | * Bugs:
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| 97 | * {note_any_bugs_here}
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| 98 | *-
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| 99 | */
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| 100 |
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| 101 | #include "pal.h"
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| 102 | #include "pal1sofa.h"
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| 103 |
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| 104 | #include <math.h>
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| 105 |
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| 106 | void
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| 107 | palDtps2c( double xi, double eta, double ra, double dec,
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| 108 | double * raz1, double * decz1,
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| 109 | double * raz2, double * decz2, int *n) {
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| 110 |
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| 111 | double x2;
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| 112 | double y2;
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| 113 | double sd;
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| 114 | double cd;
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| 115 | double sdf;
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| 116 | double r2;
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| 117 |
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| 118 | x2 = xi * xi;
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| 119 | y2 = eta * eta;
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| 120 | sd = sin(dec);
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| 121 | cd = cos(dec);
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| 122 | sdf = sd * sqrt(x2 + 1. + y2);
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| 123 | r2 = cd * cd * (y2 + 1.) - sd * sd * x2;
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| 124 | if (r2 >= 0.) {
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| 125 | double r;
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| 126 | double s;
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| 127 | double c;
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| 128 |
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| 129 | r = sqrt(r2);
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| 130 | s = sdf - eta * r;
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| 131 | c = sdf * eta + r;
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| 132 | if (xi == 0. && r == 0.) {
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| 133 | r = 1.;
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| 134 | }
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| 135 | *raz1 = eraAnp(ra - atan2(xi, r));
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| 136 | *decz1 = atan2(s, c);
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| 137 | r = -r;
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| 138 | s = sdf - eta * r;
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| 139 | c = sdf * eta + r;
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| 140 | *raz2 = eraAnp(ra - atan2(xi, r));
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| 141 | *decz2 = atan2(s, c);
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| 142 | if (fabs(sdf) < 1.) {
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| 143 | *n = 1;
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| 144 | } else {
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| 145 | *n = 2;
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| 146 | }
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| 147 | } else {
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| 148 | *n = 0;
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| 149 | }
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| 150 | return;
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| 151 | }
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