| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palDs2tp
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| 5 |
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| 6 | * Purpose:
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| 7 | * Spherical to tangent plane projection
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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 | * palDs2tp( double ra, double dec, double raz, double decz,
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| 17 | * double *xi, double *eta, int *j );
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| 18 |
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| 19 | * Arguments:
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| 20 | * ra = double (Given)
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| 21 | * RA spherical coordinate of point to be projected (radians)
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| 22 | * dec = double (Given)
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| 23 | * Dec spherical coordinate of point to be projected (radians)
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| 24 | * raz = double (Given)
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| 25 | * RA spherical coordinate of tangent point (radians)
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| 26 | * decz = double (Given)
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| 27 | * Dec spherical coordinate of tangent point (radians)
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| 28 | * xi = double * (Returned)
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| 29 | * First rectangular coordinate on tangent plane (radians)
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| 30 | * eta = double * (Returned)
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| 31 | * Second rectangular coordinate on tangent plane (radians)
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| 32 | * j = int * (Returned)
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| 33 | * status: 0 = OK, star on tangent plane
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| 34 | * 1 = error, star too far from axis
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| 35 | * 2 = error, antistar on tangent plane
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| 36 | * 3 = error, antistar too far from axis
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| 37 |
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| 38 | * Description:
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| 39 | * Projection of spherical coordinates onto tangent plane:
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| 40 | * "gnomonic" projection - "standard coordinates"
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| 41 |
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| 42 | * Authors:
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| 43 | * PTW: Pat Wallace (STFC)
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| 44 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 45 | * {enter_new_authors_here}
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| 46 |
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| 47 | * History:
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| 48 | * 2012-02-08 (TIMJ):
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| 49 | * Initial version with documentation taken from Fortran SLA
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| 50 | * Adapted with permission from the Fortran SLALIB library.
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| 51 | * {enter_further_changes_here}
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| 52 |
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| 53 | * Copyright:
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| 54 | * Copyright (C) 1996 Rutherford Appleton Laboratory
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| 55 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 56 | * All Rights Reserved.
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| 57 |
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| 58 | * Licence:
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| 59 | * This program is free software: you can redistribute it and/or
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| 60 | * modify it under the terms of the GNU Lesser General Public
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| 61 | * License as published by the Free Software Foundation, either
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| 62 | * version 3 of the License, or (at your option) any later
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| 63 | * version.
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| 64 | *
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| 65 | * This program is distributed in the hope that it will be useful,
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| 66 | * but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 67 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 68 | * GNU Lesser General Public License for more details.
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| 69 | *
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| 70 | * You should have received a copy of the GNU Lesser General
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| 71 | * License along with this program. If not, see
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| 72 | * <http://www.gnu.org/licenses/>.
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| 73 |
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| 74 | * Bugs:
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| 75 | * {note_any_bugs_here}
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| 76 | *-
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| 77 | */
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| 78 |
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| 79 | #include "pal.h"
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| 80 | #include <math.h>
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| 81 |
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| 82 | void
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| 83 | palDs2tp ( double ra, double dec, double raz, double decz,
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| 84 | double *xi, double *eta, int *j ) {
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| 85 |
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| 86 | const double TINY = 1.0e-6;
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| 87 |
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| 88 | double cdec;
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| 89 | double sdec;
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| 90 | double radif;
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| 91 | double cdecz;
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| 92 | double denom;
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| 93 | double sdecz;
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| 94 | double cradif;
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| 95 | double sradif;
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| 96 |
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| 97 | /* Trig functions */
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| 98 | sdecz = sin(decz);
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| 99 | sdec = sin(dec);
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| 100 | cdecz = cos(decz);
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| 101 | cdec = cos(dec);
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| 102 | radif = ra - raz;
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| 103 | sradif = sin(radif);
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| 104 | cradif = cos(radif);
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| 105 |
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| 106 | /* Reciprocal of star vector length to tangent plane */
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| 107 | denom = sdec * sdecz + cdec * cdecz * cradif;
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| 108 |
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| 109 | /* Handle vectors too far from axis */
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| 110 | if (denom > TINY) {
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| 111 | *j = 0;
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| 112 | } else if (denom >= 0.) {
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| 113 | *j = 1;
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| 114 | denom = TINY;
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| 115 | } else if (denom > -TINY) {
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| 116 | *j = 2;
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| 117 | denom = -TINY;
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| 118 | } else {
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| 119 | *j = 3;
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| 120 | }
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| 121 |
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| 122 | /* Compute tangent plane coordinates (even in dubious cases) */
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| 123 | *xi = cdec * sradif / denom;
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| 124 | *eta = (sdec * cdecz - cdec * sdecz * cradif) / denom;
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| 125 |
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| 126 | return;
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| 127 | }
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