| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palDh2e
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| 5 |
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| 6 | * Purpose:
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| 7 | * Horizon to equatorial coordinates: Az,El to HA,Dec
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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 | * palDh2e( double az, double el, double phi, double * ha, double * dec );
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| 17 |
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| 18 | * Arguments:
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| 19 | * az = double (Given)
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| 20 | * Azimuth (radians)
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| 21 | * el = double (Given)
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| 22 | * Elevation (radians)
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| 23 | * phi = double (Given)
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| 24 | * Observatory latitude (radians)
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| 25 | * ha = double * (Returned)
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| 26 | * Hour angle (radians)
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| 27 | * dec = double * (Returned)
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| 28 | * Declination (radians)
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| 29 |
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| 30 | * Description:
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| 31 | * Convert horizon to equatorial coordinates.
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| 32 |
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| 33 | * Authors:
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| 34 | * PTW: Pat Wallace (STFC)
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| 35 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 36 | * {enter_new_authors_here}
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| 37 |
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| 38 | * Notes:
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| 39 | * - All the arguments are angles in radians.
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| 40 | * - The sign convention for azimuth is north zero, east +pi/2.
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| 41 | * - HA is returned in the range +/-pi. Declination is returned
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| 42 | * in the range +/-pi/2.
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| 43 | * - The latitude is (in principle) geodetic. In critical
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| 44 | * applications, corrections for polar motion should be applied.
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| 45 | * - In some applications it will be important to specify the
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| 46 | * correct type of elevation in order to produce the required
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| 47 | * type of HA,Dec. In particular, it may be important to
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| 48 | * distinguish between the elevation as affected by refraction,
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| 49 | * which will yield the "observed" HA,Dec, and the elevation
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| 50 | * in vacuo, which will yield the "topocentric" HA,Dec. If the
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| 51 | * effects of diurnal aberration can be neglected, the
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| 52 | * topocentric HA,Dec may be used as an approximation to the
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| 53 | * "apparent" HA,Dec.
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| 54 | * - No range checking of arguments is done.
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| 55 | * - In applications which involve many such calculations, rather
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| 56 | * than calling the present routine it will be more efficient to
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| 57 | * use inline code, having previously computed fixed terms such
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| 58 | * as sine and cosine of latitude.
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| 59 |
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| 60 | * History:
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| 61 | * 2012-02-08 (TIMJ):
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| 62 | * Initial version with documentation taken from Fortran SLA
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| 63 | * Adapted with permission from the Fortran SLALIB library.
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| 64 | * {enter_further_changes_here}
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| 65 |
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| 66 | * Copyright:
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| 67 | * Copyright (C) 1996 Rutherford Appleton Laboratory
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| 68 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 69 | * All Rights Reserved.
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| 70 |
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| 71 | * Licence:
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| 72 | * This program is free software: you can redistribute it and/or
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| 73 | * modify it under the terms of the GNU Lesser General Public
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| 74 | * License as published by the Free Software Foundation, either
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| 75 | * version 3 of the License, or (at your option) any later
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| 76 | * version.
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| 77 | *
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| 78 | * This program is distributed in the hope that it will be useful,
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| 79 | * but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 80 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 81 | * GNU Lesser General Public License for more details.
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| 82 | *
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| 83 | * You should have received a copy of the GNU Lesser General
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| 84 | * License along with this program. If not, see
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| 85 | * <http://www.gnu.org/licenses/>.
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| 86 |
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| 87 | * Bugs:
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| 88 | * {note_any_bugs_here}
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| 89 | *-
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| 90 | */
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| 91 |
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| 92 | #include "pal.h"
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| 93 | #include <math.h>
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| 94 |
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| 95 | void
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| 96 | palDh2e ( double az, double el, double phi, double *ha, double *dec) {
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| 97 |
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| 98 | double sa;
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| 99 | double ca;
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| 100 | double se;
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| 101 | double ce;
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| 102 | double sp;
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| 103 | double cp;
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| 104 |
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| 105 | double x;
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| 106 | double y;
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| 107 | double z;
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| 108 | double r;
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| 109 |
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| 110 | /* Useful trig functions */
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| 111 | sa = sin(az);
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| 112 | ca = cos(az);
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| 113 | se = sin(el);
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| 114 | ce = cos(el);
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| 115 | sp = sin(phi);
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| 116 | cp = cos(phi);
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| 117 |
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| 118 | /* HA,Dec as x,y,z */
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| 119 | x = -ca * ce * sp + se * cp;
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| 120 | y = -sa * ce;
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| 121 | z = ca * ce * cp + se * sp;
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| 122 |
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| 123 | /* To HA,Dec */
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| 124 | r = sqrt(x * x + y * y);
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| 125 | if (r == 0.) {
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| 126 | *ha = 0.;
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| 127 | } else {
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| 128 | *ha = atan2(y, x);
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| 129 | }
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| 130 | *dec = atan2(z, r);
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| 131 |
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| 132 | return;
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| 133 | }
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