| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palPa
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| 5 |
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| 6 | * Purpose:
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| 7 | * HA, Dec to Parallactic Angle
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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 | * double palPa( double ha, double dec, double phi );
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| 17 |
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| 18 | * Arguments:
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| 19 | * ha = double (Given)
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| 20 | * Hour angle in radians (Geocentric apparent)
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| 21 | * dec = double (Given)
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| 22 | * Declination in radians (Geocentric apparent)
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| 23 | * phi = double (Given)
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| 24 | * Observatory latitude in radians (geodetic)
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| 25 |
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| 26 | * Returned Value:
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| 27 | * palPa = double
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| 28 | * Parallactic angle in the range -pi to +pi.
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| 29 |
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| 30 | * Description:
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| 31 | * Converts HA, Dec to Parallactic Angle.
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| 32 |
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| 33 | * Authors:
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| 34 | * PTW: Patrick T. Wallace
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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 | * - The parallactic angle at a point in the sky is the position
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| 40 | * angle of the vertical, i.e. the angle between the direction to
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| 41 | * the pole and to the zenith. In precise applications care must
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| 42 | * be taken only to use geocentric apparent HA,Dec and to consider
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| 43 | * separately the effects of atmospheric refraction and telescope
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| 44 | * mount errors.
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| 45 | * - At the pole a zero result is returned.
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| 46 |
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| 47 | * History:
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| 48 | * 2012-03-02 (TIMJ):
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| 49 | * Initial version
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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) 1995 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 General Public License as
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| 61 | * published by the Free Software Foundation; either version 3 of
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| 62 | * the License, or (at your option) any later version.
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| 63 | *
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| 64 | * This program is distributed in the hope that it will be
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| 65 | * useful, but WITHOUT ANY WARRANTY; without even the implied
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| 66 | * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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| 67 | * PURPOSE. See the GNU General Public License for more details.
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| 68 | *
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| 69 | * You should have received a copy of the GNU General Public License
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| 70 | * along with this program; if not, write to the Free Software
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| 71 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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| 72 | * MA 02110-1301, USA.
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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 |
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| 81 | #include <math.h>
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| 82 |
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| 83 | double palPa( double ha, double dec, double phi ) {
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| 84 | double cp, sqsz, cqsz;
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| 85 |
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| 86 | cp = cos(phi);
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| 87 | sqsz = cp * sin(ha);
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| 88 | cqsz = sin(phi) * cos(dec) - cp * sin(dec) * cos(ha);
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| 89 | if (sqsz == 0.0 && cqsz == 0.0) cqsz = 1.0;
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| 90 | return atan2( sqsz, cqsz );
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| 91 | }
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