| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palAirmas
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| 5 |
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| 6 | * Purpose:
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| 7 | * Air mass at given zenith distance
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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 palAirmas( double zd );
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| 17 |
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| 18 | * Arguments:
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| 19 | * zd = double (Given)
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| 20 | * Observed zenith distance (radians)
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| 21 |
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| 22 | * Description:
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| 23 | * Calculates the airmass at the observed zenith distance.
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| 24 |
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| 25 | * Authors:
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| 26 | * PTW: Patrick Wallace (STFC)
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| 27 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 28 | * {enter_new_authors_here}
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| 29 |
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| 30 | * Notes:
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| 31 | * - The "observed" zenith distance referred to above means "as
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| 32 | * affected by refraction".
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| 33 | * - Uses Hardie's (1962) polynomial fit to Bemporad's data for
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| 34 | * the relative air mass, X, in units of thickness at the zenith
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| 35 | * as tabulated by Schoenberg (1929). This is adequate for all
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| 36 | * normal needs as it is accurate to better than 0.1% up to X =
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| 37 | * 6.8 and better than 1% up to X = 10. Bemporad's tabulated
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| 38 | * values are unlikely to be trustworthy to such accuracy
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| 39 | * because of variations in density, pressure and other
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| 40 | * conditions in the atmosphere from those assumed in his work.
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| 41 | * - The sign of the ZD is ignored.
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| 42 | * - At zenith distances greater than about ZD = 87 degrees the
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| 43 | * air mass is held constant to avoid arithmetic overflows.
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| 44 |
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| 45 | * See Also:
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| 46 | * - Hardie, R.H., 1962, in "Astronomical Techniques"
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| 47 | * ed. W.A. Hiltner, University of Chicago Press, p180.
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| 48 | * - Schoenberg, E., 1929, Hdb. d. Ap.,
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| 49 | * Berlin, Julius Springer, 2, 268.
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| 50 |
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| 51 | * History:
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| 52 | * 2012-03-02 (TIMJ):
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| 53 | * Initial version from the SLA/F version including documentation.
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| 54 | * Adapted with permission from the Fortran SLALIB library.
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| 55 | * {enter_further_changes_here}
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| 56 |
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| 57 | * Copyright:
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| 58 | * Copyright (C) 1999 Rutherford Appleton Laboratory.
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| 59 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 60 | * All Rights Reserved.
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| 61 |
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| 62 | * Licence:
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| 63 | * This program is free software; you can redistribute it and/or
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| 64 | * modify it under the terms of the GNU General Public License as
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| 65 | * published by the Free Software Foundation; either version 3 of
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| 66 | * the License, or (at your option) any later version.
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| 67 | *
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| 68 | * This program is distributed in the hope that it will be
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| 69 | * useful, but WITHOUT ANY WARRANTY; without even the implied
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| 70 | * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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| 71 | * PURPOSE. See the GNU General Public License for more details.
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| 72 | *
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| 73 | * You should have received a copy of the GNU General Public License
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| 74 | * along with this program; if not, write to the Free Software
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| 75 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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| 76 | * MA 02110-1301, USA.
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| 77 |
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| 78 | * Bugs:
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| 79 | * {note_any_bugs_here}
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| 80 | *-
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| 81 | */
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| 82 |
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| 83 | #include "pal.h"
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| 84 | #include "palmac.h"
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| 85 |
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| 86 | double palAirmas ( double zd ) {
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| 87 | double seczm1;
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| 88 | double airmass;
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| 89 |
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| 90 | /* Have maximum zenith distance of 87 deg */
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| 91 | const double MAXZD = 87.0 * PAL__DD2R;
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| 92 |
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| 93 | zd = fabs(zd);
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| 94 | zd = ( zd > MAXZD ? MAXZD : zd );
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| 95 |
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| 96 | seczm1 = (1.0 / cos(zd)) - 1.0;
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| 97 | airmass = 1.0 + seczm1*(0.9981833 - seczm1*(0.002875 + 0.0008083*seczm1));
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| 98 | return airmass;
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| 99 | }
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