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76 lines
2.5 KiB
Fortran
76 lines
2.5 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_AIRMAS (ZD)
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*+
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* - - - - - - -
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* A I R M A S
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* - - - - - - -
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*
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* Air mass at given zenith distance (double precision)
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*
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* Given:
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* ZD d Observed zenith distance (radians)
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*
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* The result is an estimate of the air mass, in units of that
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* at the zenith.
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*
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* Notes:
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*
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* 1) The "observed" zenith distance referred to above means "as
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* affected by refraction".
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*
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* 2) Uses Hardie's (1962) polynomial fit to Bemporad's data for
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* the relative air mass, X, in units of thickness at the zenith
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* as tabulated by Schoenberg (1929). This is adequate for all
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* normal needs as it is accurate to better than 0.1% up to X =
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* 6.8 and better than 1% up to X = 10. Bemporad's tabulated
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* values are unlikely to be trustworthy to such accuracy
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* because of variations in density, pressure and other
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* conditions in the atmosphere from those assumed in his work.
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*
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* 3) The sign of the ZD is ignored.
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*
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* 4) At zenith distances greater than about ZD = 87 degrees the
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* air mass is held constant to avoid arithmetic overflows.
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*
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* References:
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* Hardie, R.H., 1962, in "Astronomical Techniques"
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* ed. W.A. Hiltner, University of Chicago Press, p180.
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* Schoenberg, E., 1929, Hdb. d. Ap.,
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* Berlin, Julius Springer, 2, 268.
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*
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* Original code by P.W.Hill, St Andrews
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*
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* P.T.Wallace Starlink 18 March 1999
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*
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* Copyright (C) 1999 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION ZD
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DOUBLE PRECISION SECZM1
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SECZM1 = 1D0/(COS(MIN(1.52D0,ABS(ZD))))-1D0
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sla_AIRMAS = 1D0 + SECZM1*(0.9981833D0
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: - SECZM1*(0.002875D0 + 0.0008083D0*SECZM1))
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END
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