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