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+      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
+*-
+
+      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