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+.help atmdsp Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slATMD (TDK, PMB, RH, WL1, A1, B1, WL2, A2, B2)
+
+     - - - - - - -
+      A T M D
+     - - - - - - -
+
+  Apply atmospheric-dispersion adjustments to refraction coefficients.
+
+  Given:
+     TDK       d       ambient temperature, degrees K
+     PMB       d       ambient pressure, millibars
+     RH        d       ambient relative humidity, 0-1
+     WL1       d       reference wavelength, micrometre (0.4D0 recommended)
+     A1        d       refraction coefficient A for wavelength WL1 (radians)
+     B1        d       refraction coefficient B for wavelength WL1 (radians)
+     WL2       d       wavelength for which adjusted A,B required
+
+  Returned:
+     A2        d       refraction coefficient A for wavelength WL2 (radians)
+     B2        d       refraction coefficient B for wavelength WL2 (radians)
+
+  Notes:
+
+  1  To use this routine, first call slRFCO specifying WL1 as the
+     wavelength.  This yields refraction coefficients A1,B1, correct
+     for that wavelength.  Subsequently, calls to slATMD specifying
+     different wavelengths will produce new, slightly adjusted
+     refraction coefficients which apply to the specified wavelength.
+
+  2  Most of the atmospheric dispersion happens between 0.7 micrometre
+     and the UV atmospheric cutoff, and the effect increases strongly
+     towards the UV end.  For this reason a blue reference wavelength
+     is recommended, for example 0.4 micrometres.
+
+  3  The accuracy, for this set of conditions:
+
+        height above sea level    2000 m
+                      latitude    29 deg
+                      pressure    793 mB
+                   temperature    17 degC
+                      humidity    50%
+                    lapse rate    0.0065 degC/m
+          reference wavelength    0.4 micrometre
+                star elevation    15 deg
+
+     is about 2.5 mas RMS between 0.3 and 1.0 micrometres, and stays
+     within 4 mas for the whole range longward of 0.3 micrometres
+     (compared with a total dispersion from 0.3 to 20.0 micrometres
+     of about 11 arcsec).  These errors are typical for ordinary
+     conditions and the given elevation;  in extreme conditions values
+     a few times this size may occur, while at higher elevations the
+     errors become much smaller.
+
+  4  If either wavelength exceeds 100 micrometres, the radio case
+     is assumed and the returned refraction coefficients are the
+     same as the given ones.
+
+  5  The algorithm consists of calculation of the refractivity of the
+     air at the observer for the two wavelengths, using the methods
+     of the slRFRO routine, and then scaling of the two refraction
+     coefficients according to classical refraction theory.  This
+     amounts to scaling the A coefficient in proportion to (n-1) and
+     the B coefficient almost in the same ratio (see R.M.Green,
+     "Spherical Astronomy", Cambridge University Press, 1985).
+
+  P.T.Wallace   Starlink   6 October 1995
+
+  Copyright (C) 1995 Rutherford Appleton Laboratory
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp