.help atmdsp Jun99 "Slalib Package"
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      SUBROUTINE slATMD (TDK, PMB, RH, WL1, A1, B1, WL2, A2, B2)

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      A T M D
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  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.

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