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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.
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