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.help refz Jun99 "Slalib Package"
.nf
SUBROUTINE slREFZ (ZU, REFA, REFB, ZR)
- - - - -
R E F Z
- - - - -
Adjust an unrefracted zenith distance to include the effect of
atmospheric refraction, using the simple A tan Z + B tan**3 Z
model (plus special handling for large ZDs).
Given:
ZU dp unrefracted zenith distance of the source (radian)
REFA dp tan Z coefficient (radian)
REFB dp tan**3 Z coefficient (radian)
Returned:
ZR dp refracted zenith distance (radian)
Notes:
1 This routine applies the adjustment for refraction in the
opposite sense to the usual one - it takes an unrefracted
(in vacuo) position and produces an observed (refracted)
position, whereas the A tan Z + B tan**3 Z model strictly
applies to the case where an observed position is to have the
refraction removed. The unrefracted to refracted case is
harder, and requires an inverted form of the text-book
refraction models; the formula used here is based on the
Newton-Raphson method. For the utmost numerical consistency
with the refracted to unrefracted model, two iterations are
carried out, achieving agreement at the 1D-11 arcseconds level
for a ZD of 80 degrees. The inherent accuracy of the model
is, of course, far worse than this - see the documentation for
slRFCO for more information.
2 At ZD 83 degrees, the rapidly-worsening A tan Z + B tan**3 Z
model is abandoned and an empirical formula takes over. Over a
wide range of observer heights and corresponding temperatures and
pressures, the following levels of accuracy (arcsec) are
typically achieved, relative to numerical integration through a
model atmosphere:
ZR error
80 0.4
81 0.8
82 1.5
83 3.2
84 4.9
85 5.8
86 6.1
87 7.1
88 10
89 20
90 40
91 100 } relevant only to
92 200 } high-elevation sites
The high-ZD model is scaled to match the normal model at the
transition point; there is no glitch.
3 Beyond 93 deg zenith distance, the refraction is held at its
93 deg value.
4 See also the routine slREFV, which performs the adjustment in
Cartesian Az/El coordinates, and with the emphasis on speed
rather than numerical accuracy.
P.T.Wallace Starlink 19 September 1995
Copyright (C) 1995 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
.fi
.endhelp
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