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.help refv Jun99 "Slalib Package"
.nf
SUBROUTINE slREFV (VU, REFA, REFB, VR)
- - - - -
R E F V
- - - - -
Adjust an unrefracted Cartesian vector to include the effect of
atmospheric refraction, using the simple A tan Z + B tan**3 Z
model.
Given:
VU dp unrefracted position of the source (Az/El 3-vector)
REFA dp tan Z coefficient (radian)
REFB dp tan**3 Z coefficient (radian)
Returned:
VR dp refracted position of the source (Az/El 3-vector)
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 algorithm used here is equivalent to
one iteration of the Newton-Raphson method applied to the above
formula.
2 Though optimized for speed rather than precision, the present
routine achieves consistency with the refracted-to-unrefracted
A tan Z + B tan**3 Z model at better than 1 micro-arcsecond within
30 degrees of the zenith and remains within 1 milliarcsecond to
beyond ZD 70 degrees. The inherent accuracy of the model is, of
course, far worse than this - see the documentation for slRFCO
for more information.
3 At low elevations (below about 3 degrees) the refraction
correction is held back to prevent arithmetic problems and
wildly wrong results. Over a wide range of observer heights
and corresponding temperatures and pressures, the following
levels of accuracy (arcsec) are achieved, relative to numerical
integration through a model atmosphere:
ZD error
80 0.4
81 0.8
82 1.6
83 3
84 7
85 17
86 45
87 150
88 340
89 620
90 1100
91 1900 } relevant only to
92 3200 } high-elevation sites
4 See also the routine slREFZ, which performs the adjustment to
the zenith distance rather than in Cartesian Az/El coordinates.
The present routine is faster than slREFZ and, except very low down,
is equally accurate for all practical purposes. However, beyond
about ZD 84 degrees slREFZ should be used, and for the utmost
accuracy iterative use of slRFRO should be considered.
P.T.Wallace Starlink 26 December 1994
Copyright (C) 1995 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
.fi
.endhelp
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