.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