Apply atmospheric-dispersion adjustments to refraction coefficients palAtmdsp
To use this routine, first call palRefco specifying WL1 as the wavelength. This yields refraction coefficients A1,B1, correct for that wavelength. Subsequently, calls to palAtmdsp specifying different wavelengths will produce new, slightly adjusted refraction coefficients which apply to the specified wavelength.
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.
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.
If either wavelength exceeds 100 micrometres, the radio case is assumed and the returned refraction coefficients are the same as the given ones. Note that radio refraction coefficients cannot be turned into optical values using this routine, nor vice versa.
The algorithm consists of calculation of the refractivity of the air at the observer for the two
wavelengths, using the methods of the palRefro 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).