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From: Marcel E. Tschudin (no email)
Date: Sat Aug 13 2005 - 12:48:14 EDT
Fred,
Thank you for the additional address. Hopefully they may help...
> Also, I don't believe relative humidity has much influence on
> atmospheric refraction. As George H. pointed out, it's a direct
> function of density, which is primarily controlled by air temp and
> pressure.
Have a look here:
http://emtoolbox.nist.gov/Wavelength/Documentation.asp
It seems to me that one needs an additional program just for calculating the
refractive index ...
The reference data for Bennett's formula seem to have been calculated with a
refractive index of n approx. = 1.0003. Using this value provides refraction
values which correspond well with Bennett's original formula in the altitude
range of 0° to 90°. This refraction index provides however to high
refraction values than those mentioned in table 6. Could it be that the
n=1.0003 is the refractive index of air at the higher (blue) end of the
visible specrum (380nm) n = 1.000285 with some contribution of a "wet term"?
Using the refractive index of air at the lower (red) end of the visible
specrum (750nm), which is n=1.000276, the refraction values of table 6 for
maximal altitudes (-3°) could fairly well be approximated; but the
refraction values for the lower range, 0° to 90°, are then systematically to
low.
Probably one should calculate refractive index for the dominant wavelength
depending on the object's zenith distance?
This leads to still an other question: Is there a "certain factor", with
which the refractive index for a certain wavelength could be adjusted to the
conditions of the standard atmosphere?
Marcel
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