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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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