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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 13, 19:48 +0300

    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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