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    Re: Refraction correction
    From: Marcel Tschudin
    Date: 2009 Apr 14, 13:01 +0300

    Peter,
    
    The refraction tables are generally made based on a standard
    atmosphere and assuming "astronomical viewing conditions". Refraction
    depends on the temperature gradients in the atmosphere, temperature
    and pressure but also - and this is important - on the angle of
    incidence of the viewed ray. This is the reason for not doing
    astronomical measurements below about 5 degrees altitude.
    
    The presence of clouds may cause various different disturbances of the
    viewed ray, which - depending on the situation - may more or less
    disturb the measurement. This would require a distinction between
    critical and non-critical cases. I don't know if someone already has
    looked into this in more detail.
    
    Marcel
    
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