Sounds like differential refraction through the atmosphere- Red on top of the disc and blue on the bottom. At low altitudes the atmosphere looks like a wedge of atmonphere and as such will cause dispersion. BTW large telescopes have atmospheric dispersion compensators and for spectroscopy guide on a particular wavelength.
The differential refraction effect looks wonderful in an APO telescope you might see the whole spectrum not just one colour. Its easy to tell the difference between chromatic aberration(CA) and differential refraction. CA has a violet fringe of unfocused red and blue light and the atmospheric dispersion will be vertical (mostly).
An APO 'scope cannot correct for atmospheric dispersion as the light has been dispersed before it reaches the optics but it gives one a better view of this aberration!!!
Jerry.
