In short: if you correct for aberrations a refractor scope at 0m MSL, are aberrations also corrected at e.g. 2000m MSL?
I had an interesting situation (see another post I wrote), were my scope was not giving me a sharp image in Mono Lake, at an altitude of 2000m. It was as if there was a sort of fuzzy double image overlapped with the main image; as if my scope had suddenly developed something similar to a huge coma distortion. It was distinct from the usual tremor caused by heat and air turbulence.
I attributed this to some diffraction effect caused by heat over the lake. I was looking at Osprey nests, maybe 80-100m away, and about 4m over the water level.
But then I started to wonder: do scopes work equally well at altitude as they do at sea level? The problem is that the glass of the scope is unchanged, and so is the density of the gas (supposedly nitrogen) that they contain. But the density of the outside air changes, from 0m to 2000m over sea level, and so does its refractive index. And the refracting power of a lens is tied to the difference between its refractive index and the one of the medium around it. So it might be the case indeed that if one corrects for all aberrations taking into account the air density at 0m, some aberrations may no longer be corrected at 2000m.
Of course, for reflector telescopes this is not an issue. But for a refractor bird scope, with large aperture : focal length, and in need of correcting aberrations, it might be?
I had an interesting situation (see another post I wrote), were my scope was not giving me a sharp image in Mono Lake, at an altitude of 2000m. It was as if there was a sort of fuzzy double image overlapped with the main image; as if my scope had suddenly developed something similar to a huge coma distortion. It was distinct from the usual tremor caused by heat and air turbulence.
I attributed this to some diffraction effect caused by heat over the lake. I was looking at Osprey nests, maybe 80-100m away, and about 4m over the water level.
But then I started to wonder: do scopes work equally well at altitude as they do at sea level? The problem is that the glass of the scope is unchanged, and so is the density of the gas (supposedly nitrogen) that they contain. But the density of the outside air changes, from 0m to 2000m over sea level, and so does its refractive index. And the refracting power of a lens is tied to the difference between its refractive index and the one of the medium around it. So it might be the case indeed that if one corrects for all aberrations taking into account the air density at 0m, some aberrations may no longer be corrected at 2000m.
Of course, for reflector telescopes this is not an issue. But for a refractor bird scope, with large aperture : focal length, and in need of correcting aberrations, it might be?
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