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Are scopes equally sharp at high altitude? (1 Viewer)

LucaPCP

Happy User
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?
 
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The refractive index of air does change due to water, temperature and co2 changes (IIRC)(Edlen corrections). Remember that the refractive index of the glass will also change with temperature too (thermo-optic effect). I’d have to do some digging to find the scale of these, but I’m not sure they’d be noticable. If the nitrogen was well sealed then it would impart a varying internal pressure as you changed altitude.
Did your scope return to normal when you came down the hill and did it behave correctly in similar (lower altitude) situations?

Peter
 
The refractive index of air is 1,00027717, so at altitude, if you can still breathe, it's still going to be somewhere between that and 1.
I suspect that tolerances of optical glass refractive indices might be of that order of magnitude.
The effects of different focus settings, which are also dependent on the viewer's eyesight, would be much greater but I can nevertheless see no loss in performance in my scopes at 5 m.

John
 
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Astro scopes are usually better at 2,000m, but they are not waterproof.

Large astro lenses work well at anything up to 40,000ft, 70,000ft and 85,000ft as well as in space.

750mm, 900mm, 1200mm focal lengths.
Also 100 inch, 180 inch, 240 inch focal lengths.

The Carlsberg 6 inch transit refractor on La Palma was at 7,800ft.
We had no problem with scopes, binoculars or cameras at 7,800ft.
Other observatories are at 10,000ft and 14,000ft.

It may be that sealed spotting scopes have problems?

Regards,
B.
 
The refractive index of air does change due to water, temperature and co2 changes (IIRC)(Edlen corrections). Remember that the refractive index of the glass will also change with temperature too (thermo-optic effect). I’d have to do some digging to find the scale of these, but I’m not sure they’d be noticable. If the nitrogen was well sealed then it would impart a varying internal pressure as you changed altitude.
Did your scope return to normal when you came down the hill and did it behave correctly in similar (lower altitude) situations?

Peter

Yes, at sea level, my scope is sharp.
For the moment, my leading explanation is that the air was differentially heated on the lake surface, in spite of the wind, and that caused a diffraction effect that gave the double image / halo. In a bit, I will have a chance to go to the mountains again, and I will test the scope again (and compare with a non-sealed scope).
 
For the moment, my leading explanation is that the air was differentially heated on the lake surface, in spite of the wind, and that caused a diffraction effect that gave the double image / halo.
And that's the explanation. Nothing to do with the altitude at all. Ant certainly nothing to do with the fact that it's a sealed scope. The atmospheric conditions, especially on sunny days and over water, can do all sorts of funny things.
In a bit, I will have a chance to go to the mountains again, and I will test the scope again (and compare with a non-sealed scope).
Just use the scope. No need to do any testing, let alone comparison testing.

Hermann
 

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