Spotting scope aperture and heat waves. Seeing? (1 Viewer)

[Swampy Sam]

Just wondering what you guys thought? Smaller aperture show more details than larger aperture on hot days?

 

[Binastro]

It is possible.

For astro telescopes bad Seeing can result in poorer images in larger scopes.
But if the Seeing is that bad one simply does not observe, at least not planets.

If the Seeing was bad I would not bother to set up my 12.5 inch or 14.5 inch planetary scopes, but might use a low power 5 inch refractor.

Bird watchers seem to typically observe in poor conditions.
An 85mm scope at low power might still be better than a smaller scope.
Perhaps put a towel over the top of the scope, and observe from higher up.

Square wooden telescopes typically do better than compact round scopes.

Regards,
B.

 

[has530]

I would think the opposite would be true although it's unlikely it would make a real difference. The biggest factor is magnification. Larger aperture may help a tiny bit as if the turbulence/thermal flows are very localized/small scale you may be able to kind of see around them with the parallax from a large objective but I doubt this would ever really help in reality.

 

[Binastro]

The cells of air are reckoned to be 4 inch across, although I have seen mention of 2 inch cells.

The turbulence will not be localised.

With the small size of bird watching scopes, I think we are talking of terrible turbulence near the ground.

I think someone who actually uses different size spotting scopes in these poor conditions may know better.

Regards,
B.

 

[Swampy Sam]

Thanks guys. I have 85 and 65mm scopes the smaller one does seem to give a sharper image on extremely hot days.. of course most of the days are hot in Florida.

 

[kabsetz]

There was quite a bit of talk about this many years back as Better View Desired optics review site was pretty strongly endorsing this idea that smaller scopes would cut through heat haze better than bigger ones.

I did quite a bit of testing and trials on this with literally dozens of scopes, and ended up being convinced that what really matters is freedom from optical aberrations. Since smaller scopes have usually been easier to manufacture to stricter quality standards, they used to generally have lower aberrations than bigger scopes and would perform better, or at least not degrade as much relatively speaking, when seeing was poor. In heat haze, a scope with lots of aberrations (spherical aberration, coma, astigmatism) will show a significantly worse image and be harder to focus than a scope with low aberrations, irrespective of whether they are the same size or same or different makes or models.

When I went through the process of acquiring my previous "reference scope," a Nikon Fieldscope ED 82 A, one of my testing methods was to compare specimen directly against a known super-low-aberration specimen of a Fieldscope ED 78. Not until I found a sample with even more close to perfect optics, did it out-perform the smaller scope in heat-haze.

My current Swaro ATX 95 has even lower aberrations, and is the best scope in heat haze I have ever used despite its large front lens.

I would like to eventually upgrade to the X115, but haven't found a sample yet that would pass this test.

Regards,
Kimmo

 

[jring]

Hi,

as has been noted by others, two perfect scopes (or at least examples with the same amount of aberrations) of different sizes will probably show better perfomance of the smaller scope in bad seeing. That's the conventional wisdom of the astro crowd. Even better would be large binoculars as you have two tubes then with each having a chance of a good view and the brain will merge the two images...

So if you want a stationary scope, a second look at big bins (Highlander, APM and so on, 10x56 need not apply) might offer the best views...

Joachim

 

[Swampy Sam]

Thanks Joachim, I often glass with my 10x42 Swarros on my tripod. I love everything about it except the magnification Drop off. everything is razor sharp and no seeing issues at all, unless you are going for miles away.. I've always wondered why some of the bigger bino companies don't make zoom binoculars.. I've looked at Celestron and Nikon but am always reluctant to by more optics..

 

[4th_point]

Just wondering what you guys thought? Smaller aperture show more details than larger aperture on hot days?

I asked a similar question last year.

How can 60mm be better than 80mm (help needed)?

In post #10 of that thread I described one situation where the smaller scope was definitely better than the larger scope, however I have not been able to repeat it since. That one situation was really bad viewing with smoke plus heat haze. And I don't know for sure if the apertures per se were really the reason for the differences observed. In hindsight I wish I would have placed a mask on the larger scope's objective.

 

[beacon hill]

"In hindsight I wish I would have placed a mask on the larger scope's objective."

In really bad seeing conditions, stopping my 12" Dob with a aperture mask gives much steadier views of
the Moon and Planets. Also despite the smaller aperture can resolve double stars more clearly. Again, this is only on
nights of really bad seeing. On steady nights aperture always wins.

Have had a number of people question this on other forums. But usually from users who have not tried
it for themselves.

 

[Hermann]

Since smaller scopes have usually been easier to manufacture to stricter quality standards, they used to generally have lower aberrations than bigger scopes and would perform better, or at least not degrade as much relatively speaking, when seeing was poor. In heat haze, a scope with lots of aberrations (spherical aberration, coma, astigmatism) will show a significantly worse image and be harder to focus than a scope with low aberrations, irrespective of whether they are the same size or same or different makes or models.

That's a perfect summary.

Hermann

 

[DRodrigues]

Besides all said I add 2 details:

• usually smaller scopes have less minimum magnifications so looking better within hot days, due to lower magnifications;
• being all equal, smaller scopes work better since have higher depth of field, so are less affected by thermal waves. Within my search for different solutions, I tested a 80mm triplet apo-refractor that was used for bird dslr photography and had a manual aperture regulation (with a circular guillotine, as on tele-lenses...), and visual image quality and resolution increased reducing aperture within bad seeing conditions!

 

[Fedster]

When I went through the process of acquiring my previous "reference scope," a Nikon Fieldscope ED 82 A, one of my testing methods was to compare specimen directly against a known super-low-aberration specimen of a Fieldscope ED 78. Not until I found a sample with even more close to perfect optics, did it out-perform the smaller scope in heat-haze.

How did you do the actual testing? and, did you just go to a shop and run the gamut of their scopes?

 

[has530]

Besides all said I add 2 details:

• usually smaller scopes have less minimum magnifications so looking better within hot days, due to lower magnifications;

I think this is spot on.

• being all equal, smaller scopes work better since have higher depth of field, so are less affected by thermal waves.

I dont think depth of field really comes into play much here as thermal waves are more of a lateral refraction problem rather than a light path difference from the refractive index (as evidenced by the fact that similar things are seen in astronomy which is certainly all done beyond the hyperfocal distance). Also objective diameter does not play a big role in DOF as has been discussed ad nauseam in the binocular forum.

• Within my search for different solutions, I tested a 80mm triplet apo-refractor that was used for bird dslr photography and had a manual aperture regulation (with a circular guillotine, as on tele-lenses...), and visual image quality and resolution increased reducing aperture within bad seeing conditions!

Did you duplicate the test in good seeing conditions? Is it possible you are just increasing the resolution by stopping down your scope (ie increasing the f/ of the scope) or using only the best central part of your eye by reducing the size of the exit pupil?