TheSkySearchers

I am placing this in "Off Topic" because I just realise we don't have a "General Observing" sub forum!
I have come across this notion, in different forms about aperture and "seeing", here is one example:

"The larger the aperture, the larger the volume of air that the light rays coming into the scope must pass through. Thus scopes of larger aperture tend to be more sensitive to turbulent air ("seeing") than scopes of smaller aperture."

I don't understand the logic because it seems to insinuate that small aperture telescopes are less affected by bad seeing conditions than large aperture telescopes ?! I have a 4" and 15" reflector and I have never experienced a situation were I could ever observe more detail with the 4" than with the 15" under bad seeing conditions. When comparing telescopes I think it is important to do so at more or less the same magnification, for example if I want to compare my 4", with a 600mm focal length, to my 15", with a 1727mm focal length, I would have to use a 10mm eyepiece in the 4" and a 28mm in the 15", to make a fair comparison. With a 28mm in the 4" bad seeing conditions would definitely be less noticeable .....at a 21 magnification not much detail of anything would be noticeable. I basically use 3 eyepieces with my 15" telescope : 27mm, 13mm and a 7mm, with or without a 2X Barlow. The size of the object and seeing conditions would determine which. This year in June I experienced the best seeing condition while observing thus far : I observed Saturn with the 7mm and could, for the first time, see more detail as with the 13mm. I then added the 2x Barlow to the 7mm and the level of detail was about the same as with just the 7mm, it was the first time that I could use the 7mm with the 2x barlow and get a half descent view.

The part that with larger aperture the volume of air increases that the light has to travel through, to me, this is like saying resisters in parallel will have more resistance than a single resister or that stacking multiple images will result in less detail. If this notion was true I think we would have seen a lot more small aperture telescopes at professional observatories for those bad seeing days.
So what does the experts say? folklore or fact?

Well, I am not an expert on this - but it would seem to me that light has to travel through the same amount of air no matter the aperture. The diameter of the aperture might allow a tad more air "currents", but the amount of air per length of tube (being the same, of course) is in fact the same. Now the micro climate in a newtonian reflector might have some influence until the temperature inside equalizes with the outside, but I don't see how the aperture would change the amount of air unless there is somehow a different density between the air in the two tubes. <shrug>

I don't believe having more air in the tube impacts what one can see. Seeing conditions theoretically should be the same at varying apertures, since it largely is an atmospheric phenomenon. I can think of situations when a newtonian is not acclimated to the outside temperature as causing more turbulence, but this should dissipate once temperature equilibrium is achieved.

I'm no expert either, but I think the "fair comparison", in this context, is usually between magnifications relative to the maximum. I also guess that a big aperture scope has better resolution, so it magnifies even problems that a smaller aperture scope doesn't "see". Kind of similar effect that when you look at a rasterized picture too close.

We had a discussion about this here a while ago but I could not find it.
A air cell (average sized) is 6-8" so a smaller aperture in turbulent air will hold a slightly steadier image longer as the skies deteriorate.

It is a scientific fact (not folklore) that large aperture scopes are more affected by seeing conditions.

Some of the "arguments" upthread are really depressing because of the defects in logic. Small aperture telescopes certainly are affected less by air currents than large aperture scopes. The logic for this is that poor seeing causes the disk of the star to expand. (It is the volume of air outside the tube that is relevant here. The only time the volume of air inside the tube is relevant is when thermal equilibrium isn't achieved.) Now if the seeing is so poor that stars are boiling 2 arc second disks then the 0.5 arc second resolution of a 10" scope isn't going to happen and neither is the 1 arc second of a 5" scope. You will not get a diffraction limited Airy disk if the atmosphere won't let it.

Networks of resistors are a really bad analogy for this and are irrelevant. What is happening is that differences in index of refraction in the air column causes waveform errors that prevent a diffraction limited image. You cannot resolve what the atmosphere won't let you resolve.

Resolution is not the only function of a telescope though. They also collect light. Even on a bad seeing night a bigger light bucket still collects more light. This is why large observatories still operate large telescopes on bad seeing nights. If you are feeding a spectrograph the slit of the instrument maintains the resolution since the spectrogram is imaging the slit, not the star. You can compensate for the poor seeing wasting more of the light outside the slit by taking a longer exposure.

Comparing the resolution of two telescopes should be done at the same exit pupil NOT the same magnification. A trained observer can get all the resolution a scope can provide at 2mm exit pupil. If you need a smaller exit pupil (higher magnification) to distinguish more details then you have not practiced observing enough to be proficient yet. It is a fact of human physiology of the eye that the resolution maxes out at 2mm exit pupil. It is a fact of human psychology that you need to practice a skill to attain that.

The path length os of course, the same, but the path volume is different and that larger volume has more turbulent eddies and more differences in atmospheric index of refraction happening.

"It is a scientific fact (not folklore) that large aperture scopes are more affected by seeing conditions." : this statement involves long exposures and the magnitude by which its affecting a large aperture telescope...not really what I am interested in. I am specifically referring to visual observations and the question, for me, is if a small aperture telescope could ever outperform a large aperture telescope, by showing more detail, of a object during bad seeing conditions, because this is what some people are deducting from these rather complex statements that are being presented by some academics.

The KISS answer to my question, for me, is still....no!

"The larger the aperture, the larger the volume of air that the light rays coming into the scope must pass through. Thus scopes of larger aperture tend to be more sensitive to turbulent air ("seeing") than scopes of smaller aperture."
I'm not sure but...
I don't think the scope can be affected by seeing.
I think it's about the quality of image - whether drawn on film or retina.
It's just that better quality image doesn't necessarily mean more detailed image. It could also mean less detailed image that doesn't jump around.

If seeing is as large as 5 arc seconds, no amateur-telescope on earth will show details smaller than 5 arc seconds. The 4 inch telescope won't "outperform" the 8 inch telescope.

If seeing is expressed in diameter of the disturbing airbubble, the story changes. Two 5 centimeter air bubbles of different temperature that pass in front of a 4 inch telescope disturb the wavefront 4 times. The same in an 8 inch. If the disturbing air bubbles come in one long continuous row, the 4 inch telescope is still disturbed 4 times, and the 8 inch 8 times in the same time. It is obvious that the front in the 8 inch is more frequently disturbed per second.
The image in the 4 inch will be calmer, but resolution still not better.

The term "outperforming" is not specific enough.That is the problem.

Piet Le Roux wrote: ↑Wed Sep 30, 2020 8:03 pm "It is a scientific fact (not folklore) that large aperture scopes are more affected by seeing conditions." : this statement involves long exposures and the magnitude by which its affecting a large aperture telescope...not really what I am interested in.

False. It also applies to visual observations see below.

I am specifically referring to visual observations and the question, for me, is if a small aperture telescope could ever outperform a large aperture telescope, by showing more detail, of a object during bad seeing conditions, because this is what some people are deducting from these rather complex statements that are being presented by some academics.

The KISS answer to my question, for me, is still....no!

Well, you're wrong. Factually scientifically wrong and you are pushing folklore against scientific facts.

What happens with a larger aperture and poor seeing for visual observations is that you are playing a lottery. Because turbulence is dynamic there are random times when the mean error in the wavefront due to the atmosphere is small. These are the "moments" of good seeing that skilled visual observers wait patiently for. Then and only then the eye and larger aperture scope can capture the resolution that the scope is capable of achieving. The wait time between moments of good seeing is longer for larger aperture scopes.

If that were not the case I would always take out the 12" scope. But on nights of average or worse seeing the wait times between those moments becomes unacceptable. I just will not bother with the 12" since for average seeing conditions it's a waste of effort. For average conditions I'll use something in the 4-8" class since the wait times between those moments of good seeing are acceptably brief and the scope is not so much work to set up.

I suppose if you are willing to bear any heavy burden of set up and willing to wait indefinitely for a moment of clarity you won't care what the facts are. I know from experience and the science that larger aperture scopes are more affected by poor seeing for visual use in that the wait times to get a stable image are longer. That's a fact. It's also the science.

turboscrew wrote: ↑Wed Sep 30, 2020 8:38 pm "The larger the aperture, the larger the volume of air that the light rays coming into the scope must pass through. Thus scopes of larger aperture tend to be more sensitive to turbulent air ("seeing") than scopes of smaller aperture."
I'm not sure but...
I don't think the scope can be affected by seeing.
I think it's about the quality of image - whether drawn on film or retina.
It's just that better quality image doesn't necessarily mean more detailed image. It could also mean less detailed image that doesn't jump around.

This is false. If the seeing is producing a 2" disk then resolution is limited to 2" by the atmosphere.

https://weather.gc.ca/astro/seeing_e.html

https://en.wikipedia.org/wiki/Astronomical_seeing

My own personal experiences can easily back up the claim that a smaller aperture can bring better results in not so good conditions .
My main interest in astronomy is solar imaging , I have 3 Ha solar scopes , 60mm 90mm and 150mm . My 60mm scope is very forgiving and can give good results even in poor conditions , My 90mm is less forgiving in poor conditions but works very good in my average conditions . Now by comparison my 150 mm scope requires almost pristine conditions for me to use at all and I am lucky if I see these conditions more than a couple of times a year . The larger aperture not only resolves all of the fine detail I want to capture it also does a very good job of resolving the distortion between my scope and the target. I know this is not night time viewing but I do believe this one example of how a larger aperture can work against you .

Solsearcher wrote: ↑Wed Sep 30, 2020 9:55 pm My own personal experiences can easily back up the claim that a smaller aperture can bring better results in not so good conditions .
My main interest in astronomy is solar imaging , I have 3 Ha solar scopes , 60mm 90mm and 150mm . My 60mm scope is very forgiving and can give good results even in poor conditions , My 90mm is less forgiving in poor conditions but works very good in my average conditions . Now by comparison my 150 mm scope requires almost pristine conditions for me to use at all and I am lucky if I see these conditions more than a couple of times a year . The larger aperture not only resolves all of the fine detail I want to capture it also does a very good job of resolving the distortion between my scope and the target. I know this is not night time viewing but I do believe this one example of how a larger aperture can work against you .

The physics of turbulence and its effects on optical performance doesn't care whether it is day or night.

One final thought. It also depends on the observing target, for me anyway.

OTOneHand if the evening is devoted to DSOs there is no point to using the largest aperture available when the seeing doesn't permit. I like to wait for nights of good seeing for resolving globular clusters and close doubles.

OTOtherHand the Mars opposition is coming up. That is a rare window of opportunity. My approach to that will be to use the largest aperture I can manage and tough it out waiting for those random moments when the seeing is good for a split second.

I found this explanation on the "Sky and Telescope" website:

Sky & Telescope: BY: ALAN MACROBERT AUGUST 14, 2006
Large or slow-moving eddies cause slow seeing, but they don't stay large forever. No matter what size the eddies are when they originate, they break up into smaller and smaller ones. When these finally become small enough to measure in millimeters, they die out and dissipate their energy as heat via the air's fluid friction (viscosity).

A light wave from a star is distorted on many size scales by the atmosphere. When the wavefront enters a telescope, its 'tilt' determine's the star's apparent position, while its 'roughness' determines how fuzzy the star looks.
A light wave from a star is distorted on many size scales by the atmosphere. When the wavefront enters a telescope, its 'tilt' determine's the star's apparent position, while its 'roughness' determines how fuzzy the star looks. Generally a small telescope sees a relatively sharp star dancing around, while a large one sees a relatively steady but fuzzy star.

This complex situation belies an often-repeated piece of astronomer's lore: that seeing cells are 10 centimeters (4 inches) in size. In fact they come in all sizes. But cells in this middle range do have an important property: they affect a large telescope more seriously than a small one. If you have a 4-inch scope, cells 4 inches and larger passing through its line of sight will make an image move around while staying relatively intact. The same cells passing in front of a 12-inch aperture will superpose multiple images at once.

This fact has led to another piece of folklore: that when the atmospheric seeing is bad, a large telescope shows less detail than a small one. Therefore, supposedly, you can improve the view in poor seeing by stopping down a large aperture with a cardboard mask.

Technically there is a bit of truth in this, but in practice the improvement is nonexistent. I have never seen any improvement by stopping down a telescope when the problem was poor seeing. The most that can usually be said is that on a really rotten night, large- and small-aperture views will be equally poor. Even then, if you constrict the aperture you miss the chance for the momentary high-resolution views that the full aperture will provide if the air briefly steadies.

There are reasons why you may indeed see more sharply through a stopped-down telescope. Most of them are bad — and have nothing to do with the atmosphere. Maybe your eye was dazzled by a too-bright planet; in that case an eyepiece filter would solve the problem better than a reduced aperture. Maybe the aperture stop is masking off the optical errors of a flawed objective. Maybe it's just allowing a mediocre eyepiece to perform better by increasing the telescope's f/ratio. Poor collimation of the optical parts is also less damaging when the f/ratio is increased.

The whole article : https://skyandtelescope.org/astronomy-e ... he-seeing/

Piet Le Roux wrote: ↑Thu Oct 01, 2020 11:59 am
.......

This fact has led to another piece of folklore: that when the atmospheric seeing is bad, a large telescope shows less detail than a small one. Therefore, supposedly, you can improve the view in poor seeing by stopping down a large aperture with a cardboard mask.

Technically there is a bit of truth in this, but in practice the improvement is nonexistent.

........

The whole article : https://skyandtelescope.org/astronomy-e ... he-seeing/

As far as it goes, it’s correct but it is still incomplete. It never addresses the effect of aperture on the mean time between moments of good seeing. That’s what makes the wait time for good moments longer for larger aperture when seeing is poor.

I’m not a fan of the aperture mask technique, btw. I will cut my setup labor overhead and use a smaller aperture scope altogether. The rule I ordinarily apply is to use an aperture that can resolve the seeing disk by a factor of two. So a night of average seeing has 1-2 arc seconds of seeing and a 5-6” aperture is the most I’ll set up.

It’s not folklore that the wait time for a good seeing moment is longer for a larger aperture scope. It’s a pity that the author of the article neglected that effect. Of course if one enjoys a permanent observatory one might be a little spoiled by that and not consider setup and take down time. I prefer to continue enjoying the hobby. I don’t want to bother with 120 lbs of kit on a poor seeing night.

notFritzArgelander wrote: ↑Wed Sep 30, 2020 9:15 pm

turboscrew wrote: ↑Wed Sep 30, 2020 8:38 pm "The larger the aperture, the larger the volume of air that the light rays coming into the scope must pass through. Thus scopes of larger aperture tend to be more sensitive to turbulent air ("seeing") than scopes of smaller aperture."
I'm not sure but...
I don't think the scope can be affected by seeing.
I think it's about the quality of image - whether drawn on film or retina.
It's just that better quality image doesn't necessarily mean more detailed image. It could also mean less detailed image that doesn't jump around.

This is false. If the seeing is producing a 2" disk then resolution is limited to 2" by the atmosphere.

https://weather.gc.ca/astro/seeing_e.html

https://en.wikipedia.org/wiki/Astronomical_seeing

What's false in that? I don't think the scope itself changes in any way depending on seeing.
Or is it about image quality being all about details and resolution?
Or is it that sharp "dancing" image vs. steady fuzzy blob?

turboscrew wrote: ↑Thu Oct 01, 2020 2:31 pm

notFritzArgelander wrote: ↑Wed Sep 30, 2020 9:15 pm

turboscrew wrote: ↑Wed Sep 30, 2020 8:38 pm "The larger the aperture, the larger the volume of air that the light rays coming into the scope must pass through. Thus scopes of larger aperture tend to be more sensitive to turbulent air ("seeing") than scopes of smaller aperture."
I'm not sure but...
I don't think the scope can be affected by seeing.
I think it's about the quality of image - whether drawn on film or retina.
It's just that better quality image doesn't necessarily mean more detailed image. It could also mean less detailed image that doesn't jump around.

This is false. If the seeing is producing a 2" disk then resolution is limited to 2" by the atmosphere.

https://weather.gc.ca/astro/seeing_e.html

https://en.wikipedia.org/wiki/Astronomical_seeing

What's false in that? I don't think the scope itself changes in any way depending on seeing.
Or is it about image quality being all about details and resolution?

The atmosphere is part of the optical system. It’s a mistake to neglect that. Indeed when you include the atmosphere details and resolution are affected.

notFritzArgelander wrote: ↑Thu Oct 01, 2020 2:52 pm

turboscrew wrote: ↑Thu Oct 01, 2020 2:31 pm

notFritzArgelander wrote: ↑Wed Sep 30, 2020 9:15 pm

This is false. If the seeing is producing a 2" disk then resolution is limited to 2" by the atmosphere.

https://weather.gc.ca/astro/seeing_e.html

https://en.wikipedia.org/wiki/Astronomical_seeing

What's false in that? I don't think the scope itself changes in any way depending on seeing.
Or is it about image quality being all about details and resolution?

The atmosphere is part of the optical system. It’s a mistake to neglect that. Indeed when you include the atmosphere details and resolution are affected.

Yes, but in the text: "thus scopes ... tend to be more sensitive to turbulent air". (That's why the emoticon,)

This is a good discussion for me since I was just contemplating getting a 10" or 12" F/4 Newt astrograph for AP. I was aware of the facts mentioned and I suppose they apply for AP straight so there is no point in going above 8". For visual observing of DSOs on average nights it may be different because a larger image affects the perception by the brain so that the loss in resolution is offset by the increased perceived brightness, and a few arc seconds more or less does not matter because you can't tell the difference anyway, for faint nebulosity. Not so for observing planets because brightness is not an issue, or for AP because the faint details do matter once enough light is collected.

Of course in my case where I am imaging with a DSLR where the pixels for each color are spaced further apart, a larger image projection may still result in better resolution because the pixel spacing is a limiting factor. Then there's the practical aspect of weight and how often I can set up drag it to the Sierras. The 12" has a Losmandy dovetail that I would like, but it weighs 50 lbs whereas the 10" weighs only 34 lbs. The 12" is probably too heavy for thieves to carry so I can safely leave it up on my driveway.