300mm Dobsonian...blur

[laowho]

Good news Loeina,

There's the idea that gets bandied around that a large mirror increases the thermal fluctuations in the atmosphere, and that smaller apertures cut through these thermal pockets better. And often the line that gets drawn for the largest aperture that doesn't suffer under this detriment is around 12". Now, if you read Sky and Telescope's article on common telescope myths, you'll find this particular contention. TelescopeOptics.com also dismisses this, though it takes two pages of tortuous math to reach the same conclusion. But we were recently out with a very experienced observer who insisted that experience had nevertheless proven to him the validity of the claim.

[AlexB67]

Good news Loeina,

There's the idea that gets bandied around that a large mirror increases the thermal fluctuations in the atmosphere, and that smaller apertures cut through these thermal pockets better. And often the line that gets drawn for the largest aperture that doesn't suffer under this detriment is around 12". Now, if you read Sky and Telescope's article on common telescope myths, you'll find this particular contention. TelescopeOptics.com also dismisses this, though it takes two pages of tortuous math to reach the same conclusion. But we were recently out with a very experienced observer who insisted that experience had nevertheless proven to him the validity of the claim.

Any links to that article ?,  I would like to see it. I wonder what assumptions are made and how that result drops out. 

[laowho]

Wish I knew how to copy and paste on Nexus 7...pathetic, I know. So I'll write as much of it as I can here...

www.skyandtelescope.com/howto/visualobserving/3305656.html?page=5&c=y

Page 5 of "Four Infamous Telescope Myths" by Gary Seronik...(myth #4..."large scopes are more adversely affected by seeing than small ones")

[AlexB67]

Thank you, I was looking for the big math section   but anyway now I see, but it is all sort of  nailed in the last paragraph and a bit, I don't think he is actually denying it in the way I understand it, a little play with words as much as anything in that article.

[laowho]

He says itsa myth for two reasons...that he's never noticed such, and that there's no plausible mechanism to explain why it should be so. He made a mask and experimented for several years, never once noticing any better detail with the mask, not even on the planets. The other article I referenced is much more involved and very scientific, but concludes that although conditions could theoretically be such where smaller aperture would perform better, other conditions dictate that this can almost never happen...see telescopes-optics.net chapter 5 "induced aberrations" section 1 "air medium errors" and. "Seeing error" and "seeing aperture"

5.1.1.1 Seeing and Aperture (but read through to the bottom of the following page or you'll think that it's true...)

http.www.telescope-optics.net/seeing_and_aperture.htm

[AlexB67]

Thank you. I'll read it in some more detail when I am fresher, I had that series of articles in my bookmarks but never got to reading that part of it so far.

[laowho]

Anybody who can decipher the math in the above could maybe paraphrase the findings/dynamics at play and whether/where/when a smaller aperture is advantageous for bad seeing. But I'm glad to get this site out for everyone's benefit...it really is a remarkable wealth of information and quite the labor of love...something I see a lot of in astronomy generally as compared with other hobbies?

Cheers

[astronymonkey]

I havent looked at the article yet but I always thought that it was all down to the resolution of the scope. As bigger scopes can resolve finerdetails you were more likely to be affected by smaler atomospheric disturbances that couldnt be resolved or seen in a smaller scope. So I didnt think it was due to a small aperture cutting through poor seeing better but was more down to the smaller scope being less likely to be able to resolve the ripples in the first place.

With this in mind id agree that a big scope cout have it full potential restricted by poor seeing but only to the point that it would be limited to the resolution of the ripples in the atomosphere rather than the resolution of the mirror.

like I said I havent read the article but this is what I always thought was the explanation ....

cheers

[Moonshane]

Seronik is far more experienced than me but he completely contradicts himself as far as I can tell:

"Most often I would wind up using my telescope's full aperture because even on nights of substandard seeing, occasional brief moments of stillness would allow me to see details that simply could not be seen with the aperture mask in place."

My point about seeing is that the larger the aperture, and the faster the focal ratio the longer you have to wait for the moments of good seeing if it's mediocre. Granted when the seeing pings into sharpness the detail with more aperture is stunning but with a smaller aperture, especially one with a slower focal ratio, you get a more stable image more of the time so can 'soak up' the detail more effectively. If we all agreed on aspects of astronomy, we'd have nothing to discuss on forums like SGL

Don't get me wrong, the detail through the full aperture in great seeing is simply jawdropping. Also, with things like moons, the crepe ring on Saturn etc you can certainly see them more easily with more aperture no matter what the seeing but my experience dictates that for me at least the view through my 16" scope masked to 170mm is more often than not a better all round view than with the full aperture. With great seeing the mask comes off immediately and I enjoy what the aperture provides.

[Loeina]

I assume "seeing" is the conditions of the weather around the observer? interesting discussion thought!

[Loeina]

nevermind, first reply before

[laowho]

nevermind, first reply before

Lol. Me too!

Here's what the more scientific report concluded:

"What all this implies is that it is possible for a significantly smaller aperture to outperform the larger one, but it requires them to be, and to remain, at a specific seeing error level, generally involving compromised seeing conditions. It is only possible when ... in the larger aperture is ..., in which case a smaller aperture with ...will have, all else equal, better resolution and contrast transfer. In other words, smaller aperture could perform better while the seeing error is large enough, but if it lessens sufficiently due to seeing fluctuations, larger aperture would rebound and perform better. Typically, seeing fluctuations are wide enough for that to happen."

[laowho]

P.s. Moonshane,

I would agree about your insightful comment re: Seronik's apparent contradiction, but it seems that he made it by misunderstanding the full dynamics of what he was testing and seeing. The other article, which has so much heady stuff in it, treats of short- vs. long- visual (only) exposures to SO MANY factors occurring that Seronok's--indeed, all of our--characterizations of the seeing conditions dynamics are falling short and failing in one way or another. Which is why the best that I can do is supply the conclusion that, under certain static atmospheric considerations a smaller aperture could perform better theoretically, but that in real-time the atmospheric fluctuations (which is all but the absolute dead-steadiest air) will average out all the advantages and disadvantages of either aperture.

[Crebles]

I had a similar problem once, and it turned out to be exactly as AlexB67 describes earlier (page 1 of this thread). I'd somehow connected the 2" eyepiece adaptor AND the 1.25" one together. DoH!

Whatever the problem turns out to be, I hope you get it sorted. Best of luck.

[mdstuart]

I also agree. The 14 inch is much more fussy to seeing than my 10 inch and the 90mm refractor seems to give sharper views in most conditions than the 10 inch.

So Shanes stopping down plan I think is the right way to go for sharp views with a big dob unless the seeing is v good.

BUT

As a galaxy hunter looking at those fuzzies I need ALL the light I can get...A fuzzy fuzzy is better than no fuzzy at all!

Mark

[laowho]

I read a little further:

"Considering that seeing fluctuates, and that it will always be better than the average within an appreciable portion of time, an optimum aperture, which would perform best during those times, should be somewhat larger...Since it is the contrast level that determines resolution limit to extended details, like those on planetary surface, this implies that larger apertures, in general, outperform smaller ones in this respect, despite having a larger seeing error...larger apertures tend to retain edge in contrasts related resolution..."

I've never used a mask but may try it just to see...as I said, and as some here have said, some people report masking gains, but when reading the in-depth analysis cited above, I can't understand how they're doing it, which makes me wonder whether they really are. Guess I'll have to "see" for myself.

Cheers

[Moonshane]

I have a theory (anecdotal only) that it's partly due to exit pupil/focal ratio. e.g. with a fast ratio like f4, the focus is much more critical than a f11. this suggests that there is more depth of field to a slower ratio in a way similar to a camera system. therefore sharper detail remains sharper for longer as seeing waxes and wanes.

also a slower ratio gives a smaller exit pupil and therefore creates a 'refractor like view'  with more contrast and detail visible due to reduced glare.

at the end of it all, I like my big fast dob and my small slow dob and the convenience of masking my big dob means I don't have to have both out at the same time.

[laowho]

Well,

I'm all for anecdotal. Even science can be said to be an accumulation of anecdotal evidence, and I much prefer such to "airy abstraction." That of which you speak isn't touched upon in the article, unless it's subsumed under one of the many things there that I don't understand. Our scope is 12.5" and probably too small to notice the difference, and it's also f/6, another strike against finding any performance as per your factors. But we're ideally situated for the experiment, having identical optics side by side for comparison, and who can resist the chance to optimize their equipment?

[John]

I have a theory (anecdotal only) that it's partly due to exit pupil/focal ratio. e.g. with a fast ratio like f4, the focus is much more critical than a f11. this suggests that there is more depth of field to a slower ratio in a way similar to a camera system. therefore sharper detail remains sharper for longer as seeing waxes and wanes.

also a slower ratio gives a smaller exit pupil and therefore creates a 'refractor like view'  with more contrast and detail visible due to reduced glare.

at the end of it all, I like my big fast dob and my small slow dob and the convenience of masking my big dob means I don't have to have both out at the same time.

Interesting theory Shane. I've read some similar viewpoints regarding the performance of long, slow refractors v's faster ones.

With a shallowly tapering light cone, the zone of apparently sharp focus (depth of field ?) would be somewhat broader than with the steeply tapering cone of the faster scope as far as I can see. 

When I've been observing with my largest aperture scope and also my fastest, I've seen, as I'm sure many of us have, a sharply focused object, shift in and out of focus, due to atmospheric disturbances. With a broader zone of sharp focus I guess such interruptions would be less apparent and some would not be perceived, thus delivering what appears to be a more stable and consistent image.

During the moments of fine, settled, seeing the larger aperture scope should still show more contrast and resolution though. You just have to be more patient to pick those moments up !

[laowho]

I just remembered,

(And it's hearsay, I know), that Carl Zambuto is supposed to have said that "the magic begins at f/8," so you may be onto something.

[Alveprinsen]

about 10 meters. but im not looking through an eyepiece at the moment. just down the focuser

Try ... 1 km?

[iksobarg]

You need to use the telescope outside to get it to cool down properly to the ambient temperature. Using it inside just does not work because of the warmer air currents inside the house.

Hello Robin and Friends,

I read through this thread and got most of the parts about temperature and atmospheric fluctuations but please explain this comment further.

John

[Moonshane]

warm air escaping from a warm room completely destroys any sharpness in the eyepiece image. try observing outside near a closed back door and looking at stars above it. then open the door and see how the view changes.

[rwilkey]

Hi John, temperature affects the performance of the mirror, a well-cooled mirror will inevitably show sharper, more well-corrected images.  The tube is another consideration.  Cooled to the outside (ambient) temperature you will stand a good chance of getting better views.  A larger mirror takes longer to cool down, but half-an-hour is fairly average, but longer in many cases.  Refractors tend to require less cool down time.  Just plan your viewing around this, low power targets can be viewed while the scope is cooling down.

[astronymonkey]

Id agree with the comments regarding cool down as this causes an element of heat shimmer through the warmer air which significantly degrades the view.

For my 16inch mirror at least 30mins is needed for things to begin to sharpen and after 90mins things tend to be settled.

As for the tolerance of a fast mirror for remaining in focus , id agree with shanes view on this in that the depth of field is so shallow any minor variations in the in the way the atomosphere refracts the light will cause the image to drift in and out of focus.

At first I thought this was just my eyes but ive come to realise that its the viewing that causes the oroblem

Cheers