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The Skinny on Seeing

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#1 Asbytec

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Posted 25 April 2013 - 01:04 PM

This is a topic I am trying to understand better. Currently reading some references on the topic, but would like to engage in conversation alongside the study. Hey, we all know seeing sucks. But how badly? The Airy pattern is inversely proportional to aperture. It gets smaller in larger apertures, but seeing favors smaller apertures. So, what's the net FWHM as aperture increases in varying seeing? That kind of thing...interested to discuss.

So, how does one reconcile seeing favoring smaller apertures (pleasing views) and higher resolution of larger apertures? Does seeing really affect larger apertures so adversely? Okay, so stars do seem to bloat, disperse into speckle patterns, magnitude might even be a factor, and waver in some seeing conditions using some apertures. Understanding the Fried parameter, R0, and aperture is not so difficult, per se, but how does it change over given aperture ranges, obstructed or otherwise, and across varying seeing conditions? When R0 is smaller than aperture, the atmosphere determines resolution? I know what that means, but what does it mean at the eyepiece of a 10" reflector? Is all lost even in average seeing because the atmosphere essentially limits resolution to 6" or aperture averaging those wonderful steady moments?

I'm curious because I'm trying to understand the wonderful tropical seeing patterns that seem to fit my 6" aperture so nicely. According to the Pickering scale, diffraction patterns are almost always 8/10 or better. There are moments of image blurring, yet steady moments are very frequent. And, sometimes the image seems to jump around within a few arc seconds. The latter effect can be troublesome observing doubles. And I suspect that same seeing might be evaluated differently, probably, using a different apertures. Is 8/10 for a 6" aperture really 5/10 for a 10" aperture?

Okay, so how does all this come together? Don't be afraid to use MTF (LOL) or PSF, and even some lite math if you feel it applies. Thanks. (Is there a sticky thread somewhere on CN I have not found, yet?)

#2 Cotts

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Posted 25 April 2013 - 09:57 PM

Well Norme, my take on seeing is entirely anecdotal. I have found it convenient to put seeing into three categories.

First, the best one, is when the diffraction pattern is more or less visible, as in the Pickering scale but the centre point of the pattern is stationary. i.e. there is no movement of the star in RA or Dec. There may be movement of the rings and 'hair' around the pattern but it is stationary.

The second category still shows the diffraction pattern but the image 'dances' around in RA and Dec. There may be good looks at the rings but the movement makes close splits of double hard to get. It is as if someone was slightly jiggling the telescope if I may describe it that way. The center of the pattern does not remain stationary.

The third category is scintillation where the diffraction pattern breaks into 'speckles' and blooms out to (sometimes) alarming multiples of the telescope's resolving power. Under these conditions scope diameter becomes a significant factor, at least more so than the other two. Smaller scopes may not be able to resolve the speckles and show a more 'stable' image.

These categories do not have hard and fast boundaries and can change on many different time scales. I wonder how each is caused..... I would not want to have to put empirical 'rules' on these three situations. It would be a difficult task....

Dave.

#3 FirstSight

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Posted 25 April 2013 - 10:09 PM

+1 for that explanation Dave. Particularly important is that the seeming lower sensitivity of smaller vs larger apertures to imperfect seeing is due to the inherent coarser resolving ability of smaller apertures

#4 Starman1

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Posted 26 April 2013 - 12:30 AM

Seeing is never a constant. It is always fluctuating. Out of every minute of observing, the seeing will vary. Over hours, the seeing will vary significantly.
During moments of stillness, and they do occur, even during bad seeing, the larger aperture will catch fleeting glimpses of greater resolution than the smaller scope.
During moments of poor seeing, the larger scope will have reduced resolution, but not to a point worse than the small scope.
So what is is that is preferable about the small scope, to some?
It is probably that the variation in the seeing is not revealed to as great a degree, and the constancy of the seeing is apparently less annoying than having the seeing seem to go from horrible to decent and back to horrible again.

#5 Jon Isaacs

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Posted 26 April 2013 - 03:48 AM

I'm curious because I'm trying to understand the wonderful tropical seeing patterns that seem to fit my 6" aperture so nicely.



My thinking:

It if it is really that wonderful (and hopefully it is), it should fit a 12.5 inch that was cooled down and collimated even more nicely. The analysis says that for viewing the planets, it takes pretty poor seeing for a 6 inch to be optimal.

Regardless of fans and shrouds and various attempts to control thermal issues, I do think most larger scopes are not fully thermally equilibrated.

Jon

#6 davidpitre

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Posted 26 April 2013 - 07:58 AM

... seeing favors smaller apertures... Does seeing really affect larger apertures so adversely?


I have never heard a convincing argument or explanation in regards to smaller scopes being less affected by seeing. I don't believe it to be true.
I frequently hear that some telescope "cuts through the seeing" because of it's quality or aperture. Someone convince me.

#7 Asbytec

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Posted 26 April 2013 - 08:15 AM

Dave, agreed. Seeing is such a dynamic effect, continously changing, and hardly predictable that empirical rules or mathematical treatment can be inaccurate. When I evaluate seeing, I do try to nail the 'best moments' to the Pickering scale as accurately as possible. Maybe a little fudging to the downside. There are nights when the pattern is absolutely still (making presice collimation a snap.)
Such nights are beautiful.

Don, I guess that preference for "pleasing" views and fleeting moments of higher resolution is kind of what spurred this interest. My 6" is often "pleasing" (with the other 2 C's well controlled), and it seems to be putting up everything a 6" can offer...which is quite a bit and putting up full resolution, as well. I do sometimes wonder if a larger aperture would fare as well under those same conditions.

As you said:

During moments of poor seeing, the larger scope will have reduced resolution, but not to a point worse than the small scope.


We've all seen those speckle patterns and bloated stars, but what approximate aperture is effectively in use as the Airy pattern bloats with seeing and shrinks with aperture. That empirical 'thing' might be approximated on average. Does a 12" with it's tiny Airy pattern bloat so much during some seeing conditions that it's essentially useless for moderately to hi res observing? Or does the shrinking Airy pattern keep it competitive in, say, 6/10 Pickering?

Does it really take some *BLEEP* poor seeing (Pickering <6 or 5/10) to bloat those star images enough so the image begins to speckle so badly as to be "displeasing" as well as ruining resolution? And if so, to what extent would a smaller aperture with it's already diminished resolution be likewise affected under those same conditions? From what I've read, and in line with what Dave said above, it seems the Airy pattern breaks down into a Speckle pattern at about Pickering 3 or 4/10. That's probably the death nail for higher res viewing (on average.) Would someone still get a pleasing view in a 6" scope when the larger aperture's Airy pattern disinigrated at that level of seeing?

It if it is really that wonderful (and hopefully it is), it should fit a 12.5 inch that was cooled down and collimated even more nicely.


Makes some sense, but stars here do seem to twinkle pretty well (something is playing with the light path to my 5mm pupil.) It kind of goes against that old addage of good seeing when the stars are not twinkling much. That is another aspect if the seeing question, why do they twinkle as if seeing is terrible yet when a 6" aperture is applied the patterns are rock steady and Jupiter puts up her best views. Mars and Saturn and the moon, too, pushing higher power.

...it takes pretty poor seeing for a 6 inch to be <less than?> optimal.


Seeing is influenced by tube currents, too. It's all air density churning in the light path, so tube currents need to be minimized. Maybe that's the defining point, though, as tube currents can be controlled to some extent whereas seeing really cannot (except by picking an efficient location.)

Anyway, I realize my thoughts are not coherent, yet, while putting this all together. I am familiar with seeing, just would like to understand it's application at different apertures from eye ball twinkling, to 6" pleasing and rock steady Airy patterns, to total disintigration of the Airy pattern in a 12", for example. If you're 12" is in average seeing, would it still be worth keeping outside getting 8" views of plantets (on average when those fleeting moments are very far apart?)

#8 Asbytec

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Posted 26 April 2013 - 08:19 AM

Someone convince me.


Me too. While I understand the ideas behind it, and might even be experiencing it in the real world proverbial "field," I'm curious to know the dynamics between "pleasing" views and high resolution. I mean, is my 6" under what appears to be excellent seeing conditions really beating the *BLEEP* out of a 10" under average seeing conditions somewhere else? I dont think so, but...is it? LOL I might agree it's more pleasing, but is it resolving better?

Seems the seeing really breaks down when the speckle pattern rears it's ugly head at about Pickering 3 or 4/10. Here the seeing D/R0 has a ratio of about 3, which means the seeing cells are about 4" across (D=12"/R0=4" = 3.) So, what angular diameter are the Airy patterns in a 12" at this level of seeing...where 4" seeing cells pretty much match a 4" aperture which /should be/ experiencing nearly perfect seeing? In theory...in reality?

#9 Starman1

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Posted 26 April 2013 - 10:44 AM

Well, after 35 years of observing at the same site, with scopes of 90mm through 32", I can give you some idea what I see and have seen when viewing planets and double stars.
1) Small refractors usually beat small reflectors. I don't believe this is primarily due to the reflector vs. refractor debates but most often because the small reflectors are of poor optical quality (and made to a very cheap price) and poorly collimated, while small refractors are usually of high optical quality (where I observe, likely to be triplet apos), and good optics beat poor optics.
2) Reflectors of 10" and larger nearly always show more details on planets and resolve closer double stars, even in poor seeing. I say nearly always because the average 10" reflector is an inexpensive instrument while the average 7" refractor is likely to be an AstroPhysics where I observe, and there the optical quality will usually win.
3) Even at this late a date, most reflector owners are totally unaware of the thermal problems in their own scopes. I measure temperature differentials, mirror-to-air, of 15 degrees or more on a lot of scopes, and you simply can't expect the "local seeing" (i.e. in the scope) to allow the image to be good.
4) properly cooled and collimated, I ALWAYS see more planetary details in the big scopes, whatever the seeing. I've seen details on Jupiter in some optimized 12.5" scopes that a nearby 9" refractor could not see. And one image of Jupiter in a 28" was the closest thing I've see to a Hubble picture in a visual instrument. The really big difference between large scopes and small scopes is the color in the image. You would need an artist's palette to begin to depict the huge range of colors visible with larger apertures. Jupiter has corals, ochers, whites, grays, greens, blues, yellows, beiges, sages, salmons, blacks, reds, pinks, and a ton of other pale tints that are simply not seen in small apertures.
5) Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors. The reasons are multiple: thermal issues, optical quality, light scatter, poor collimation, dirty optics, etc.
But, cooled, collimated, and cleaned, a 10" reflector sees it just fine, even at f/4.5.

So, in retrospect, the issues I see with reflectors aren't due to secondary obstruction, or even seeing problems in the atmosphere. The bigger aperture always does better at "cutting through seeing" than the small aperture. The main problems are poor attention to thermal issues, collimation, cleanliness and light scatter suppression, and something that is often the "luck of the draw"--optical quality.
It makes sense, really. The typical 4' apo is what, $4000? What does the image look like in a 10" $4000 newtonian? Pretty good, I'd say, handily beating the 6" apo. (I've seen one or two over 35 years)
But is it fair to compare a normal $500 10" reflector with a $10,000 6" apo? Probably not. That so many of those 10" scopes give the 6" apo a run for the money says that aperture is a big thing, but it only gets you so far.

In the real world, then, where things are as they are, if my choice were between a 5" reflector or an inexpensive 4" achromatic refractor, I'd pick the reflector to avoid chromatic aberration (my old 4" f/15 Unitron was a great scope, but who builds f/15 refractors any more?). And I would read up on optimizing that reflector by cooling it, collimating it, cleaning it, and optimizing the suppression of light scatter.
But if the choice were between a 12.5" f/5 newtonian and a 4" apo (comparing scopes at roughly the same prices), is there really a contest? I own both, but the images in the 12.5" are simply better, in all seeing conditions, than the 4" apo.

The 4" apo gives "pleasing" views of the planets, and stupendous, almost-pictorial, views of the Moon. But the 12.5" on the same targets makes it seem like the 4" apo is viewing the universe through "fuzzy glasses". Even when the seeing is really bad. True, in the 4", the image is dancing around, but staying in focus, while in the 12.5", the image is more stationary but going in and out of focus more. But the 12.5" still sees more, even at the same magnification. The resolution difference is too big a hurdle for the 4" to cross.

#10 FirstSight

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Posted 26 April 2013 - 01:12 PM

Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors. The reasons are multiple: thermal issues, optical quality, light scatter, poor collimation, dirty optics, etc.


A significant part of the reason Sirius B is often harder to see in a 10"-12" reflector is the frustrating tendency of Sirius B to be problematically near one of the diffraction spikes. Higher magnifications in reflectors are often needed as much or more to create enough separation of Sirius B from diffraction spikes than to resolve it or split it from Sirius A itself.

#11 Asbytec

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Posted 26 April 2013 - 01:21 PM

Don, thank you for the reply.

I don't believe this is primarily due to the reflector vs. refractor debates but most often because the small reflectors are of poor optical quality...


You mean, MTF? LOL - (which I am a fan of, btw...a tool for what it's worth.) Makes sense, a good quality scope should put up a noticeably better image than a poorer quality scope at roughly the same aperture, especially when induced aberrations such as seeing are favorable. This is my experience with a descent Strehl, diffraction limited peak intensity, and 8/10 seeing regularly.

..p.roperly cooled and collimated, I ALWAYS see more planetary details in the big scopes, whatever the seeing. I've seen details on Jupiter in some optimized 12.5" scopes that a nearby 9" refractor could not see.


Cooling and collimation always come up and seem to be attached to reflectors (and the venerable CAT) almost as if they are intrinsic to the design. So, I agree through experience, when those aberrations (including focus) are optimal...and in good seeing...scopes tend to perform as advertised. "Textbook."

Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors.


Its interesting you bring that up. It does seem for some deltaM and separations, an obstructed scope does struggle with unequal pairs. It might be interesting to "crunch the numbers" on Sirius and the pup and how seeing might affect their resolution. I have yet to resolve the pair, yet folks state they've done so in 4" scopes. Sirius is bright and we've all seen it's jumbled speckle patterns low on the horizon. In a 10", it should be a very bright glowing mess...and is at times. Yet, a 10" should be able to do it. I wonder if seeing induced "blobs" prevent it sometime.

The bigger aperture always does better at "cutting through seeing" than the small aperture. The main problems are poor attention to thermal issues, collimation, cleanliness and light scatter suppression, and something that is often the "luck of the draw"--optical quality.


That's what I am studying and finding /should/ be the case, given reasonable Strehl or diffraction limited at least in all samples holding seeing constant between both. I am tempted to do another in a long line "comparison" threads, but trying to avoid it as much as possible and just look at affects of seeing across apertures. Inevitably there will be comparisons, testimonials, and the like...that's understandable. But I hope the focus as this thread develops stays on the affects of seeing (and vs "pleasing" if needed.)

I am still looking for footing on this topic, but I cannot escape the idea seeing is so important we might like to understand such things. Having found myself retired under some very nice skies, it's become interesting to dive into it. To understand it.

Thank you for chiming in, mulling over your comments.

I am almost ready to toss out some math, some theory. Some folks hate theory because it is cold, hard, impartial, and probably doesn't exist in the real world. Seeing is the real world, though. Even though the models are simplistic, you can make them as complex as you wish and they will still "apply" across apertures and designs. Plus, theory is derived from observations and measurements of this very real world "thing" that hampers all who live in the atmosphere. Some prety smart folks dug this stuff up. So, while it's simple and might not accurately reflect someone's real world experience, it really does put the topic on footing we can discuss objectively, whether it "pleases" everyone or not. :)

#12 Starman1

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Posted 26 April 2013 - 01:53 PM

I've also noticed an improvement of contrast with improved optical quality, too. My current 12.5" Zambuto (20.8% secondary) did not amaze me in what I could see with 12.5", since I had spent the previous 8 years with the same aperture.

What did impress me, though, was the improvement in contrast. The much lower light scatter has made some very low contrast galaxy features more visible than they were in my previous scope. For example, the small spikes that stick out of M82 in directions perpendicular to the line of the galaxy, or the faint "fingers" that stick out from the companion of M51, or the outer arms of M81, or the bends at the end of UGC3697. All of these were more easily seen with the Zambuto mirror, despite the fact it has "standard" coatings, where my previous scope had "enhanced" coatings.

I made the comment, years ago, that I was amazed how much the seeing had improved when I learned to collimate well (Catseye tools). I can say the same thing about the Zambuto. I seem to be running into above average or exceptional seeing a lot more of the time. Part of that is simply cooling of the (much) thinner mirror (I start out the night, now, with the mirror AT the ambient temperature). But part of it is the suppression of light scatter from the mirror's surface.

Ironically, improved contrast also comes with a price. Diffraction spikes on stars are all much longer now because the exceedingly faint outer lengths are visible against the darker background and because the light of the spikes is not scattered. I can focus using the spikes if the star disks are a little soft. The wave pattern in the brightness and color of the spikes is also more evident.

To be more specific in my implications, I think a lot of the seeing problems people experience aren't seeing problems at all. When you observe, as I often do, in a field with a lot of other scopes and you can see that one scope seems to be experiencing poor seeing when one right beside it in the field is not, then you know it isn't really seeing at all.

#13 Dave Mitsky

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Posted 26 April 2013 - 02:27 PM

As I said a number of times in the past, my very best planetary views have been through large classical Cassegrains and premium Newtonian Dobs when the seeing has been excellent.

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#14 Asbytec

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Posted 26 April 2013 - 02:45 PM

Good point. I'm fully with you on the 3 C's (cooling, collimation and conditions.)

I've always been attentive to collimation, but thermals were not as easy to deal with. The 6" MCT I have now is well cooled and collimated. Tending to that does seem to make focuing easier and with dead calm diffraction 'perfect' collimation is a breeze (pun intended.) So, maybe part of the jaw dropping experience is factors other than seeing, too.

However, I can tell when seeing is very good because neither Jupiter nor the moon waver or boil and I can count out to five rings on the brightest stars. The central disc is clearly visible and the rings are not a blurry mess. Diffraction patterns roll and the rings blur, sometimes even speckling badly. When it's not so good, Jupiter can be soft while the limb wavers a bit or worst case simply a blur with two dark bands. Other times, the object will shudder rapidly then calm down.

But, with such good seeing, and all other C's controlled, those wow moments are very common - more so than uncommon. So, yea, all those factors have to be minimal to get those esquisit views allowing a 6" (at least) to strut her stuff.

So, is this a result of seeing favoring small aperture? Dunno...seems, as Jon might have been pointing out above, a 12" would rock under those same conditions, too, and despite the naked eye twinkling warning of terrible seeing above.

#15 Asbytec

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Posted 26 April 2013 - 03:49 PM

Okay, tossing out some math based on modeling observed seeing behavior. Starting with a 12" aperture and purposfully choosing Pickering 3/10 because that seems to be where speckling begins (D=12"/R0=4" = D/R0 = 3.) At this level of seeing, those modeled seeing 'windows' of relatively homogenous air are about 4" across. There will be several that overly a 12" aperture, each with a slight tilt sending star light in various directions. As these points try to reach focus, they interfere with each other destroying the textbook Airy pattern...the speckle pattern emerges.

At this point, Pickering 3/10, the speckle pattern is "about the same diameter of the third diffraction ring." (Quoting Vlad mentioning a 5" refractor and assuming the same for a 12".) The radius of the third ring is 419/Dmm in arc seconds. So, the diameter of the seeing induced diffraction in 3/10 seeing spans 6.6" arc for a 5" scope. Using the same 419/dmm, the diameter of the third ring for a 12" scope is ~3" arc, nearly half the diameter of the same seeing in a 5".

However, D=5"/R0=4" = 1.25, which equates to 7/10 seeing in a 5" scope. Under those conditions, the speckle pattern does not exist and the Airy pattern is pretty well formed. The first ring raduis in a 5" is 184/Dmm ~ 1.5" arc, or about 3" arc in diameter.

The result seems to be, if the math is correct, both a 12" scope and a 5" scope - accounting for a smaller Airy pattern with aperture - put up the same angular size Airy pattern (both at 3" diameter) despite the 12" showing some speckle pattern(3/10 seeing due to severe roughness) while the 5" remained at 7/10 Pickering (with 4" 'seeing cells.') That appears to be basically the same resolution (on average, not counting those moments of better seeing) in both apertures under the SAME conditions (4" 'seeing cells' flying over each aperture.)

Is that right? Hmmm...still thinking and throwing that out there for discussion. Its part of the learnig process.
http://www.telescope...d.htm#surfaces.

#16 Starman1

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Posted 26 April 2013 - 04:22 PM

Norme,
I did a cursory reading of your post, but wish to point out that, in 3" seeing, the seeing is not always 3". It varies over a range. The seeing conditions typically refer to the maximum photographic resolution, which is why the tall mountaintop observatories are usually limited to 1" seeing, despite their vastly superior resolutions.
Visually, though, we are not so limited. We may see resolution go from 0.2" to 2" in 1" seeing. The fluctuations can sometimes be severe.
Without changing magnification, I have gone from seeing sharply delineated white swirls within Jupiter's GRS to barely being able to tell the GRS is on the side of the planet I'm looking at a few seconds later and then back to the first situation a few minutes later. What was the seeing that night? Poor, or spectacular? The answer is Yes. :grin:

So, whereas it is possible that seeing conditions, on average, might show that the larger scope is seeing limited to the same level as the smaller scope, they are never so constrained over a period of time. And when the seeing drops to, say, 0.4", the resolution of the 5" will still be around a second, while the 12.5" will see much much more.

I have tried, to drive the point home, to evaluate my seeing, on the Pickering Scale, at my home, using a high enough power that seeing is definitely affecting the image quality. I have seen, at that site, seeing so bad my 5" Maksutov could not resolve Albireo (34" !!!) and I have seen seeing so good the Mak was resolving the dark intrusion into the Polar Ice cap on Mars at 328X (over 65X/inch). Now I will admit that that swing did not occur on the same night, or in the same hour. But I have seen times when epsilon Lyrae was easily resolved at 96X and a few minutes later not resolvable at all, at any power. And that separation should be a cinch for a 5". The seeing was a lot better than 2" one minute and worse than 6" a minute later. The Pickering Scale variation would be considered to be severe over the span of moments.

The weatherman Todd Gross wrote a few articles on maximizing seeing for CN a couple years ago, and they're still worth reading:
http://www.cloudynig...p?author_id=594

We CAN take steps to improve our seeing conditions, even if it's just keeping an eye on the weather forecasts.

Yes, it is definitely possible the larger scope may not see seeing any better than the small scope some of the time, in some places, under some conditions.
But it is also true that the larger scope will see seeing better than the small scope some of the time, in some places, under some conditions.
In my estimation, that says the larger scope is not only superior to the small scope from the standpoint of maximum resolution, but also in the seeing conditions experienced some of the time.
And since good seeing does occur, that makes the larger scope superior to the small scope, at least with those considerations.

There may be reasons to prefer the small scope: portability, maximum field of view, suitability for photography, daytime use, etc. But at least seeing conditions and maximum resolution are not two of them.

IMO, of course.

#17 jrbarnett

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Posted 26 April 2013 - 06:02 PM

I too am interested in your topic. But I am even more interested in how changes in seeing affect instruments with differing optical figure quality, but in all other respects are equal. My field-based experience suggests to me that a slower scope (refractor anyway) will resist seeing induced errors better than a faster one, all else being equal, BUT a better figured scope of a given focal ratio will likewise resist seeing induced errors than a scope with a poorer figure, all else being equal. Assuming arguendo that there is some science to support that empirical experience, what I would really like to understand is how important, relatively, is figure quality versus focal ratio in determining how well a scope of a given aperture beats poor seeing.

My hunch is that figure quality (again, at least among refractors) is even more important than focal ratio in allowing an optic to beat the seeing.

I'll follow this thread with interest.

- Jim

#18 Asbytec

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Posted 26 April 2013 - 08:22 PM

Don, yes, seeing will vary from good to bad. The models give about half the time on either extreme, which of course is why we wait for those good moments that come randomly. I've experienced those same wild swings in seeing, too. I have seen Jupiter go from crystal clear to mush on the same night. Something weird happens along my western sky about half way to the horizon. Jupiter often, not always, becomes a fuzzy ball with two indistinct bands...a far cry from colorful festoons and such a few hours earlier.

Ya, a cursury glance at the math can be confusing. It's still kind of rough figuring what they are saying - in theory.

Jim, the site I am reading talks about corrections to various aberrations in terms of RMS and Strehl. I haven't begun to sort that just yet, even in aggregate (which seems the easiest.) Here's the site (Strehl, page forward a couple times.)

http://www.telescope...d.htm#surfaces.

Often I've wondered, and believe is true, a scope with a better Strehl (or even peak intensity) can handle seeing a little better. Since seeing is an aberration, a marginal scope will drop below difraction limited - adding total RMS - more quicky than one that has some spare margin for error - well corrected, low RMS, in other words. But, I have not read that anywhere.

Focal ration? Hmmm...okay, I'd like to know more about that, too. Could it be related to the linear size of the diffraction pattern, larger at longer focal lengths? Easier to see?

Don! "...and I have seen seeing so good the Mak was resolving the dark intrusion into the Polar Ice cap on Mars at 328X (over 65X/inch)." Oh, yes!! I have seen and sketched that, too! Six-inch MCT, about the same mag/inch. Beautiful, huh?! (Still reading your comment.)

I suspect you're correct, a small scope can match a larger scope, but very infrequently. Maybe as infrequently as a larger scope excells when seeing calms down. So, on average...what can we expect? Does it warrant folks saying, "hey, my 4" stomped other 12" scopes that night?"

Seeing is so varied a topic, it's probably hard to say much that wont be refuted by this or that condition. That's why I was sticking to averages with the understanding it get's better sometime. It gets better and stays better as seeing improves, say both larger and smaller apertures operating in 8/10 seeing. Here, aperture rules (provided reasonable Strehl and the 3 C's are held or assumed constant.)

Another interesting thing that might play a role is Pickering devised his scale using a 5" refractor. That means the scale is determined using homogenous seeing cells of about 5" or less in diameter. In a 5". D=5"/R0=5" = 1. That translates into 8/10 Pickering. Those same seeing cells translate into lesser seeing on the Pickering scale for larger apertures. A 5" seeing cell might bring a 10" scope to D/R0 = 10"/5" = 2, or something closer to 5/10 seeing. So, it's almost required to state the aperture when defining seeing. It's not the same Pickering scale in different apertures.

Thanks, Don, let me read your link.

#19 Jon Isaacs

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Posted 26 April 2013 - 09:14 PM

Seeing is influenced by tube currents, too. It's all air density churning in the light path, so tube currents need to be minimized. Maybe that's the defining point, though, as tube currents can be controlled to some extent whereas seeing really cannot (except by picking an efficient location.)


I think the working assumption with a larger Newtonian is that the tube currents have been addressed and the scope is thermally stable. But that's a big assumption and I think that true thermal stability, not only the mirror but the scope itself, takes longer than most expect.

Anyway, I realize my thoughts are not coherent, yet, while putting this all together. I am familiar with seeing, just would like to understand it's application at different apertures from eye ball twinkling, to 6" pleasing and rock steady Airy patterns, to total disintigration of the Airy pattern in a 12", for example. If you're 12" is in average seeing, would it still be worth keeping outside getting 8" views of plantets (on average when those fleeting moments are very far apart?)



I am an opportunistic observer, there are plenty of interesting objects to view, if the seeing does not support planetary viewing, then I will point my telescope elsewhere.

Jon

#20 Asbytec

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Posted 26 April 2013 - 09:21 PM

Yes, it's a big assumption while trying to isolate seeing from other aberrations. We can put them back later. :)

For those of us who are fortunate enough to have no thermal problems (in a MCT, no less), and when seeing is good snough to afford nearly perfect collimation, seeing becomes the dominate aberration. That's why I want to isolate it and understand it by itself.

I tend to specialize with planetary, lunar, and double stars these days. Your point is well taken, though. Right scope, right conditions, right observing.

#21 Jon Isaacs

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Posted 27 April 2013 - 07:51 AM

For those of us who are fortunate enough to have no thermal problems (in a MCT, no less), and when seeing is good snough to afford nearly perfect collimation, seeing becomes the dominate aberration. That's why I want to isolate it and understand it by itself.



I think it is worth understanding seeing but in general, "seeing" and clouds are very similar, they are just something that one has to live with. Coming to terms with the seeing and understanding ones particular location is good thing to do.

In my particular situation, what I understand about seeing is that if it's not good enough for a 10 inch, it's not good enough for a 6 inch either.

Jon

#22 Asbytec

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Posted 27 April 2013 - 09:29 AM

...it's not good enough for a 10 inch, it's not good enough for a 6 inch either.

Jon


I think that's in part what I am curious about. Again, my seeing is about 8/10 or better almost nightly. But, is that because seeing favors smaller apertures? Can seeing drag performance of a 10" down so much (assuming hi res observing) to where a 4" outperforms it? Or is it simply more pleasing in a smaller aperture? How does the Airy pattern behave at such times spoiling resolution and contrast? And why?

I know my own 6" is very "pleasing" in terms of nice Airy patterns and planetary detail (both contrast and resolution.) Once I even caught a glimpse of a very faint belt in Jupiter's NNTB below below the brownish northern polar hood. It was faint even in images. So, at least in good seeing, it seems to be offering up everything the small aperture can offer. Its more than pleasing, it's performing...and that is pleasing. :)

So, ya, I guess this thread is kind of exploring what it means to be pleasing and performing and under what conditions.

One thing mentioned above, that under some conditions a 5" and 10" /seem/ to offer up about 3" Airy pattern - on average. As seeing calms, both scopes improve but as Don mentioned the 5" improves slightly while the 10" really begins resolving closer to it's aperture. On the other half of average, when seeing is worse that 3", both scopes seem to be afflicted...and seeing is not good for either.

So, it would seem larger aperture is at least equal to a smaller scope (assuming good Strehl, etc) somewhat infrequently while at some points the larger aperture dominates when seeing improves.

We all know that, but why do some folks claim a smaller aperture trumps a larger aperture under some seeing conditions when it might, at best, seem to equal it infrequently. Is it a statement on optical quality or peak intensity (obstructed vs unobstructed debate)? Does it boil down to care and maintenance of induced aberrations such as focus, cooling, collimation, and what have you? (Its probably most of that, most of the time.)

Anyway, sorry for rambling...thinking out loud.

#23 JKoelman

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Posted 27 April 2013 - 10:06 AM

I applaud the attempt of OP to elevate the discussion on atmospheric seeing and telescope performance above the level of anecdotal evidence.

I recently discovered that loads of relevant material (including Fried's paper) is available for free on the Internet. I started to look into those papers. These tend to considers the effect of atmospheric turbulence on perfect (diffraction-limited and aberration-free) optics (no luck, Jim). A key result that I reported on in a thread in the refractors forum is perhaps worth repeating here:

Taking into account the effects of seeing, a large aperture diffraction-limited telescope will deliver diffraction-limited snapshots only a very small fraction of the time. In other words, the time one has to wait for what is referred to as a “lucky image” increases exponentially with aperture.

Some numbers for poor seeing (at the lower end of typical seeing conditions described by optical turbulence characteristic size r0 in the range 0.1-0.2m), with the leftmost figures giving the aperture in inches, and the rightmost figures the fraction of snapshots that yield "lucky images" (diffraction-limited performance):

4 100%
8 100%
12 98%
16 46%
20 11%
24 2.1%
28 0.27%
32 0.026%

When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time. The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area. That's a steep uphill battle...

#24 Cotts

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Posted 27 April 2013 - 12:47 PM

With regard to the size of the "cells" as they relate to aperture, I doubt very much one can simply say, "the cells are 4" tonight....." These are natural phenomena which will form a normal distribution around a mean size, which may be 4" but the "cells" may have a huge range in size...

Dave

#25 JKoelman

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Posted 27 April 2013 - 01:21 PM

With regard to the size of the "cells" as they relate to aperture, I doubt very much one can simply say, "the cells are 4" tonight....." These are natural phenomena which will form a normal distribution around a mean size, which may be 4" but the "cells" may have a huge range in size...

Dave

Absolutely. That would be naive, wrong and misleading. One should not describe the Fried parameter r0 as a cell size. The Fried parameter characterizes the size distribution of the turbulent refractive index variations in the atmosphere. It is the "huge range in sizes" that renders possible the 'lucky images' for large aperture scopes.






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