Seeing vs Aperture

[Peter Ward]

Not wanting to hijack Mike's Deep Sky image thread, I did make the observation of larger aperture being a bugger when it comes to seeing.

I have only been blessed with a handful of nights where I could truly say
my 36cm aperture was producing razor-sharp imagery, yet my 13cm aperture scope performs very well on most nights.

For those newbies who are interested there was a good write-up on this subject by Andrew Young at JPL in the September 1971 edition of Sky & Telescope.

Young noted "the aperture must be matched to the seeing conditions"
and I have to say I agree with him....but am curious as to whether others
have not found this to be the case.

[h0ughy]

did he come up with a chart/make/seeing type of diagram Peter

[Satchmo]

If you are after a constant sharpness in your image and that is your criteria then stop your scope down until you see a stable airy disc at high power. for most nights it will probably be 6" aperture or less .

That being said I always see more with a larger aperture regardless of the seeing or sky brightness.

Fleeting moments always show more detail than that which is constant in a smaller aperture IMHO.

Also I believe that instruments with larger central obstructions show less in inclement seeing than more modestly obstructed ones.

Does anyone have similar experience ?

[Bassnut]

Thats a grand myth according to this (http://www.skyandtelescope.com/howto/visualobserving/3305656.html?c=y&page=5) , but thats visual (dont understand why that should matter actually, visual vs imaging, if its wobby then it moves around visually and blurs images).

Do you mean large aperture is affected more by local disturbances (eg thermal) or seeing generally?.

In a simple way I thought it was image scale that should be matched to seeing, not aperture, sounds odd..

I havent imaged with a tiny aperture, I wouldnt want to in a fit. But wouldnt the seeing on a large aperture just show up more due to its higher optical res? (with the same end result).

[Peter Ward]

I should have made it more clear that I was refering to the imaging case.

I agree with Mark (and that other S&T piece) in that for visual observing larger aperture wins every time.

For imaging, the isoplanic wave (ie block of air that is stable across the aperture of your telescope) needs to be larger to appear stable in larger apertures....but the rub is, the refractive index changes for these waves are often directly proportional to their wavelength.

Smaller apertures as a result often have a more optimum (ie smaller) refractive index changes.

What is intestesting is that high speed tip-tilt optics did not exist for amateurs back in 1971, and (without trying to sound like an advert) these can and do correct slow seeing changes that were impossible to compensate with film based systems.

[RobF]

Fascinating topic, and enjoying the rational debate on this. I don't have imaging experience on anything other than my trusty 8" Newt alas to add any useful info. Doesn't mean I won't happily soak up the info on offer though.....:)

[Peter Ward]

Should I ask Bert to chime in?? :lol:

[Peter Ward]

In case you don't have access to a 1971 copy of S&T, Young also mentions
you can make you own luck by making the local area and instrument itself
congenial to seeing.

Imaging off grassy surface rather than concrete, cooling fans for the optics etc. all help.

It is surprising how much local thermal conditions can break up a starry wave-front.

[Terry B]

When David Malin imaged with film on the 3.9m AAO he seemed to get very good resolution despite the seeing at Coona being average mostly. This is a big scope and much bigger than the so called "air cell size" that was purported to reduce the resolution through large scopes.
Maybe the low sensitivity of the film helped to minimise the blurring. Any thoughts?

[RobF]

I was toying with adding some sort of quantitative estimate of seeing to my set up routine recently (e.g. Maxim FWHM value on an image of 1 second or so on say a 5th mag star). Haven't played with it too much as I expect it could change lots during the night as scope cools and local conditions change (hopefully improve). Supposedly on a long exposure such values are relatively meaningless as they reflect worse case over many "wobbles".

Does anyone routinely try to measure their seeing, or is it just something you learn to gauge by experience?

[Peter Ward]

The resolution wasn't great....sure better than amateur hacks at sea level.

...BUT.... the signal ! Aye carrumba!...was excellent.

OK you want proof Here it is
(http://www.atscope.com.au/BRO/gallery96.html)

[Peter Ward]

Actually the roll-over is a little quick and dirty, I wouldn't take too much notice of positional shifts of the background stars as I simply eyeballed the two images in PS5, rather that do a time consuming re-map.

C-11 image?? Sure, but I haven't used a C-11 for about a decade! But if you must... :)

[Satchmo]

Comparing visual and imaging you need to make a distinction between the two extremes of bad seeing , one type is the very high frequency turbulence that makes stars look soft vs more slow undulating seeing through which you can still see a lot of detail but often a little distorted, or in little bursts of better seeing. In both cases time exposures used for imaging will show similarly bloated star image although as Peter suggests the lower frequency shifting can be taken out to some extent with adaptive optics.

[Peter Ward]

Sorry... missed your question....Yes...but it didn't really cover the imaging case.

[multiweb]

I remember reading about the relative size of those so called 'air cells' at high altitude having to be bigger than the aperture of your telescope, hence the bigger aperture, the bigger the chance to capture one of more of those wobbly buggers in your field.

Do you think the active optics technology is close to make it to the consumer market? I see Edmond Optics is selling kits of such small mirrors but it's still very involved and pretty hands on. (Hardware, soldering/wiring, software SDK, etc...)

[Peter Ward]

Again depending on the seeing, you have hit upon a technique not dis-similar to that used by planetary imagers i.e grab lots of short exposure frames and stack them. Each indicidual frame is statistically likely to have a small wavefront error, compared to one equivalent long exposure ( tip-tilt guiding can only do so much)

It's a high resolution technique that clearly can be used for deep sky, the only down side I could see would be noise supression (particularly with small apertures or if you shoot through a filter or ) and the ultimate depth of the image.

[Paul Haese]

Sorry, but this has got to be one of the biggest urban myths in astronomy. As I read this, I take your argument to mean that using a larger aperture is more affected by seeing than a smaller one?

Large aperture is more likely affected by optics that are not at ambient than seeing itself. Cells of air movement are many kilometers across and would not have the type of effect suggested here. Nor will stopping down a scope make the seeing better. It might make the view better in an uncooled scope but not the seeing.

I use a 14" at ultra high resolutions and the difference between a naturally cooled telescope and one with active cooling is like chalk and cheese. You cannot ever get a naturally or fan cooled scope to ambient temperature. Anything above 0.5 degrees C will have an effect on the image quality or the way the image is presented. I have demonstrated this to many times over the years and the difference is staggering.

Imaging at the native focal length with an uncooled image will have less effect than at high resolutions. I have viewed Jupiter at very high resolutions in good seeing with an 18" telescope and seen detail visible in images. Anthony has too. Use active cooling on your scope Peter and you will see the difference.

Sorry if anyone takes offence. I am passionate about this topic.

[Satchmo]

Paul, there is certainly a description of seeing which defines the average size of seeing cells in the atmosphere which will produce diffraction limited wavefront with perhaps only tilt variation that can be taken out with a tip tilt adaptive optics device ( in other words a stable airy disc which is moving about )

A long term study using interferometry by the Sydney University Stellar Interferometer found diffraction limited seeing at 150mm aperture at Lindfield on an acceptable number of nights and at Narrabri closer to 200mm.

There are some good academic papers around on 'seeing' . I'll have to see if I can locate some online.

Your post suggests that thermal management is all that is required to get superb views out of a large instrument. This is an oversimplification. Bad seeing will limit large scopes aesthetically on most nights. If you are happy to accept fleeting glimpses of fine detail with a large scope ( like I am ) or be prepared to stack your good frames ( like you do ) and manage thermal issues , then large scope will always be more useful than smaller ones.

[Paul Haese]

No Mark I am suggesting that without thermal management how can you possibly know what the seeing is actually like. Having good thermal management will not give you good seeing, but it will allow you to determine what the seeing actually is like.

I will see if I can get hold of a DIM and test this theory thoroughly by testing the seeing via dim and image before and after cooling. That way the dim measurements can confirm what the seeing was at each point, so that the argument cannot be made that the seeing changed rapidly.

I am also talking about visual use too. A thoroughly cooled scope will give better views than a naturally cooled scope. Stick it outside and using a fan for two or so hours is not going to cut the mustard. There are a lot of us now that have the definitive proof of this. When the seeing is good and the mirror is at ambient you get steady views for minutes at a time, not just fleeting moments. At high resolution this can also be for 10-30 seconds at a time.

edit BTW the studies you are talking about are suggesting that seeing at each locality supports a particular size telescope. Therefore it is the seeing at the location which is the determinative factor and not the scope.

[Peter Ward]

I disagree.



Yes, I also use active cooling on my scope...plus have an extractor fan in the dome so air is always being pulled in via the observing slit.... thermal control is indeed important!

Mark's remarks on SUSI are a great example and typical of my experience with different aperture instruments....*when used for imaging*.

A lot of work was done by Kolmogorov who quantified the spectrum of turbulence.

A small aperture will typically only be looking through a small part of the wavefront distortion, which has a small image distortion but a larger positional shift at the focal plane than that seen by the larger instrument- which sees just the opposite: stable position but with more distortion.

Hope that makes sense... :)

[Satchmo]

Paul - We are not in disagreement , and I acknowledge all your points about thermal management.

My points and observations all assume that you have good thermal management- I'm talking about the nature of seeing 'cells' in relation to aperture and the aesthetics of different kinds of seeing. No amount of thermal management can give you a sharp steady view in a large telescope if the wavefront is only good for a crisp `visual' view in a 6" aperture ( and here I mean a clear visible airy pattern that is just moving around slowly ) .