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Magnification Suffers as Aperture Increases

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#1 Glory Eye

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Posted 02 June 2020 - 09:32 PM

I have been told that larger aperture instruments suffer under challenging seeing conditions as compared to smaller aperture counterparts. I made this graph below that not only reflects the magnification verses seeing for a given aperture, but also shows the limits of the achievable magnification before exit pupil is too small to observe an object at the eyepiece. Please take a look and scrutinize the accuracy of the "Seeing curves" for the values read on the graph. Please offer your suggestions and critiques as I feel that if a graph such as this was accurate, it could be a useful tool.

Thanks.

 

Attached File  Seeing Magnification and Aperture.pdf   106.37KB   50 downloads

 

Seeing Magnification and Aperture.png


Edited by Glory Eye, 02 June 2020 - 09:39 PM.


#2 Taosmath

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Posted 02 June 2020 - 10:31 PM

I have a couple of questions.

 

1) Is there any theoretical basis for your seeing curves or are these just arcs that you think are OK because of some empirical or experiential  reason?

 

2) Regarding your comment number 5 about premium optics:  if the seeing is poor, surely that will dominate the observational result and it won't matter if you have a mass market mirror or a premium mirror, the image will be the same. The only time a premium mirror will show better results is if the mass market mirror has optical flaws which are more deleterious to image than the seeing.

 

There was a thread I read a couple of moths ago which dealt with all of this. I'll see if I can find it.....


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#3 TOMDEY

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Posted 02 June 2020 - 10:49 PM

That's fine, as far as it goes, time-averaged performance.

 

One could easily argue that naked eye is best of all... right there on the graph, lower left... 0.3-inch aperture, 1x Rich-Field. What it doesn't take into account are the ~moments~ of good, great, excellent seeing. So, if you dwell on a target long enough, patiently enough, with a giant scope at higher mag... you will see far more than the little ones can ever see. I am noticing that effect with my 36-incher. Spend more time on each target, more and more detail manifests. Like the Enke Gap in Saturn's rings. Ummm... my seeing here tends to be often good, sometimes great.

 

Small scope --- always poor resolution.

Big scope --- more often poor resolution, but more than compensated by moments of spectacular resolution.    Tom


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#4 stargazer32864

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Posted 02 June 2020 - 11:27 PM

This is one topic that's going to be interesting, in my book anyway. I always thought that the bigger the mirror or lens is, the better the object resolves. I'm looking forward to your arguments.


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#5 CHASLX200

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Posted 03 June 2020 - 05:43 AM

I don't have that problem in my steady seeing. Bigger is better at least with my Zambuto and OMI mirrors. Top notch optics are better when bigger. 


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#6 epee

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Posted 03 June 2020 - 07:33 AM

Bigger apertures resolve finer points. Therefore they also resolve turbulence better. If seeing is poor, dial back the magnification or pick less detailed targets.


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#7 Jethro7

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Posted 03 June 2020 - 07:38 AM

Bigger apertures resolve finer points. Therefore they also resolve turbulence better. If seeing is poor, dial back the magnification or pick less detailed targets.

You got it in a nutshell.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro


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#8 Jon Isaacs

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Posted 03 June 2020 - 07:55 AM

The graph certainly doesn't agree with my experience. 

 

If the seeing limits a 16 inch scope to 200x, 350x will not be sharp in an 8 inch. It takes very good seeing for an 8 inch to be clean and sharp at 350x. 200x would be questionable in the 8 inch.

 

Typically, regardless of seeing, higher magnifications are more usable/useful in a larger scope.

 

Jon


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#9 Keith Rivich

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Posted 03 June 2020 - 10:00 AM

Big scopes...don't knock 'em til you've tried 'em.


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#10 Taosmath

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Posted 03 June 2020 - 11:05 AM

Dave Cotts kindly sent me a link to the post I was looking for.

 

https://www.cloudyni...ing/?p=10021529

 

In it he outlines the argument why on a night of bad seeing a large scope doesn't help you resolve more details than a small scope.

 

I‚Äč believe exactly the same argument applies to why on a bad night, a premium scope won't help you see more details than a mass market scope.

 

Colin


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#11 Zamboni

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Posted 03 June 2020 - 11:06 AM

I think one factor a lot of people don't take into account when discussing PERCEIVED poor high-power viewing in large scopes is thermal issues; larger scopes need correspondingly more drastic thermal management, and a large number of observers seriously underestimate the time it takes for a large mirror to stop giving up heat, especially in higher elevation environments with big day-to-night temperature swings. Even if you have a substrate with a low coefficient of thermal expansion, the material itself is still going to be giving off heat to the surrounding environment.

Just as a large precision mirror has an excellent resolving power, that sensitivity is also a double-edged sword when it comes to thermals because the disruption of the image will be correspondingly amplified.

A great illustration of this sensitivity principle is Schlieren imaging, which uses a parabolic telescope mirror for the purposes of photographing the tiniest fluctuations of heat through air.

https://youtu.be/4tgOyU34D44

Just think, if the mirror is THAT sensitive to thermal instability in the air within the optical path, what is that going to do to the image when you're trying to resolve point sources less than an arcsecond apart? Also, many large-aperture scopes these days are truss-based. That also has consequences when it comes to thermal management.

This is why a lot of observers have different scopes for different times of the night; using a smaller scope that cools quickly earlier in the evening and ONLY deploying a larger instrument deep into the night after aggressive cooling for hours.
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#12 havasman

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Posted 03 June 2020 - 11:40 AM

Spreadsheet astronomy often fails to capture the dynamic realities of observing. Core assumptions are presented as universal and without flexibility in the face of the real world where conditions dominate the potential for high magnification observing. Often these assumptions derive from statements read on the interwebthingy and are not supported by experience. Then someone else sees the new expression of a debatable or dubious concept and they are inspired to spread it further, again presenting it as factual w/o basis.

 

An example here is that exit pupils below 0.5mm are unusable. This does not match my experience as from my primary observing site on a good night bright planetary nebulae have been very successfully observed at 0.35mm exit  pupils. This is via medium-large 16" Dob. My eyepiece kit provides, by design, for exit pupils down to at least 0.5mm via discreet eyepieces in all scopes from 4 to 16" apertures and I carry a 2x Barlow to go lower when it's possible and valuable to the observation.

 

As above, aperture increase very definitely facilitates higher magnification observing. I am fortunate to have conditions that commonly facilitate high magnification observing and have used much larger scopes in even better conditions where this opinion is supported. From others' forum posts, observers cursed with commonly poorer conditions are limited pretty much across apertures.


Edited by havasman, 03 June 2020 - 11:44 AM.

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#13 Keith Rivich

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Posted 03 June 2020 - 12:38 PM

I think one factor a lot of people don't take into account when discussing PERCEIVED poor high-power viewing in large scopes is thermal issues; larger scopes need correspondingly more drastic thermal management, and a large number of observers seriously underestimate the time it takes for a large mirror to stop giving up heat, especially in higher elevation environments with big day-to-night temperature swings. Even if you have a substrate with a low coefficient of thermal expansion, the material itself is still going to be giving off heat to the surrounding environment.

Just as a large precision mirror has an excellent resolving power, that sensitivity is also a double-edged sword when it comes to thermals because the disruption of the image will be correspondingly amplified.

A great illustration of this sensitivity principle is Schlieren imaging, which uses a parabolic telescope mirror for the purposes of photographing the tiniest fluctuations of heat through air.

https://youtu.be/4tgOyU34D44

Just think, if the mirror is THAT sensitive to thermal instability in the air within the optical path, what is that going to do to the image when you're trying to resolve point sources less than an arcsecond apart? Also, many large-aperture scopes these days are truss-based. That also has consequences when it comes to thermal management.

This is why a lot of observers have different scopes for different times of the night; using a smaller scope that cools quickly earlier in the evening and ONLY deploying a larger instrument deep into the night after aggressive cooling for hours.

On both my 18 and 25" scopes I find mirror thermal problems to be a very minor to non-existent issue. If the temperature is dropping fast enough that the mirror is at some absurd temperature difference against ambient then the induced bad seeing is going to overwhelm any mirror heat problems. 

 

To address your last paragraph: 

 

Both my scopes, and the scopes of people I observe with, get just as good image quality at sunset as we do at 3:00am. We do not change scopes as the night progresses. I would give up observing if this were true. 


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#14 Mike Lockwood

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Posted 03 June 2020 - 01:14 PM

First, I see no mathematical basis for the location of the seeing curves, the curved nature of those, or the graph or the artibrary 0.5mm exit pupil limit.  I'm sorry to be blunt.  It is a complicated problem.

 

Second, larger instruments do most certainly allow you to perceive more detail when the seeing steadies up for a moment, which may only happen in a portion of the field.  However, over time, you will see those moments over the whole field.  The notion that a larger telescope can't be exploited in imperfect seeing is purely a myth pushed by those who have never exploited a good, large telescope.

 

Third, Zamboni said it very nicely above - larger mirrors take longer to equilibrate, and this is somewhat amplified by its increased resolving power.  The simple fact that thermal performance is comprehensively addressed in professional telescopes shows its importance.  We see this effect at WSP every year, a location with minimal temperature difference between day and night.  The 32" telescopes take a bit longer to cool than 25"s and 16"s, but ultimately produce more detailed images after 1-2 hours of cooling by fans and a nice ocean breeze.


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#15 TOMDEY

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Posted 03 June 2020 - 02:31 PM

Yes to the big scope fans. It takes more care, experience, and commitment to achieve and fully-utilize a truly Grand Scope --- and is well worth the effort. The bigger scope samples more of the incoming wavefront. You don't abandon the Carrier for the Canoe to avoid the waves.    Tom

 

~click on~ >>>

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#16 areyoukiddingme

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Posted 03 June 2020 - 04:26 PM

I also think the thermal part is a big reason for the difficulty with the bigger apertures.

 

I've had a 8" quartz mirror for about a year now, and routinely find myself cranking power without realizing it. Apart from the quality of the optic, the cooling situation is idiot proof. 

 

In my 12.5" scope which is on par for quality, the mirror cooling is comparatively difficult and can take well over an hour to catch up with the 8". While the 12.5" is still mushy the 8" is crisp and taking good magnification.

 

Of course, once you resolve those same issues on the bigger aperture, you are always going to see more.


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#17 Glory Eye

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Posted 03 June 2020 - 04:55 PM

I want to be clear that I do not at this point offer the seeing curves as scientific fact. The curves were sketched in (meaning they can be erased or modified) based on how some CN members argue this issue. I am a big scope fan and I purchased the largest I could afford; I'll definitely get larger aperture if opportunity allows. My hope is that this topic will result in a discussion that proves out what the seeing curves actually look like. I have never looked through anything larger than my 16" and before buying my own scope, never at all with the exception of two brief sessions with an 8", so I rely on the experience of experienced observers to learn from as I have in the posts above. PLEASE BE BLUNT.


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#18 havasman

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Posted 03 June 2020 - 05:18 PM

This is why a lot of observers have different scopes for different times of the night; using a smaller scope that cools quickly earlier in the evening and ONLY deploying a larger instrument deep into the night after aggressive cooling for hours.

Yeah, that's just weird.

 

Who? Where?  "A lot of people" say and do things on this forum string that make little or, in this case, NO sense.

 

Active cooling, like active dew control, just WORKS! And better all the time as techniques are developed and fine-tuned.

 

I do sometimes set up an NP101is when I set up the Starmaster but it has NOTHING to do with cooling or timing. In fact it is almost never used until an observation with the 16" seems like it could also be enjoyed in very wide field. Often I never look through it until session end when the big scope's packed up and ready to go.


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#19 CHASLX200

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Posted 03 June 2020 - 06:25 PM

I don't have cooling problems in my area. On any winter nite with big temps drops i never bother to do high power work and stick with deep sky views at lower powers. My summers are also never super steady for some reason, but odd that Feb seems to be the best month for the most steady views at crazy powers.

 

Many times in Feb when it is super warm at nite with no temp drops, i have used over 1000x with my 14.5" Zambuto scopes, had two of them and a 15" OMI Obsession.  One Feb nite in 2001 i maxed out my power at 1150x on a dead still nite with a 14.5" Starmaster. Jup and Sat were just dead still at over a 1000x.  Never had a another nite that still but many were close.



#20 Glory Eye

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Posted 03 June 2020 - 06:27 PM

I have a couple of questions.

 

1) Is there any theoretical basis for your seeing curves or are these just arcs that you think are OK because of some empirical or experiential  reason?

 

2) Regarding your comment number 5 about premium optics:  if the seeing is poor, surely that will dominate the observational result and it won't matter if you have a mass market mirror or a premium mirror, the image will be the same. The only time a premium mirror will show better results is if the mass market mirror has optical flaws which are more deleterious to image than the seeing.

 

There was a thread I read a couple of moths ago which dealt with all of this. I'll see if I can find it.....

No.1: Reasons for the shape of the curve: It has been stated by an experienced member that larger aperture is more affected by seeing than a small scope. This must mean that for a given magnification, seeing would distort the view in a larger scope before it affected the smaller one. I am extrapolating that the graph is a curve mainly because if the function was linear it implies a zero magnification intercept for a limiting large aperture which I am sure doesn't exist. In other words, no matter how large the aperture you can get some magnification which implies an asymptotic function.

 

No.2: I was under the assumption that if two telescopes were side by side one with a Strehl 95 mirror, the other with a Strehl 90, you could push the magnification farther with the 95 than with the 90; if this is not true then why buy premium?

 

Thanks for checking for the other thread; I'd be interested if you could find it.



#21 Glory Eye

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Posted 03 June 2020 - 06:32 PM

That's fine, as far as it goes, time-averaged performance.

 

One could easily argue that naked eye is best of all... right there on the graph, lower left... 0.3-inch aperture, 1x Rich-Field. What it doesn't take into account are the ~moments~ of good, great, excellent seeing. So, if you dwell on a target long enough, patiently enough, with a giant scope at higher mag... you will see far more than the little ones can ever see. I am noticing that effect with my 36-incher. Spend more time on each target, more and more detail manifests. Like the Enke Gap in Saturn's rings. Ummm... my seeing here tends to be often good, sometimes great.

 

Small scope --- always poor resolution.

Big scope --- more often poor resolution, but more than compensated by moments of spectacular resolution.    Tom

I went from a 5" scope to a 16" and the difference was mind blowing. I wonder how you could incorporate that into the graph or it may be that simply observing and experience makes the whole thing of none effect. It may be an interesting exercise for everyone to think about how you could represent what you know experientially into the graph.



#22 areyoukiddingme

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Posted 03 June 2020 - 06:48 PM

Yeah, that's just weird.

 

Who? Where?  "A lot of people" say and do things on this forum string that make little or, in this case, NO sense.

 

Active cooling, like active dew control, just WORKS! And better all the time as techniques are developed and fine-tuned.

 

I do sometimes set up an NP101is when I set up the Starmaster but it has NOTHING to do with cooling or timing. In fact it is almost never used until an observation with the 16" seems like it could also be enjoyed in very wide field. Often I never look through it until session end when the big scope's packed up and ready to go.

I recall interesting discussions where Daniel Mounsey reported setting up his smaller refractors and large mewlons, using the former while the latter got acclimated.

 

That's not something I do as a tactic, so much as figuring what I'd like to use and how much time I think i will have (e.g. clouds or fog closing in vs. a long clear night). 

 

Even with active cooling--a rear mounted fan and a table fan pointed at the front of my mirror--I still get going much faster in my 8" quartz than 12.5" borosilicate. 



#23 Jon Isaacs

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Posted 03 June 2020 - 07:31 PM

No.1: Reasons for the shape of the curve: It has been stated by an experienced member that larger aperture is more affected by seeing than a small scope. This must mean that for a given magnification, seeing would distort the view in a larger scope before it affected the smaller one. I am extrapolating that the graph is a curve mainly because if the function was linear it implies a zero magnification intercept for a limiting large aperture which I am sure doesn't exist. In other words, no matter how large the aperture you can get some magnification which implies an asymptotic function.

 

 

Consider this:

 

A larger aperture scope starts out with greater resolving ability. A 16 inch is capable of resolving something 1/2 the size of an 8 inch.

 

When one says the 16 inch is affected more by seeing, it means that for example, in poorer seeing, it might only be able to resolve some 2/3 the size.

 

But in general, these relationships are not simple mathematical relationships. The way seeing affects the image depends on the characteristics of the disturbances to the wave front that are affecting the image. There are models for the size of the disturbances that can then be used in numerical simulations to estimate the effect on the image as a function of aperture.

 

These are not things one can intuitively derive.

 

Jon


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#24 Volvonium

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Posted 03 June 2020 - 08:19 PM

Aperture always wins for me.  I have a variety of refractors going up to 140mm APO, as well as dobs going up to 16" with a premium optic.   My refractors can show better contrast on most objects at low-mid power... but my dobs, after 10", really start walking away from the refractors in terms of overall performance and will always show more.  Under 10" and cooldown is fairly quick for me with dobs.   Even in average seeing, there are always moments where the air is stable enough for a fraction of a second to glimpse spectacular detail with a big dob.   Lucky imagers count on those moments of good seeing, as do visual observers.   SCT is the only design where its long thermal acclimation really hindered my ability to use it on a variety of objects after setup.

 

That said, I love all telescopes and love the differences in their designs.  Just because my big dobs can usually wipe the floor with the refractors doesn't necessarily mean I undervalue them or use them less.  Being under the skies with any scope is better than none, so I enjoy any chance I get to bring out a "toy".   It's a privilege to be able to enjoy and appreciate these kinds of instruments.

 

I think that a telescope's limiting magnitude and seeing could also be a good overall performance indicator.  There are so many factors when making an observation, including central obstructions.  I had a good lesson on seeing and thermal acclimation when I challenged myself to observe Triton for the first time, across a series of nights with a 12" dob.    For the first couple of nights, with just average seeing, I could swear I could see it periodically shift in and out of view with averted vision at around 400X....but I learned as I performed my observations later in the night on subsequent days, that my scope's thermal acclimation was also a significant factor.   With my scope cooled down a bit better and by using my boundary layer fans, I could get a steady view of the faint moon without using averted vision.   I'd like to try that exercise again with my larger refractor, where its limiting magnitude is less than the big dob, but still within the realm of the refractor's capabilities.  I think the lack of a central obstruction will help me to see it, but I anticipate it will again be a unique challenge, where acclimation, seeing, aperture, telescope design, and observer experience will heavily factor in my success.  Regardless of the outcome, I'm sure I'll walk away having learned something new.



#25 TOMDEY

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Posted 03 June 2020 - 09:03 PM

I went from a 5" scope to a 16" and the difference was mind blowing. I wonder how you could incorporate that into the graph or it may be that simply observing and experience makes the whole thing of none effect. It may be an interesting exercise for everyone to think about how you could represent what you know experientially into the graph.

I came up with this attached hand-out long ago. If you are willing to observe a LOT, and await superior seeing, the aperture size advantage is quintic for smaller apertures, becoming cubic at the atmospheric seeing limit (which varies from moment to moment, night to night, and location sensitive). But it is inexorably a monotonically increasing function! Indeed, bigger is always better, just not as steeply-so beyond the seeing threshold. That's all there is to it!

 

NOTE: I also generated this telling illustration, where the simulated pupil is reasonably Rich-Field 5mm, 100 times more luminous than the posited 0.5mm. Note that, under the rich-field assumption, the bigger scopes show much more, completely no contest! And that the magnifications are not at all stressed by seeing conditions. Tom

 

~click on~ >>>

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