26 replies to this topic

[Nippon]

I have heard this all my astronomy hobby life which is about 30 + years. So this evening I put that accepted logic to the test sort of accidently. I set up my Edge 8 SCT up for a night of observing in my backyard. Both Jupiter and Saturn are slipping away and have been low in the south all year even from my Florida location.

 

We have just had a cold front come through and that usually causes poorer seeing and as usual it did. Jupiter was pretty bad but Saturn still higher in the sky was much better but certainly not as good as I've seen it recently.

I brought my 4" Stellarvue triplet out on an Alt/Az mount thinking I'd get a better view. I should say a little something about the 

two telescopes involved in this. My Edge 8 is a very good sample and I'm picky about collimation and my 4" triplet is also excellent optically.

In good seeing the 4" can approach the Edge for planets but can't quite match it as would be expected.

 

To my surprise though the Edge was still able to provide a more detailed and more pleasing view than the 4" in fact the 4" seemed to be more bothered by the poor seeing than the Edge 8 and I believe that the reason was the fact that the image is so much brighter in the Edge.

I checked Vega high in the sky with the 4" to make sure it was not a thermal equilibrium issue in the refractor and it was not but I didn't think it would be because right now in Florida there is no difference between outside and inside temperature.

 

Just my observation on this commonly accepted believe.

Paul

[TOMDEY]

In the Russel Porter era, they called it an "old wives' tale". I assume we are supposed to call it something else now-a-days. At any rate, it sure seems like one of those things that is just passed down from master to apprentice, with no particular basis in reality.    Tom

[freestar8n]

This has been discussed at length in CN several times - but the possible win of smaller vs. larger aperture has a theoretical basis in Fried's papers from the 60's - and it can be demonstrated in simulations of seeing.  It's a subtle effect but I can believe it is real and it happens when the size of the Fried cells is comparable to the aperture.  At apertures below that, diffraction will make the star spot larger - and at apertures above that the speckles will be smaller and form a larger patch.

 

I haven't really observed this myself - but I can believe it.

 

Frank

[Nippon]

This has been discussed at length in CN several times - but the possible win of smaller vs. larger aperture has a theoretical basis in Fried's papers from the 60's - and it can be demonstrated in simulations of seeing.  It's a subtle effect but I can believe it is real and it happens when the size of the Fried cells is comparable to the aperture.  At apertures below that, diffraction will make the star spot larger - and at apertures above that the speckles will be smaller and form a larger patch.

 

I haven't really observed this myself - but I can believe it.

 

Frank

I know seeing effects have different characteristics are you saying poor seeing depending on it's particular nature can look better in a small instrument while poor seeing of a different nature can look better in a large instrument?

[Ken Sturrock]

Interesting test. I guess that I was never under the impression that small telescopes performed "better" than larger telescopes in bad seeing so much as I thought that smaller telescopes were less impacted by bad seeing because they have lower resolution and so they normally didn't see many of the details (visible in a larger instrument) that the seeing would impact.

 

Thanks for posting your impressions.

[freestar8n]

I know seeing effects have different characteristics are you saying poor seeing depending on it's particular nature can look better in a small instrument while poor seeing of a different nature can look better in a large instrument?

In the Fried model, seeing is determined by a single value, the Fried cell diameter referred to as r0.  The smaller it is, the worse the seeing.  This doesn't capture how rapidly the star is moving in time - but it does capture how speckly and bloated it is.

 

Typical values of r0 are 10-20 cm.  Here is wiki talking about it:

 

https://en.wikipedia...Fried_parameter

 

https://en.wikipedia...onomical_seeing

 

but you can also read Fried's original papers.  The theory from back then is still in use.

 

Frank

[freestar8n]

Interesting test. I guess that I was never under the impression that small telescopes performed "better" than larger telescopes in bad seeing so much as I thought that smaller telescopes were less impacted by bad seeing because they have lower resolution and so they normally didn't see many of the details (visible in a larger instrument) that the seeing would impact.

 

Thanks for posting your impressions.

It's the kind of thing that sounds improbable and is wishful or confused thinking - but it does have a basis in published research in astronomical seeing.

 

But at the same time, it is at best a small effect.

 

Frank

[fcathell]

Here in Tucson where we usually have very transparent but frequently turbulent skies, I compared 3 scopes on Jupiter a few years ago over a period of about a week and in every case the smaller aperture showed improvement over the larger under poor conditions. In fact, I was surprised that even a 102mm Mak displayed better images than a 127 Mak on occasion. I would not have thought that this seemingly incremental change in aperture would make a difference, but it did.

 

Another Frank

Tucson

[sydney]

My experience generally agrees with Paul.

 

I’ve evaluated different apertures under good, average, and poor seeing conditions many times over the years.  While the smaller apertures sometimes gave a prettier and more stable appearing image, not once have I seen more actual detail in a smaller scope compared to a larger one.  And of course, during those fleeting moments when the seeing settles down,  the larger aperture really shines.

 

I remember that Questar used to advertise the Fried cell concept for their 90mm scope, but while sounding plausible in concept, I haven’t seen it supported in practice.

[Nippon]

My experience generally agrees with Paul.

 

I’ve evaluated different apertures under good, average, and poor seeing conditions many times over the years.  While the smaller apertures sometimes gave a prettier and more stable appearing image, not once have I seen more actual detail in a smaller scope compared to a larger one.  And of course, during those fleeting moments when the seeing settles down,  the larger aperture really shines.

 

I remember that Questar used to advertise the Fried cell concept for their 90mm scope, but while sounding plausible in concept, I haven’t seen it supported in practice.

I guess what surprised me was I expected the smaller scope to provide a "prettier" view but it didn't. The view through the larger scope was more aesthetically pleasing.

[Redbetter]

My experience is that the seeing has to be particularly bad before the smaller scope reveals more detail than the larger scope.  It happens frequently enough in my backyard, but at that stage the seeing is bad enough that it really isn't suitable for planetary anyway.  A 60 or 80mm scope will be showing far less than they normally would, so it becomes a matter of:  what is the point of observing like that other than to try to get a glimpse of some transitory event?  I do however, use 80 to 127mm scopes as "seeing scouts" to determine when the seeing will reward larger aperture.

 

In very poor seeing the 20" really gets hammered for planetary because much of the image is turned to glare by what are effectively many different "cells" each with differing focus and little of it centralized.  Even on the best nights the seeing here is never really settled, not like what I have seen in other regions.  For example, here I always see undulating flare patterns off of Jovian moons no matter how well equilibrated the scope is.  (This is true even when I get 5 or 6/10 seeing with the 20" and am able to make double star splits that are sub 0.4 arc second.)  There is the central disk of a moon and typically three or more shifting patterns from it-- ghostly partial disk extensions.  The relative intensity, degree of offset, and speed of movement change with the level of the seeing even when 400x is productive, but never settle to an extent I used to consider normal for a good night.   I see the same sort of thing when I resolve large asteroids. 

[freestar8n]

If you do very long focal length and high speed video of a star to see its changing speckle pattern - you can see how this might work.  With very large aperture it is a large blob that is buzzing with speckles everywhere.  And for small aperture it is just a big single blob limited by diffraction.  But in between there is a point where it is pretty much one speckle that is moving around - and the overall size is smaller.  So it's similar to being able to draw the object with a finer pen - at this intermediate aperture.

 

This all may sound hand-wavy - but it's how the theory works - approximating the MTF of the system combined with a model for seeing.

 

Frank

[Jared]

It’s not an old wives’ tale, but it’s also not something I would worry about on the vast majority of nights.  I have—once—seen my 80mm refractor outperform my (then) 10” Ritchey with respect to resolution.  The seeing that night was absolutely awful.  That’s why I was taking out the 80mm in the first place, to try some low power sweeping since anything above about 60x magnification looked terrible.  On that particular night the 80mm was consistently out resolving the 10”.  Not just prettier views, but more detailed ones.  Basically, the 80mm could just show a single equatorial belt on Jupiter while the 10” showed no details at all.  Obviously, not a night for planetary views no matter what the scope.  

Edited by Jared, 12 October 2019 - 06:05 AM.

[Jon Isaacs]

If you do very long focal length and high speed video of a star to see its changing speckle pattern - you can see how this might work.  With very large aperture it is a large blob that is buzzing with speckles everywhere.  And for small aperture it is just a big single blob limited by diffraction.  But in between there is a point where it is pretty much one speckle that is moving around - and the overall size is smaller.  So it's similar to being able to draw the object with a finer pen - at this intermediate aperture.

 

This all may sound hand-wavy - but it's how the theory works - approximating the MTF of the system combined with a model for seeing.

 

Frank

As they say... In theory, there is no difference between theory and practice.  In practice, there is.

 

My own experiences agree with Paul's.  When the seeing is poor in a large aperture scope, it's bad in a small scope and the small scope shows less detail.  This is a frequent comparison I make because out in the high desert, the seeing is generally so-so and sometimes quite poor.  My typical setup is a 3 or 4 inch apo and a 12.5 inch to 22 inch Dob.  When things are not good in the Dob, I check them out in the refractor, they're not good there either.  

 

In final analysis it does come down to the fact that if the seeing is so bad that a 4 inch is better than an 8 or 10 inch, it's just bad, the views are just bad and it's time to look at something that doesn't require decent seeing. 

 

Jon

[freestar8n]

As they say... In theory, there is no difference between theory and practice.  In practice, there is.

 

My own experiences agree with Paul's.  When the seeing is poor in a large aperture scope, it's bad in a small scope and the small scope shows less detail.  This is a frequent comparison I make because out in the high desert, the seeing is generally so-so and sometimes quite poor.  My typical setup is a 3 or 4 inch apo and a 12.5 inch to 22 inch Dob.  When things are not good in the Dob, I check them out in the refractor, they're not good there either.  

 

In final analysis it does come down to the fact that if the seeing is so bad that a 4 inch is better than an 8 or 10 inch, it's just bad, the views are just bad and it's time to look at something that doesn't require decent seeing. 

 

Jon

Sometimes in threads like these I will lean toward the theory side and say that what people are observing is somewhat hard to believe - based on theory and fundamental principles.

 

But for this topic - as I have alluded - any benefit for smaller aperture is at best a small one - based on theory.  If anyone agrees or disagrees based on experience I'm fine with that.

 

At the same time, this idea isn't something conjured up by amateurs with hand wavy arguments.  It happens to be based on well established models of how seeing works.

 

So - I put it in the class of visual things that aren't really confirmable either way.  Someone says smaller aperture is better and someone else disagrees.  There are reasons to believe either conclusion.

 

Frank

[Jon Isaacs]

Sometimes in threads like these I will lean toward the theory side and say that what people are observing is somewhat hard to believe - based on theory and fundamental principles.

 

But for this topic - as I have alluded - any benefit for smaller aperture is at best a small one - based on theory.  If anyone agrees or disagrees based on experience I'm fine with that.

 

At the same time, this idea isn't something conjured up by amateurs with hand wavy arguments.  It happens to be based on well established models of how seeing works.

 

So - I put it in the class of visual things that aren't really confirmable either way.  Someone says smaller aperture is better and someone else disagrees.  There are reasons to believe either conclusion.

 

Frank

 

Frank:

 

I worked with a research group in materials science.  Most of our work was the dynamic/ballistic behavior of materials.  There is a lot of money in this field because of it's interest to the military.  My part of the group was the characterization of the materials and experimental verification of the models.  Sometimes the models predicted the behavior of the materials in complex situations, sometimes they didn't.

 

Models by their very nature are simplifications.  I have to think that if these models were running on supercomputers and clusters the way ballistic simulations are done, they would be quite complicated FEM models with many elements through the atmospheric layers with appropriate material models.  Fortunately, gas behavior is well understood and essentially linear or at least easily modeled. 

 

So the question is, how sophisticated are these models and how were these seeing models verified??  

 

A physics professor once shared this story with me.  He was at a conference and someone quite well know made the following statement.  

 

There are two kinds of papers presented at conferences, theoretical and experimental.  With a theoretical paper, no one in the room believes it, except the author.  

 

With an experimental paper, everyone in the room believes it, except the author.  

 

Being an experimentalist, I am generally on the side of being skeptical about modeling efforts but am amazed at how well sometimes the models predict the actual behavior.

 

Jon

[bobhen]

I’m sure that no 3.5, 3 or 2.5-inch telescope would best the C8  as well. But if you put a Takahashi TOA 150 next to your C8, you will see that a smaller aperture (150mm vs 200mm) can best a larger aperture (your C8) on the planets, and in any seeing conditions. 

 

Now before anyone says that the Takahashi is more bulky and much more expensive, that is of course true, but that is not the focus of the discussion or the poster's observations, which are strictly performance based.

 

Bob

Edited by bobhen, 12 October 2019 - 08:23 AM.

[freestar8n]

Frank:

 

I worked with a research group in materials science.  Most of our work was the dynamic/ballistic behavior of materials.  There is a lot of money in this field because of it's interest to the military.  My part of the group was the characterization of the materials and experimental verification of the models.  Sometimes the models predicted the behavior of the materials in complex situations, sometimes they didn't.

 

Models by their very nature are simplifications.  I have to think that if these models were running on supercomputers and clusters the way ballistic simulations are done, they would be quite complicated FEM models with many elements through the atmospheric layers with appropriate material models.  Fortunately, gas behavior is well understood and essentially linear or at least easily modeled. 

 

So the question is, how sophisticated are these models and how were these seeing models verified??  

 

A physics professor once shared this story with me.  He was at a conference and someone quite well know made the following statement.  

 

There are two kinds of papers presented at conferences, theoretical and experimental.  With a theoretical paper, no one in the room believes it, except the author.  

 

With an experimental paper, everyone in the room believes it, except the author.  

 

Being an experimentalist, I am generally on the side of being skeptical about modeling efforts but am amazed at how well sometimes the models predict the actual behavior.

 

Jon

I'm not talking about theory in a vague sense - I am talking about the actual theory used to describe the questions of this thread.  If you are interested in the models and their validity - all you have to do is consult the literature.

 

Here is a related thread from a few months ago on this topic.  In it I provided several links to follow up on:

 

https://www.cloudyni...ng-vs-aperture/

 

If you are asking about how sophisticated the models are - they have been in use in the literature for 50 years.  If you are amazed at how well they work and are still in use after 50 years - I am in agreement.  Fried's work is based on elegant simplifications of a very complex problem involving turbulence.  The results are not perfect - but they work very well and continue to be cited and referenced because they work well.

 

Frank

[Jon Isaacs]

I’m sure that no 3.5, 3 or 2.5-inch telescope would best the C8  as well. But if you put a Takahashi TOA 150 next to your C8, you will see that a smaller aperture (150mm vs 200mm) can best a larger aperture (your C8) on the planets, and in any seeing conditions. 

 

Now before anyone says that the Takahashi is more bulky and much more expensive, that is of course true, but that is not the focus of the discussion or the poster's observations, which are strictly performance based.

 

Bob

 

Of course that is not what this thread is about.  It's about seeing and aperture.  And I have to think that Takahashi might have trouble with this 8 inch SCT:

 

 

F/25 with a 20% CO.  An ATM by a professional optician with the Dutch Space Agency. 

 

Jon

[Jon Isaacs]

If you are asking about how sophisticated the models are - they have been in use in the literature for 50 years.  If you are amazed at how well they work and are still in use after 50 years - I am in agreement.  Fried's work is based on elegant simplifications of a very complex problem involving turbulence.  The results are not perfect - but they work very well and continue to be cited and referenced because they work well.

 

 

I am not amazed at how well they work.  I don't know who well they work, visual is different than imaging.   But this thread is about the visual experience viewing the planets.  I think Red said it best:

 

" My experience is that the seeing has to be particularly bad before the smaller scope reveals more detail than the larger scope.  It happens frequently enough in my backyard, but at that stage the seeing is bad enough that it really isn't suitable for planetary anyway."

 

Jon

[bobhen]

Of course that is not what this thread is about.  It's about seeing and aperture.  And I have to think that Takahashi might have trouble with this 8 inch SCT:

 

 

F/25 with a 20% CO.  An ATM by a professional optician with the Dutch Space Agency. 

 

Jon

That would be interesting.

 

I think I would still pick the Tak TOA 150 (and the more the seeing moved from good to average I think the Tak would be better still) based on these observations from Mr. Yoshida and after owning and comparing a few telescopes on Mr. Yoshida’s list.

 

The number indicates Mr. Yoshida’s designation for performance on planetary observing only.

 

Notice the smaller refractors besting some very nice and some much larger telescopes.

 

Folded designs (no matter how good) have a tad more trouble with seeing.

 

So along with aperture, both optical quality and design have to be considered when comparing an optical system in various seeing conditions.

 

(96) ASTRO-PHYSICS 160EDF
(96) Zeiss APQ150
(95) Takahashi TOA-150
(95) TMB 152mm/F8-CNC-LW
(95) Takahashiμ-250
(93) ASTRO-PHYSICS 155EDFS
(90) INTES-MICRO ALTER A-608
(90) ZEN250 Maksutov
(88) CELESTRON C-11

 

Bob

Edited by bobhen, 12 October 2019 - 09:10 AM.

[TOMDEY]

SOLUTION / Epiphany Time: That satisfies both the questionable theory and the actual use conundrum/uncertainties >>>

 

Use the bigger scope and have an adjustable off-axis stop right there! I made a 6.5-inch black foamboard stop for my trusty old 17.5-inch scope... and would just plop it on and spin it around while viewing thru the eyepiece... to find the ~sweet spot~ re' actual dynamic thermals. Sometimes it helped, sometimes it worsened. But, at least at that point... all the theory and wondering which scope to haul out --- became entirely moot! I knew that I was always optimized for what the sky had to offer, every night, all night. Here's a cartoon of the stop that I can use on my existing scope. These foamboard stops weigh and cost almost nothing... give it a spin!

 

Keep in mind... the opposite situation... the occasional nights where the seeing is superb: A good twelve-inch scope will clearly resolve a half-arc-sec. A six-inch scope will not / cannot!     Tom

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Edited by TOMDEY, 12 October 2019 - 01:01 PM.

[andycknight]

I remember directly observing my 'dodgy department store' 60mm Refractor outperformed my C8 on certain nights of poor seeing. Now I must stress that this was a 'dodgy' department store refractor, so the observation was not subtle but rather obvious. For example in the refactor Jupiter was a disk with two bands and hints of stuff in-between, in the C8 Jupiter was a fuzzball with no hints of any banding at all.

 

Curious for a reason - I made an aperture mask to stop the C8 down to 60mm. What I found was on such nights it noticeably improved the image. Not surprisingly the '60mm C8' was now giving a sharper image than the 'dodgy' refractor.

 

However on such nights I also found that if I waited long enough (using the full aperture), I would get very brief moments (for perhaps a second or so, separated by many minutes) when the C8 image dramatically improved and showed how poor the image in the 60mm aperture really was!

 

Now I must admit that this is not always the case. Many a night of poor seeing, I've found the smaller scope was no better than the larger one... Also I've observed on some nights the larger scope was always better, even though the smaller scope was clearly limited by the seeing.

 

So I have never had any difficulty in believing a smaller aperture can outperform a larger one under certain circumstances of poor seeing.

 

One possibility often thought of to explain this, is the additional time it takes the C8 to cool down. However some nights start with good seeing and then worsen and then get better etc... This can occur over relatively short periods off time compared to the time the C8 takes to thermally stabilise. I've also confirmed (when the diffraction disk is briefly visible), that collimation was good and there were no hints of a thermal 'chimney' either side of focus.

 

Regards

 

Andy.

Edited by andycknight, 12 October 2019 - 09:41 AM.

[Jon Isaacs]

That would be interesting.

 

I think I would still pick the Tak TOA 150 (and the more the seeing moved from good to average I think the Tak would be better still) based on these observations from Mr. Yoshida and after owning and comparing a few telescopes on Mr. Yoshida’s list.

 

 

Personally, if 8 could choose beyween the two, I'd pick the 8 inch F/25 SCT..

 

With enough money, any one can own a 150 mm refractor of their choice. There's only one 8 inch F/25...

 

Jon

[Eddgie]

I have provided this information in a great many threads on CN but I fear that it largely goes ub-believed or just gets dismissed out of hand but I will provide it to you and maybe based on your experience, you might be one of the few that actually accepts this point and it is a simple point. 

 

There is the telescope, and there is the human eye.  These dialogs almost always focus on things like seeing, obstruction, aperture, and on and on and on, but they almost never account for the behavior of the human eye.

 

The human eye has rods and cones.  Five rods share one neuron, so this makes the rods quite good for detecting faint objects, but because the five rods cover more area for each neuron, they lack the resolution.  Because of this, the fully scotopic eye only has a resolution of about 3 arc minutes.

 

Cones each get their own Neuron and because of this, they are far less likely to fire for a give number of photons hitting the retina, but because they each cover less area, this means they have far greater resolving power, and they also see more colors than the rods, which are mostly sensitive to green.  Now in daylight, the cones are capable of resolving 1.1 arc minute detail. 

 

When using a telescope to view planets, the eye is actually working somewhere between scoptopic mode and photopic mode. It is in a mode called mesopic mode, which is a combination of photopic and scotopic modes.  It has better low light sensitivey than pure photopic, but it looses some amount of resolution because there are not as many photons available to fire the cones.  This means that the eye is resolving better than scotopic though.

 

Enter the "Contrast Sensitivity Threshold." The contrast sensitivity threshold is the amount of contrast that has to be visible for the eye to be able to pick a feature out form the background.  As the illumination of the retina gets progressively lower, the eye's contrast sensitivity gets worse.  This means that you have to have more contrast to see a given detail.  A detail with 2% contrast is resolvable with the photopic eye, but for the scotopic eye, it can be as high as 15%.  Below this amount of contrast, and the eye simply will not be able to see it.  This is dependent on the size of the detail (frequency) and different observers will have different contrast sensitivity threasholds. 

 

The best way to increase contrast sensitivity threshold is to increase the illumination level.  Now if you use a 4" scope at 150x you are already seeing a dim image, and this means fewer rods are firing, and because of this, you have poorer resolving power.  In the much larger aperture working at the same power, you are firing you cones far more often, and as a result, your visual acuity is increased and your contrast sensitivity threshold is lowered so now, lower contrast details become easier to see.

 

See, there is resolution, and there is contrast.  Your seeing might lower your resolution, but a brighter image might allow you to more easily resolve a bigger detail that would be below your contrast sensitivity threshold in the smaller scope.

 

With more aperture, you improve your visual resolution (for a given exit pupil) and you improve your contrast sensitivity threshold.

 

Again, these forums typically totally ignore the performance of the human eye when it comes to comparing smaller telescopes to larger telescopes, and that means that the dialog is far from complete.  To really complete the picture, one has to factor in the behavior of the telescopic eye.  It is after all, a vital component in your observations.

 

I have for a very long time now been adamant that the very best thing a dedicated planetary observer can do is to buy a bigger and better telescope because one of the most important parts of the game is firing as many cones as possible and a larger aperture will almost always give this advantage. 

 

So, the larger aperture fires more cones, which increases the ability to resolve finer detail, and it raises the contrast sensitivity threshold so that larger lower contrast detail that is below that threshold in the smaller scope now exceeds that threshold in the larger scope. 

 

And last but not least, illumination gives you c o l o r. The more cones you fire, the more colors, or shades of color you will see.   A wide belt on Saturn will suddenly start to break down and be shown as two different color thinner bands next to one another.  Rather than look like featureless streaks, the belts on Jupiter will start to reveal difference densities of red/brown, and the GRS, rather than appearing as pale grey brown spot, will start to show beautiful red and gold features. 

 

I have said it over and over and over on this very forum but I keep repeating it because for the best planetary observation possible, one should recognize the role that illumination levels play on resolution and contrast detection.

 

While fine details may be lost to seeing, many structures on Jupiter, Saturn, and Mars are not so small as they are very low contrast. See the difference contrast makes?  All of the letters are the same angular size, but note how some are very difficult to see. Turn down your computer monitor brightness and watch what happens as the luminance level falls!!!.  Imagine doing the same thing to Saturn.  Get the point?  These are large details, but they are low contrast details, so even though the are more than big enough to resolve, if they fall below your contrast sensitivity threshold, they become hard to see, and when they fall below that level, they become invisible. 

 

Bottom line?  For low contrast detail on planets, bigger is almost always better because you are firing more cones and it is the cones that give use improved resolving power and contrast sensitivity.  The simple act of raising the image brightness will almost always make the image better. 

Edited by Eddgie, 12 October 2019 - 10:29 AM.