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Are long focal length refractors less prone to bad seeing?

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#1 gamma_ari

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Posted 14 September 2020 - 09:28 AM

Some people claim that long focal length refractors are less prone to seeing than short focal length refractors, resulting in more stable images. Reasons that are cited include the longer depth of focus, which makes focusing easier and allegedly more resistant to atmospheric turbulence. What are your experiences?

 

Yesterday I pointed my Vixen 80 mm f/11 achromat at Mars. The planet was only 12 degrees above the horizon so I didn't expect much. I was blown away how good the view was! I took out my TS ED 100 mm f/7 for comparison and found that the achromat offered a more stable image. I'm not sure if the smaller aperture or the longer focal length is to thank.

 

Best,

Viktor


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#2 Galicapernistein

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Posted 14 September 2020 - 09:50 AM

Slow telescopes are better on the planets no matter what the conditions.



#3 Astrojensen

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Posted 14 September 2020 - 09:52 AM

What I've found, based on numerous experiments through the years, is that a long focal length refractor doesn't offer any intrinsic advantage over a shorter one, when we consider atmospheric turbulence high in the atmosphere, but it DOES offer real advantages, when it comes to turbulence near ground level and around the observer, simply because it has the entrance aperture higher above the ground and farther from the observer (every inch counts, the first ones dramatically so). And ground-based and observer-based turbulence often has quite a large impact on the telescopic view as a whole.  

 

The increased depth of focus should in theory not offer any advantage, as telescopes used for visual observing are used as afocal instruments (IE the exit beam is focused for infinity), but in practice it makes it easier to make sure you really are focused on infinity and not on a layer of turbulence high in the atmosphere or somewhere in between. 

 

It's one of those weird cases, where both those who say it shouldn't work (in theory) and those that say it does (in practice), are right.

 

 

Clear skies!
Thomas, Denmark


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#4 chubster4

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Posted 14 September 2020 - 09:56 AM

Improved image steadiness is to be expected with smaller apertures. Larger apertures admit a wider range of turbulence wave cell sizes than smaller apertures, so whatever the atmospheric conditions, things will look steadier through a smaller scope. To see things sharply through a large scope requires much better seeing...one of the downsides of larger apertures.


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#5 Rutilus

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Posted 14 September 2020 - 09:59 AM

I would use a 100mm f/22.5 Achromat alongside a 100mm f/8 Tak apo, and to be honest I never

really noticed an advantage with the achromat.

In really bad seeing, both scopes were the same i.e. mushy view.  


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#6 ngc7319_20

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Posted 14 September 2020 - 10:06 AM

What I've found, based on numerous experiments through the years, is that a long focal length refractor doesn't offer any intrinsic advantage over a shorter one, when we consider atmospheric turbulence high in the atmosphere, but it DOES offer real advantages, when it comes to turbulence near ground level and around the observer, simply because it has the entrance aperture higher above the ground and farther from the observer (every inch counts, the first ones dramatically so). And ground-based and observer-based turbulence often has quite a large impact on the telescopic view as a whole.  

 

The increased depth of focus should in theory not offer any advantage, as telescopes used for visual observing are used as afocal instruments (IE the exit beam is focused for infinity), but in practice it makes it easier to make sure you really are focused on infinity and not on a layer of turbulence high in the atmosphere or somewhere in between. 

 

It's one of those weird cases, where both those who say it shouldn't work (in theory) and those that say it does (in practice), are right.

 

 

Clear skies!
Thomas, Denmark

Well stated.  I would only add that for a given refractor aperture, the longer F-ratio lens is likely thinner (less thermal mass) and possibly with milder aberrations (spherical, chromo-spherical).  I am also thinking of fast lens designs where the aberrations are barely suppressed, and hence might be more aggravated by mild seeing effects.


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#7 EJN

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Posted 14 September 2020 - 10:12 AM

The greatest advantage of long focal length refractors is that they are not forcing light to do tricks that light does not want to do. :yay:
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#8 JohnBear

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Posted 14 September 2020 - 10:25 AM

 

And ground-based and observer-based turbulence often has quite a large impact on the telescopic view as a whole.

New-to-me concept that I had never thought about so far. Thanks for bringing it up.

Something else interesting to check out to check out and learn about when/if the Pacific wildfire haze ever clears up.



#9 Bomber Bob

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Posted 14 September 2020 - 10:36 AM

Reasons that are cited include the longer depth of focus, which makes focusing easier and allegedly more resistant to atmospheric turbulence. What are your experiences?

 

What I've seen when comparing F7 & faster vs F10 & slower fracs is that the > F-ratio means less chasing focus.  Yes, even my F15 fracs have a (best) focal point, but the acceptable range is greater.  Once the turbulence goes above threshold, it doesn't matter how long / short the frac is -- the views will be roiling, boiling, and unenjoyable.  


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#10 SeattleScott

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Posted 14 September 2020 - 11:41 AM

New-to-me concept that I had never thought about so far. Thanks for bringing it up.
Something else interesting to check out to check out and learn about when/if the Pacific wildfire haze ever clears up.

I read a post about someone in a cold climate doing hit and run stargazing with their SCT. Locate target. Walk away from scope. Wait. Run over to scope. Observe target until his body heat started to mess with the view. Walk away from scope. Repeat.

Obviously there would be some advantage in a 4” F10 achro with that 40” long tube over his 17” long 8” SCT (presumably plus dew shield).

Scott

Edited by SeattleScott, 14 September 2020 - 11:42 AM.

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#11 gnowellsct

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Posted 14 September 2020 - 03:11 PM

Reasons that are cited include the longer depth of focus, which makes focusing easier and allegedly more resistant to atmospheric turbulence. What are your experiences?

 

What I've seen when comparing F7 & faster vs F10 & slower fracs is that the > F-ratio means less chasing focus.  Yes, even my F15 fracs have a (best) focal point, but the acceptable range is greater.  Once the turbulence goes above threshold, it doesn't matter how long / short the frac is -- the views will be roiling, boiling, and unenjoyable.  

This isn't limited to refractors.  My c8 is much more difficult to focus than my C14.  My belief is that the focusing gets harder, due to diminishing depth of focus, at about 1x per mm of aperture and over (aka diminishing exit pupils).  My C14 is easier to focus than my c8 for the simple reason that I hardly ever push it up to 350x which is the 1x per mm zone.  The c8 on the other hand requires patience and a light touch at 200x and over.

 

One of the things that refractors have going for them--the good ones anyhow, is that they have very nice focusers, even exquisite focusers, which very smoothly help you find best focus at small exit pupils.  You get help from mechanical excellence, in other words, and that impacts the view that the optics deliver.

 

I have many good things to say about SCTs but their comes-with focusers is not one of them.  The barbaric design hinders the optical performance of these very good scopes.

 

A couple of years ago some smart people here were going at it over whether longer focal ratios are better in bad seeing and I don't remember anything about all that which made me think it was anything more than it is a lot easier to focus a long focal ratio scope at high powers.

 

A Barlow is defined as something that lengthens the focal ratio of a scope and I've not heard anyone saying a barlow is better in bad seeing.  Put that way the mythical nature of the claim might seem a bit obvious.  No one says "in bad seeing put a Barlow on your scope."

 

I've never bought into the small-is-better-in-bad seeing argument either, with atmospheric cells etc.  I've spent too much time under the sky with an apo bolted on to my SCTs.  A bad night is a bad night.  You can always sharpen things up by reducing magnification.  A smaller aperture in most cases always has the potential to go to lower magnification than a larger one (since the larger aperture gets multiplied by the f/ratio etc)

 

Greg N


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#12 Galicapernistein

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Posted 14 September 2020 - 03:42 PM

Slow telescopes with wide depths of focus are very forgiving of eye position. I observe the planets with my eye well above the eyepiece. This is a very comfortable option. I can observe the planets for hours this way. Which means that, if there are only moments of good seeing, I’ll be much more likely to be able to take advantage of them.


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#13 Bomber Bob

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Posted 14 September 2020 - 03:42 PM

I've seen air columns / cells that didn't hurt the performance of a 50mm frac, but seriously affected the views in a 100mm frac...  However, in general, if the air is that turbulent, high-power observing in any aperture frac is not going to be enjoyable...

 

Besides the depth of focus, the high F-ratio / LONG achromatics are very tolerant with simple eyepieces; and, IME, the fewer pieces of glass between the object & my eyeball, the better -- especially on those sub-optimal nights.  Many of the oldest Japanese imports were built for straight-through observing for those reasons.  I try not to use Barlows for similar reasons; and, unless it's high-quality, the Barlow will add error to the system...

 

LONG fracs don't make the seeing better.  LONG fracs have a few intrinsic qualities that can help on iffy nights.  Refractor Fans didn't drop these fracs because they stank.  They swapped them for short APOs to do color digital imaging, or to use smaller & lighter mounts, or to cut false color on planetary limbs & disks... or any combination of those reasons & more...  IF you can deal with those F15+ tube lengths, there's still a place for them.

 

Or, better-yet, use one of the F11+ ED doublets!


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#14 bobhen

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Posted 14 September 2020 - 05:14 PM

HERE is a web page that really goes into detail about the impact (actually the LACK of impact) of long versus short focal ratio.

 

If you don't want to read the complete article, below is the conclusion from that page…

 

 

“Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true. While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater. In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position.

 

There is a long list of valid reasons why high f-number telescopes often perform better than faster ones. Some important reasons are:

a) Slower (i.e., high f-number) optics are exponentially easier to fabricate to the same accuracy as faster optics.
b) As already mentioned, the greater depth of focus of the high f-number telescopes makes them easier to precisely focus.
c) High f-number telescopes have a larger region of the focal plane that is diffraction limited, so off-axis performance is better. This is especially true with Newtonians, where coma and eyepiece astigmatism (mostly the latter) can be noticeable off-axis problems in fast instruments.
d) Slower optics are easier to collimate accurately, and there are less detrimental optical implications to slight misalignments.
e) Many eyepieces perform better with a higher f-number.
f) When comparing two Newtonian reflectors, slower scopes usually have smaller secondary mirrors. While the difference in image quality between, say, a 15% and 20% obstructed telescope is hard to detect, it would be a contributing factor.

 

If you happen to be observing through two telescopes of the same aperture on the same night, and the longer focus telescope is performing better, some of the reasons stated above are likely to be the explanation. Also, are the two scopes you're comparing of the same design type (i.e., reflector, refractor, SCT)? If not, they most likely have different thermal behaviors.”

 

I have never looked through a long focus achro (including 4” F15 Edmunds, 6” F15 A Jaegers, and a few others) that bested a “top quality” apo on the moon or planets. And the quality and precision of today’s high-end apos has eliminated the “slower scopes are easier to manufacture” argument. And with today’s 2-speed focusers, achieving exact focus in a faster apo is easy.

 

Sided-by-side the differences are more readily seen. That does not mean the achros put up poor images. Of course they did not. But the color purity, high quality glass, and high optical figure of modern top-end Apos is unrivaled."

 

Bob


Edited by bobhen, 14 September 2020 - 05:30 PM.

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#15 Galicapernistein

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Posted 14 September 2020 - 06:30 PM

Maybe with faster scopes, it’s harder to judge where your eye should be positioned at the eyepiece when looking at a de-focused image. This can make slow scopes seem better when seeing is bad, because eye position is much less critical.


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#16 VNA

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Posted 14 September 2020 - 07:26 PM

Hello:

Nothing to do with Chromatic aberration?

May be I did not understand the question of Gamma_Ari?

My experience with my Celestron CR6 gives me a crisper or livelier  image than my 10" SCT, but it could be power of suffusion?

Yes I understand my 6"CRT is low tech, although CA is not noticeable mostly due to my poor eye sight.

Of course one can lengthen the focal point by making the aperture smaller which is provided.



#17 John Fitzgerald

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Posted 14 September 2020 - 07:47 PM

What I've found, based on numerous experiments through the years, is that a long focal length refractor doesn't offer any intrinsic advantage over a shorter one, when we consider atmospheric turbulence high in the atmosphere, but it DOES offer real advantages, when it comes to turbulence near ground level and around the observer, simply because it has the entrance aperture higher above the ground and farther from the observer (every inch counts, the first ones dramatically so). And ground-based and observer-based turbulence often has quite a large impact on the telescopic view as a whole.  

 

The increased depth of focus should in theory not offer any advantage, as telescopes used for visual observing are used as afocal instruments (IE the exit beam is focused for infinity), but in practice it makes it easier to make sure you really are focused on infinity and not on a layer of turbulence high in the atmosphere or somewhere in between. 

 

It's one of those weird cases, where both those who say it shouldn't work (in theory) and those that say it does (in practice), are right.

 

 

Clear skies!
Thomas, Denmark

“In theory there is no difference between theory and practice – in practice there is” (Yogi Berra)


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#18 Jeff Morgan

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Posted 14 September 2020 - 11:32 PM

I live in an area of difficult seeing, and on average (that is, discounting the great nights) larger/longer refractors have not helped me break the magnification "ceiling". It might work in South Florida conditions, but not here.

 

Perhaps the smaller aperture was less affected by local seeing in your case?


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#19 Lookitup

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Posted 14 September 2020 - 11:59 PM

Not here in SW FL either currently. Using Tak 100DF and excellent SW 120 Pro the SW still mostly wins on planets. Seems like seeing condition supersedes the small aperture difference.  The 4" should show less turbulence but the last few months it did not. YMMV



#20 gamma_ari

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Posted 15 September 2020 - 02:28 AM

Thanks for the many good answers. Yesterday I repeated the experiment on Jupiter. This time the 100mm f/7 ED was clearly superior to the 80mm f/11 achro. The seeing has been very volatile in my location lately, which makes it hard to compare scopes. While you're changing eyepieces, the seeing becomes better or worse. For me, the best scope seems to be the one that allows me to spend the most time at the eyepiece, to catch those fleeting moments of good seeing.


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#21 CHASLX200

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Posted 15 September 2020 - 05:43 AM

Slow telescopes are better on the planets no matter what the conditions.

They also stick up higher so less air to blur the image.


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#22 Galicapernistein

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Posted 15 September 2020 - 09:33 AM

They also stick up higher so less air to blur the image.

And keeping body heat out of the light path is a definite advantage of slow refractors.


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#23 paulsky

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Posted 15 September 2020 - 09:38 AM

Could we say that two telescopes with the same aperture would be better for this purpose the one with the greater focal length?
And better also between two telescopes of similar focal length the one with greater diameter?
Cheers,
Paul.



#24 Hesiod

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Posted 15 September 2020 - 10:05 AM

I'd rather say the better among two telescope with the same aperture will be the one whose optical accuracy was the greatest.

The "mushyness" appears when the air-column induced "aberrations" add to the intrinsic aberrations of the telescope and bring the overall "sum" above a certain thresold (here I am in the case of a somewhat good seeing, where the diffraction pattern is not extremely disrupted since in this case the impact of the seeing is too great for optical accuracy to make a difference).

Being slower telescopes easier to craft and adjust, if have to place my bet I would not choose the faster one, especially in the case of achromats.

 

Between different apertures, the smaller one has a sort of "advantage" in being unable to detect as much detail as the larger rival: in such case it is possible that the view in the smaller telescope will look "sharp and steady" while that in the bigger one "mushy" but, if we assume equal or equivalent optical accuracy, and pay attention to what can be seen, the larger telescope will show anything which could be seen in the smaller one, and often something more, even if likely not to the very extent of its capacity.

Also, since a larger "disruption" is required to compromise the views in the smaller one, chances that the view will be steady for a longer time are much higher.

 

It is therefore not a surprise that among my telescopes it is the f/6 FL55 the best at "dodging" bad seeing, and not the TAL 100/1000: actually the FL55 does not "dodge" anything, simply it fails to take notice of the bad seeing until it reaches truly hideous levels


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#25 Astrojensen

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Posted 15 September 2020 - 02:05 PM

Many years ago, I did extensive tests between a TV-76, a 80/1200mm Vixen achromat and a 85/1572mm Zeiss apochromat (stopped down to 75mm) and the two long scopes were definitively superior for planetary observing and offered far steadier views at identical magnifications. 

 

There's a looong thread about all this buried here in the CN archives somewhere. It all started with a post by Neil English stating that long achromats offered more stable planetary views and I set out to see if it was true. He claimed it was something only achromats did (for some reason), but I found that apochromats could also do it, but that apochromats long enough to actually make the phenomena observable were rare. 

 

 

Clear skies!
Thomas, Denmark


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