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David vs Goliath

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#26 Asbytec

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Posted 23 April 2013 - 10:33 AM

(By the way, it did not that a CO reduces the size of the Airy disk slightly, the energy transferred into the rings has to come from somewhere. The disk is dimmer, therefore slightly smaller, that energy is transferred into the rings which smears the image thus reducing fine scale contrast. Of course that's a small effect... )

Jon


Jon, seeing is an interesting thing. You and Dave hit on it pretty much. All I would add is "tuning" a scope for prevelent seeing conditions might be a consideration. I conastantly rave about seeing in the tropics. Seeing a nearly perfect diffraction pattern almost constantly is "pleasing." It seems well suited for a 6" aperture, probably more, almost every night. I still have to wait for those almost perfectly still moments, but they are not so few and far between. So, calm seeing affords some jaw dropping views of lunar and planetary frequently throughout the observing session. It's both pleasing and working at full resolution and contrast a 6" is capable of.

As you alluded to discussing larger apertures shining in those calm moments, that's a scope can operating near it's theroetical, unaberrated MTF. When the seeing calms the theroetical plot settles toward "testbook," well, those are the jaw dropping moments in any aperture.

To your above, yes, the CO dims the spurious disc to some extend relative to the surface area of the obstruction, I believe (1 - co^2) normalized. However, the obstruction also adds diffraction to the system and changes the actual pattern of interference forming the Airy disc. The Airy disc is made smaller and the rings shift slightly due to added diffraction as well as obscuration. Both are small effects [in sum according to Vlad, et al, (1-co^2)^2]. When the seeing is nearly perfect those changes can be detected at smaller exit pupils and especially on equal tight doubles. And probably on lunar and planets, too, IME.

It usually bugs me a tiny bit some obstructed designs, CATs especially, and Newts are usually associated with seeing conditions, thermal issues, and collimation. These conditions, which can be minimized and optimized and are not necessarily intrinsic induced aberrations of these designs, are often toughted as reasons why CATs and Newts are Goliath while the humble refractor is David, who slew Goliath...in bad seeing. Its not a story of some noble short guy beating up the evil giant, rather its a story of an inferiority complex. A rationalization of owning 4" of aperture offering pleasing views frequently. I experience the same "pleasing" experience in my own 6" CAT because it is well tuned and operating in excellent seeing (=/> 8/10)almost nightly.

Refractors are great scopes, no doubt some of them high end, that give "pleasing" views. And yes, it is nice to use both designs where they excel. Not trouncing refractors here, just trying to put all those bad things that make Goliath a bad guy into some perspective. There is at least one observing condition I can think of where a 6" unobstructed, finely hand crafted scope can trounce a descent 12" obstructed scope...when the latter is at the bottom of the ocean. The seeing sucks down there. :)

#27 Asbytec

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Posted 23 April 2013 - 10:40 AM


I think the effect is more fundamental. By introducing a central obstruction the average distance between points within the area of light gathering increases, thereby decreasing the angular size at which zero crossings of the light amplitude occur. In other words: the central disc plus all outer rings diminish in size.



Interesting.. Do you have a reference for this. I thought the diameter of the minimums and maximums was determined by the aperture. Looking Rutten and Venrooij's section on COs, I see no mention of it.

Jon


Sorry, Jon, yes...one reference follows. Johannes is correct.

http://www.telescope...obstruction.htm

He discusses it in some detail, but here is a snippit.

"However, due to the smaller central maxima (primarily; the overall pattern is smaller), the obstructed pattern rebounds in contrast transfer not only above the aberrated clear aperture, but also above clear aberration-free aperture in the range of MTF frequencies where the dominant factor of contrast transfer becomes the size of central maxima, generally from about 0.4 to 1. Moreover, with the central maxima and overall pattern smaller (approximately) by a factor 1-co^2, the obstructed PSF becomes similar to that of unobstructed aperture larger by a factor 1/(1-co^2) having its Strehl reduced to (1-co^)^2 by spherical aberration. In other words, contrast-transfer-wise, an aperture D with central obstruction o compares to a clear aberration-free aperture of D/(1-co^2) diameter with (1-co^2)^2 Strehl due to spherical aberration."

#28 Asbytec

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Posted 23 April 2013 - 11:53 AM

Who can forget the glorious Questar ads?


I dont have their pamplet handy, but Questar also claimed to resolve below Dawes (pretty sure it mentioned Dawes...) limit. They are likely they are warranted in saying so for the reasons found in the discussion between Jon and Johannes: the scope have a very high Strehl and an obstruction, both contributing to that tight PSF that manifests itself at the high frequency end of the MTF.

#29 BillP

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Posted 23 April 2013 - 12:21 PM

4" 'fractor against a 14" of any type???
Come on, that's no comparison.


It is fair when you need to show that you can beat the 'fractor :lol:

#30 BillP

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Posted 23 April 2013 - 12:36 PM

Johannes. I cannot envisage a situation where a 6" will out-resolve a 12", assuming equal optical quality. The smaller scope is less affected by seeing and will give views that seem more stable or pleasing*.

If the seeing is scintillating to 5" size (I have never seen seeing this bad in 50 years of observing, btw) both scopes will give terrible views on high-resolution targets like planets, the moon and double stars.

...

There is no type or category of seeing where my 3.1 will resolve what my 6 will. But the view in the 3.1 is steady, stable motionless and textbook. Of an unresolved double star. (And, by extension to details on Jupiter, etc.) It is this that some call "pleasing".


FWIW, I've come across plenty of an evening here on the East Coast where my 10" shows no more than my 4". So it can easily be the case and strongly location local.

For the *pleasing* category, not only steadiness but also apparent contrast/scatter as well I think can be included. Many refractors are easy to keep their main objectives clean. On the other hand, a typical Dob is a bit of a pain to clean the objective so they are typically more dust encrusted I would say, especially for solid tube designs. This being the case, quite often refractors are seen to produce not only steadier images, but cleaner higher contrast ones as well.

I think many state their preference for the refractor, even though smaller, because of this more pleasing view. It is not something to be taken lightly either IMO, and often trumps resolving power for many. Think of it in another context, if everytime you watch a movie and went to a theater you had this great superwide IMAX screen but what also came with it was an audience constantly chatting on their cell phones during the movie, going to a small home-theater instead, with its much smaller screen, would in the end be the better experience if it was always nice and quiet and calm there so you could get into the movie. So in this analogy, the smaller screen, although less capable than the IMAX, in the end gives a better overall experience, and just plain better! By analogy, same is for refractors for many, and validly so. Taming all the gremlins in a Newtonian, SCT, or Mak takes a lot more skill and patience than it does for a refractor which is fairly effortless in this regard. I think the majority of observers are not in the class that have learned how to handle the gremlins well so the refractor always shows the best view, regardless of aperture.

In an often seen real-world experience, many evenings I view doubles with my 10" that it splits quite well. However those doubles are the typical "wooly" stars that are ill-defined due to thermal activity. My smaller refractor may not split the double on the same evening however it is a textbook airy disk that is perhaps just elongated. In the end, on these evenings, I will ditch the 10" because it is more of a thrill splitting the less close doubles given the "precision" at which the refractor shows the image. So the *pleasing* characterization often trumps many other things, including resolving power.

#31 Mark Harry

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Posted 24 April 2013 - 01:45 PM

The break point for my area here-
Planetary; a good 6" will hold the image very well, and maintain detail with an image that quivers around, but doesn't lose the detail until really bad. Then, the image will suddenly "blow" and become a large fuzzball.
8" is different. When it gets to the point of quivering around as in the 6, the detail has become "soft". Fine stuff is simply gone; almost as if it was de-focused. Regardless, when it gets soft you ain't gonna see much of anything useful, than what a 6 will have a good chance of providing useful information.
M.

#32 gnowellsct

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Posted 24 April 2013 - 08:22 PM

4" 'fractor against a 14" of any type???
Come on, that's no comparison. (no offense)
M.


That's entirely the point. It's no comparison, as you say. You have people saying here they prefer to stay under six inches in aperture because large apertures are mush. I've never seen that. When I've side by sided with other apos up to nine inches, I've still preferred the larger apertures.

So the question is at what point it becomes a comparison. There, the likely answer is: subtract the diameter of the CO from the obstructed instrument to get the refractor equivalent; bearing in mind that the smaller instrument will show less color on Jupiter, etc.

GN

#33 Asbytec

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Posted 24 April 2013 - 09:45 PM

This is an interesting topic. Anyone well versed on the size of the Airy pattern in various apertures and seeing? The Airy pattern varies inversely with aperture, but probably bloats with seeing and maybe magnitude. It would be interesting to understand those dynamics better.

#34 moynihan

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Posted 25 April 2013 - 02:27 PM

:gotpopcorn:


+1

#35 Jared

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Posted 25 April 2013 - 09:48 PM


Under seeing limited conditions it is a fairly routine occurrence that a larger scope cannot resolve severely unequal magnitudes multiple stars (Sirius A and B, Trapezium A through F, etc.) when a much smaller unobstructed scope can.


I asked you this before, when was the last time you split Sirius B with an 80mm?

My experience is that when seeing poor, a large scope often out performs a small scope on widely unequal doubles. Rigel is a good example. When it first rises in fall and is low on the horizon, I am often unable to split it with my 80mm's. My old 12.5 inch picks out it ever time.

Jon


As it happens, I can answer your first question on Jim's behalf because I was there. March 31, 2011 was the last time, and we were in the Mojave desert--we first picked out Sirius B in my 80mm LOMO and later were able to confirm it in Jim's TEC 140, but it was an easier split in the 80mm for whatever reason. Better glare control? Seeing conditions that were a better match for the smaller scope? Couldn't say for sure. But I know we weren't imagining the Pup because I recorded and later checked the position angle.

#36 Jon Isaacs

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Posted 26 April 2013 - 04:18 AM

As it happens, I can answer your first question on Jim's behalf because I was there. March 31, 2011 was the last time, and we were in the Mojave desert--we first picked out Sirius B in my 80mm LOMO and later were able to confirm it in Jim's TEC 140, but it was an easier split in the 80mm for whatever reason.



Jared:

Thanks for clarifying that. As I understand it, that was also the first time??? The reason I wanted a clarification, was that reading Jim's post, the implication seemed to be that this was a regular occurrence rather than a very remarkable observation.

And it is worthwhile noting that this observation was done with a 80mm F/6 apochromat, short and fast, and not a long focal length 80mm achromat. It is consistent with my experience with faster apos versus long focal length achromats..

Jon

#37 Jared

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Posted 26 April 2013 - 05:02 PM

That was the most recent occurrence for me, and I think for Jim, though with his acquisition of a new-to-him 80mm recently I may be overstepping a bit. Certainly, Sirius B is not a common split for me in my little refractor.

Why it worked that night in particular I couldn't say. Obviously, skies were dark with very low moisture in the air. Zodiacal light was easy to identify. Seeing conditions were mediocre at best, and there was a very stiff breeze with plenty of wind gusts. I was amazed that I was able to do any astro photography at all with my 10" RC since it is a giant sail under these circumstances.

One thing I can confirm is that it's possible for a smaller scope to out resolve a larger one under SOME circumstances. I'm not talking about a more aesthetic or more "contrasty" view here, either--actual detail visible in a small scope that is totally absent in a larger scope at the same magnification. I have seen my 60mm and 80mm refractors show more to me than my 10" and 12.5" scopes when seeing was really bad and the larger scopes hadn't had adequate time to cool. Not certain how much this matters, thoug, since I tend to pack it in when seeing is really bad--even deep sky stuff just isn't rewarding.

#38 Ziggy943

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Posted 26 April 2013 - 07:10 PM

Last Saturday I watched a heavyweight fight between Tyson Fury of England and former Cruiserweight champion Steve Cunningham. Fury weighed in at 254 pounds and stands 6’ 9” (Goliath). Cunningham weighed 210 pounds and stands 6’ 3” (David).

This was a fight between a good Goliath and a good David. The fight ended in the 10th round with Goliath knocking out David, BUT, in the second round David knocked Goliath to the ground. There were times when David was dominating the fight.

With our telescopes it’s the same thing, a good big telescope should beat a good little telescope overall. But there are instances where the little telescope will knock down the big telescope.

#39 JKoelman

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Posted 27 April 2013 - 08:56 AM

This is an interesting topic. Anyone well versed on the size of the Airy pattern in various apertures and seeing? The Airy pattern varies inversely with aperture, but probably bloats with seeing and maybe magnitude. It would be interesting to understand those dynamics better.

I agree. I am no expert, but the reactions in this thread made me wet my toe into the Kolmogorov theory describing the effects of atmospheric turbulence on telescope performance. One of the outcomes reported in the literature is that 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 cell sizes 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%

When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time.

#40 Asbytec

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Posted 27 April 2013 - 09:33 AM

Johannes, wow, great stuff. Yes, I agree on getting diffraction limited views most of the time. That is my experience. And under such conditions, the aperture is doing what it can in terms of resolution and contrast, for the most part. So, "pleasing" views tend to be performing views at that aperture.

I started a thread in the General Observing forum, "The Skinny on Seeing" trying to explore that concept.

#41 Jon Isaacs

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Posted 27 April 2013 - 12:19 PM

This is an interesting topic. Anyone well versed on the size of the Airy pattern in various apertures and seeing? The Airy pattern varies inversely with aperture, but probably bloats with seeing and maybe magnitude. It would be interesting to understand those dynamics better.

I agree. I am no expert, but the reactions in this thread made me wet my toe into the Kolmogorov theory describing the effects of atmospheric turbulence on telescope performance. One of the outcomes reported in the literature is that 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 cell sizes 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%

When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time.


Johannes:

I have to think if 98% of the snapshots are diffraction limited, that qualifies as a "pleasing view." If one is getting diffraction limited performance in a 12 inch scope 98% of the time, I have to rate that as very good seeing... But diffraction limited performance implies Dawes limit splits or at least Rayleigh splits, a 12 inch, that's less than a half arc-second.

I don't see being diffraction limited in even a 4 inch 100% of the time as indicative of poor seeing. In the first post, poor seeing was given as 5 arc-seconds... Diffraction limited in a 4 inch is about 1.4 arc-seconds.

Jon

#42 JKoelman

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Posted 27 April 2013 - 02:53 PM

If one is getting diffraction limited performance in a 12 inch scope 98% of the time, I have to rate that as very good seeing...

In the first post, poor seeing was given as 5 arc-seconds...


Terms like "good seeing" and "poor seeing" are subjective. Obviously, Rutten and Van Venrooij consider seeing conditions worse than those considered by Fried. That creates no problem whatsoever, provided one quantifies the seeing objectively. Rutten and Van Venrooij as well as Fried do so, the first via "seeing disks", the latter via the "Fried parameter" r0.

Let's not fall in the trap of equating person A's "bad seeing" to person B's "bad seeing". We can keep the discussion objective by limiting it to well-defined measures (such as the Fried parameter r0) that quantify the seeing.

The key message I hoped to convey is that overcoming atmospheric conditions and reaching distraction-limited performance in a telescope is a challenge that increases exponentially with the scope's light gathering area. This is a statement build on a body of scientific work that forms the basis for billions of dollars of investments in adaptive optics.

#43 Jon Isaacs

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Posted 27 April 2013 - 03:55 PM

Let's not fall in the trap of equating person A's "bad seeing" to person B's "bad seeing". We can keep the discussion objective by limiting it to well-defined measures (such as the Fried parameter r0) that quantify the seeing.

The key message I hoped to convey is that overcoming atmospheric conditions and reaching distraction-limited performance in a telescope is a challenge that increases exponentially with the scope's light gathering area. This is a statement build on a body of scientific work that forms the basis for billions of dollars of investments in adaptive optics.



Honestly, I get it and I don't get it. A few thoughts:

- You provided some analysis that suggested that even in "poor seeing" a 12 inch scope would be diffraction limited 98% of the time. That is contrary to my experience. I personally think any amateur astronomer would consider diffraction limited performance in a 12 inch 98% of the time as good seeing. Something doesn't compute, that's all.

- As someone who has a fair number of telescopes over a wide range of apertures, I have no difficulty understanding that as the aperture increases so does the seeing required to achieve diffraction limited performance. In my largest telescope, 25 inches, I have no expectations of ever achieving diffraction limited viewing, that would be approximately 0.2 arc-second seeing.

On the other hand, even without the possibility of diffraction limited images, the real world question is how much of the time will the 25 inch provide superior planetary views to say the 12.5 inch or the 16 inch or a 6 inch... This is the issue I believe we are struggling with.

In the simplest terms, this could be thought of as how often does aperture X achieve diffraction limited status for aperture Y. Of course X > Y.

- Except anecdotally, I am not quite sure how "pleasing views" fits into an objective discussion. By definition, "pleasing views" would be subjective.

Jon

#44 Asbytec

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Posted 27 April 2013 - 07:00 PM

...the real world question is how much of the time will the 25 inch provide superior planetary views to say the 12.5 inch or the 16 inch or a 6 inch... This is the issue I believe we are struggling with.

Yes. One of them. LOL

#45 buddyjesus

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Posted 27 April 2013 - 07:36 PM

very useful post Johannes






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