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#126 stevew

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Posted 12 December 2011 - 11:13 PM

In an area that does not have good seeing, a small scope will often present an aesthetically more pleasing image.

I am convinced that this is why there are two completely different opinions on this subject.

Steve

#127 Cotts

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Posted 12 December 2011 - 11:28 PM

So, we have an 11" SCT and a 6"APO. The SCT has MUCH better resolution, almost double that of the APO. The APO has SLIGHTLY better (but noticeable to most eyes) contrast than the SCT.

The SCT will resolve details that the 6" is incapable of using its better contrast on.

The fact that these extra details will be seen with the SCT's poorer contrast is moot. The SCT will see them! And the APO will be blissfully unaware of them.

"There is no substitute for aperture" - Peter Ceravolo

Dave


How did you calculate the resolving power of an 11" SCT?


Rayleigh criterion will do. Or Dawes' limit. Or the Sparrow limit. All three of these are based upon aperture. Since 11:6 = 1.833:1 the 11 incher has 1.833 times better resolution than the 6 does. Which was referred to as 'almost double' in my post quoted above.

Dave

#128 Cotts

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Posted 12 December 2011 - 11:51 PM

Hi, Bob.

Addressing a couple of your points:

If the seeing is 1 arc second then both telescopes are limited to that level of detail - the performance of each scope will be identical. The Apo can't outperform the SCT under those conditions. Both scopes are limited equally by the 1" seeing.

Seeing varies over many timescales, right down to second by second. Experienced planetary observers wait for these moments of excellent seeing to squeeze out that last little bit of detail. It is not out of the question that when the average seeing is 0.75" on a particular evening (admittedly infrequent in temperate latitudes) there will be moments when the seeing improves to 0.6" or 0.5" or 0.4" The 11 inch will make more of those moments than the 6-inch can. Rare moments but they do happen!

An analogy. Suppose you and I were a law-abiding but thrill-seeking motorcycle drivers on a cross-country trip. The speed limit for the entire trip is 55 except in one state out of 30 where it is 150mph..
If you had a 125cc bike the 'thrill' of that one state would be lost to you but if I had a 900cc bike I'd be able to go 150 and get the thrill. In 29 out of thirty states the two bikes would drive side-by side. In one state
only I would get the 'thrill'.

that, to me, is how Aperture rules in the case of resolution.

The light-gathering argument is MUCH easier to make.

Dave

#129 JJK

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Posted 13 December 2011 - 12:37 AM

So, we have an 11" SCT and a 6"APO. The SCT has MUCH better resolution, almost double that of the APO. The APO has SLIGHTLY better (but noticeable to most eyes) contrast than the SCT.

The SCT will resolve details that the 6" is incapable of using its better contrast on.

The fact that these extra details will be seen with the SCT's poorer contrast is moot. The SCT will see them! And the APO will be blissfully unaware of them.

"There is no substitute for aperture" - Peter Ceravolo

Dave


How did you calculate the resolving power of an 11" SCT?


Rayleigh criterion will do. Or Dawes' limit. Or the Sparrow limit. All three of these are based upon aperture. Since 11:6 = 1.833:1 the 11 incher has 1.833 times better resolution than the 6 does. Which was referred to as 'almost double' in my post quoted above.

Dave


The 11" SCT primary has a 34% obstruction. Doesn't that impact the instrument's resolving power?

#130 Cotts

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Posted 13 December 2011 - 07:05 AM

Central obstruction does not affect resolution. The Rayleigh criterion (perhaps the resolution limit most rooted in actual physics) is a measure of the radius from the center of the diffraction pattern to the center of the first minimum (dark ring). This distance is blissfully unaffected by central obstruction.

Central obstructions of varying sizes affects the distribution of the light in the central spurious disc and the surrounding rings. Generally, the larger the central obstruction the more light is thrown into the rings and the less remaining in the center. This reduces contrast.

Dave

#131 rmollise

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Posted 13 December 2011 - 07:48 AM

If, as in most backyards, the seeing is 1 arc second then both scopes have effectively been limited by the seeing to the resolving power of a 5-inch scope. If we were to compare a 5” ¼ wave SCT with a 1/10 wave 5” Apo would anyone doubt the Apo would produce a better image with more planetary detail? This is why A 6-inch Apo can out-perform an 11-inch SCT under most seeing conditions. And even if the seeing gets to be really great at .5 arc seconds - allowing the SCT to theoretically out-resolve the 6” Apo, you would still have to contend with the SCT’s obstruction, and lesser optical quality. This is why people that own both an 11” SCT and a 6” Apo say that the 2 scopes are close in rare excellent seeing but the Apo delivers better planetary detail and sharpness in good and average seeing.


That seems to argue in favor of forgetting this whole thing and keeping my 5" refractor.


If you like your 5-inch, keep it. HOWEVER...if you expect it to show you as much of anything as an 8-inch SCT, you are going to be disappointed. ;)

#132 JJK

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Posted 13 December 2011 - 08:07 AM

Central obstruction does not affect resolution. The Rayleigh criterion (perhaps the resolution limit most rooted in actual physics) is a measure of the radius from the center of the diffraction pattern to the center of the first minimum (dark ring). This distance is blissfully unaffected by central obstruction.

Central obstructions of varying sizes affects the distribution of the light in the central spurious disc and the surrounding rings. Generally, the larger the central obstruction the more light is thrown into the rings and the less remaining in the center. This reduces contrast.

Dave


So if the CO radius is any value (e.g., 95% that of the primary), the resolution will still be unaffected? The CO redistributes the light from the central peak of the Airy disk to the outer rings. If a greater degree of the light from each of two stars is in the outer rings, how doesn't that impact the resolving power of the instrument?

#133 t.r.

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Posted 13 December 2011 - 10:11 AM

Hi, Bob.

Addressing a couple of your points:

If the seeing is 1 arc second then both telescopes are limited to that level of detail - the performance of each scope will be identical. The Apo can't outperform the SCT under those conditions. Both scopes are limited equally by the 1" seeing.


I'm no expert but IMHO, this is misleading. The larger scope (C11 in this case) will still show more vivid color and planetary belt saturation in this situation. Even though the level of macro (polar caps, maria) and micro (white ovals,swirls) detail will be seeing limited, the larger scope still shows "something" more. The images in the C11 and 6" apo at the same magnification will not be identical. The C11 will be brighter with better saturation. That is overlooked in this statement. You did however say "performance"(resolution I take) is identical and not image so perhaps I misunderstood, just didn't want others to do the same.

#134 coutleef

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Posted 13 December 2011 - 10:24 AM

If, as in most backyards, the seeing is 1 arc second then both scopes have effectively been limited by the seeing to the resolving power of a 5-inch scope. If we were to compare a 5” ¼ wave SCT with a 1/10 wave 5” Apo would anyone doubt the Apo would produce a better image with more planetary detail? This is why A 6-inch Apo can out-perform an 11-inch SCT under most seeing conditions. And even if the seeing gets to be really great at .5 arc seconds - allowing the SCT to theoretically out-resolve the 6” Apo, you would still have to contend with the SCT’s obstruction, and lesser optical quality. This is why people that own both an 11” SCT and a 6” Apo say that the 2 scopes are close in rare excellent seeing but the Apo delivers better planetary detail and sharpness in good and average seeing.


That seems to argue in favor of forgetting this whole thing and keeping my 5" refractor.


If you like your 5-inch, keep it. HOWEVER...if you expect it to show you as much of anything as an 8-inch SCT, you are going to be disappointed. ;)


I agree with this. there is theory and practice. And cost. I would keep the 5 inch for the wider view it provides. But would expect to be disapointed for viewing DSOs But the view of open cluster would be different so interesting with both.

I prefer to see a reflector and a refractor as complementary scopes rather than competing scopes.

#135 bobhen

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Posted 13 December 2011 - 10:28 AM

Hi, Bob.

Addressing a couple of your points:

If the seeing is 1 arc second then both telescopes are limited to that level of detail - the performance of each scope will be identical. The Apo can't outperform the SCT under those conditions. Both scopes are limited equally by the 1" seeing.


Hi Dave,

Respectfully-

The performance will NOT be identical because the Apo is of much higher optical quality, has no central obstruction, and has much less light scatter because of its refractive optics. As we AGREE that both scopes will be limited EQUALLY by the seeing, then the scope with the higher quality optics will prevail as long as the smaller scope has enough aperture to only be limited by the seeing. This is exactly how a smaller but higher quality scope can outperform a larger scope of lesser optical quality especially when object brightness is not an issue – like the moon and planets.

As far as seeing goes. On a night of 1 arc second seeing most of the night is not close to 1 arc second - only the brief seemingly sharp moments are – the mush you see when the object is not sharp is not close to 1 arc second let alone .5 arc seconds.

This quote from previous poster jgraham hits the nail on the head:

I knew Dick Wessling for many years and as a master optician and planetary observer he sought for decades to build The Perfect Planetary Telescope. He went to great lengths (literally) with monster long Newtonians with tiny secondaries and sophisticated unobstructed designs. His conclusion? Visually, seeing rarely supports anything much over 8". Unless you live in an area blessed with exceptional seeing you'll get dimenisioning returns once you get much above 8", the largest benefit being image brightness rather than increased resolution (RESOLUTION BEING SEEING LIMITED).

Bob

#136 Mitchell Duke

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Posted 13 December 2011 - 10:35 AM

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.

#137 rmollise

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Posted 13 December 2011 - 11:10 AM

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.


:goodjob:

#138 Arizona-Ken

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Posted 13 December 2011 - 11:44 AM

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.



Soooo obvious, but this always seems to get lost in the discussions.

The advantage of refractors over Cats is that the shorter focal length results in a wider field of view. SCTs are limited in an FOV of just over a degree although the vast majority of DSOs fit well within this view. If you want to look at more extended objects, a refractor has the advantage (of course for wide views with light gathering power, get a fast Dob!).

I often piggyback an 80 mm refractor on my CPC1100 to provide complimentary views. Of course the refractor cannot gather much light as compared to the larger scope, but the combination works for me.

Arizona Ken

#139 lightfever

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Posted 13 December 2011 - 12:07 PM

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.


:goodjob:


:waytogo:

#140 simpleisbetter

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Posted 13 December 2011 - 12:20 PM

Happy Birthday Ken.

I'm in complete agreement with you, Rod, and WHDuke on this statement. My C9.25 gave me better planetary views than any 6" APO I've looked through. No refractor has ever shown me the details that my old C9.25 showed me when cooled down and seeing allowed. And that brings up cooldown, which is the other big advantage of refractors over SCT's. It's why I like my 102ED so much, it affords me many more viewing sessions a cat or reflector would not allow. My experiences lead me to see a 6" APO equivalent to an 8" SCT wrt resolution. Contrast will likely be better on the 6" APO vs the C8. And I've never seen any 6-8" APO give the level of detail that Damien Peach is getting with his SCT. There's one more user from Aussie who posts SCT pix here that compete with HST images, but I can't remember his name off-hand.

The other thing I keep seeing in these discussions, and was brought up in the thread on the refractor page is that we're comparing mass produced scopes to hand-built premiums here. We're not only comparing apples and oranges, it's like we're comparing a Ford Focus to a Rolls Royce; it's a pointless comparison IMO.

#141 t.r.

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Posted 13 December 2011 - 02:16 PM

You can't really use pics as a valid comparison to visual...they are not apples to oranges either. ;)

#142 Arizona-Ken

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Posted 13 December 2011 - 02:21 PM


The other thing I keep seeing in these discussions, and was brought up in the thread on the refractor page is that we're comparing mass produced scopes to hand-built premiums here. We're not only comparing apples and oranges, it's like we're comparing a Ford Focus to a Rolls Royce; it's a pointless comparison IMO.


Thanks for the wishes, Steve.

Yes, with a 6 inch APO OTA going for over $12K and a C9.25 Edge going for less than $2500, or even a C14 Edge going for $5800, a high quality SCT is still a bargain.

Arizona Ken

#143 simpleisbetter

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Posted 13 December 2011 - 02:57 PM

You can't really use pics as a valid comparison to visual...they are not apples to oranges either. ;)


Well...I'm inclined to disagree with that statement, though I think we're actually more in agreement here than one might think. What I didn't post above is the biggest strength of an APO beyond cooldown, size advantage, and lower maintenance needs, is it excels in wide-field deep-space AP work. If I were into deep-sky AP, and had the money, then a 6-7" triplet or quad APO on an AP-1 or 900 would be my choice platform.

However, the SCT (like the C11 in question, or especially a larger newtonian) are better lunar/planetary/double star scopes for very fine details (where they excel) than the smaller refractors (not their forte). Aperture does rule, can't change that; and that was to the point I was making using the pictures as an example. Whether imaging or visual, I find the same rules re aperture vs resolution apply. You can't cheat the laws of physics, at least it's never worked for me.

#144 alrosm

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Posted 13 December 2011 - 03:14 PM

The problem is about money, that's all, SCT is versatile, the quality is good enough that you don't have to buy many scopes that you cannot afford.

The SCT is a great deal for planets if seeing is good.

The question is not to compare a SCT to other designs but is the quality good enough for what you want to do.

The bottom line is that you gonna have to go to a star party and check for yourself.

#145 simpleisbetter

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Posted 13 December 2011 - 03:22 PM

The bottom line is that you gonna have to go to a star party and check for yourself.


:like:

I couldn't agree more with that statement, I've learned a lot about my own preconceptions of what I thought vs what is real from doing just that.

#146 George Methvin

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Posted 13 December 2011 - 03:53 PM

In all fairness to my Meade 10 inch SCT it never has a chance to show me its best images. Living where I do the only clear view is from my driveway and in the summer you can fry a egg on it. In the summer the drive way never cools down so heat wave every where. Plus all the houses around are throwing up heat waves. Its a wonder it shows as good a view as it doe's. So much of what makes up the image throught a scope is where you view from, it can make or break a good scope.

#147 John Kasianowicz

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Posted 13 December 2011 - 04:45 PM

Its always scope size, seeing, and collimation that's king!! I know a lot of people love refractors, and I do as well, however the image scale is too small in a smaller scope. This is why the larger scope always shows the most detail.


The image scale in a high quality apo can be made reasonably large. A high quality 2x Barlow would increase the FL of my AP155 f/7 to 2,170 mm and with a 4 or 6 mm Zeiss Abbe Ortho (I have three sets) provide about 450x and 360x, which is greater magnification than a C-11 can usually tolerate where I live. The 2x Barlow increases the FL of my AP180 f/9 to 3,240 mm.

#148 John Kasianowicz

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Posted 13 December 2011 - 05:08 PM

Happy Birthday Ken.

I'm in complete agreement with you, Rod, and WHDuke on this statement. My C9.25 gave me better planetary views than any 6" APO I've looked through. No refractor has ever shown me the details that my old C9.25 showed me when cooled down and seeing allowed. And that brings up cooldown, which is the other big advantage of refractors over SCT's. It's why I like my 102ED so much, it affords me many more viewing sessions a cat or reflector would not allow. My experiences lead me to see a 6" APO equivalent to an 8" SCT wrt resolution. Contrast will likely be better on the 6" APO vs the C8. And I've never seen any 6-8" APO give the level of detail that Damien Peach is getting with his SCT. There's one more user from Aussie who posts SCT pix here that compete with HST images, but I can't remember his name off-hand.

The other thing I keep seeing in these discussions, and was brought up in the thread on the refractor page is that we're comparing mass produced scopes to hand-built premiums here. We're not only comparing apples and oranges, it's like we're comparing a Ford Focus to a Rolls Royce; it's a pointless comparison IMO.



FWIW, I've never seen a C9.25 outperform an AP or Zeiss APO in the 6" class on planetary or Lunar views. In my book, that never meant a C9.25 was useless.

I don't think the comparison is pointless. Different folks have different observing "needs", and discussions like this can help others decide which instrument better serves their purposes.

Peach uses a CCD camera to image. It's transfer function is quite different than that of the human eye.

#149 John Kasianowicz

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Posted 13 December 2011 - 05:11 PM

The bottom line is that you gonna have to go to a star party and check for yourself.


:like:

I couldn't agree more with that statement, I've learned a lot about my own preconceptions of what I thought vs what is real from doing just that.


Of course, if you go to the WSP and the seeing is perfect there, but at home it isn't, your conclusions drawn there may not apply where you live.

#150 Paul G

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Posted 13 December 2011 - 05:16 PM

Hi, Bob.

Addressing a couple of your points:

If the seeing is 1 arc second then both telescopes are limited to that level of detail - the performance of each scope will be identical. The Apo can't outperform the SCT under those conditions. Both scopes are limited equally by the 1" seeing.


Hi Dave,

Respectfully-

The performance will NOT be identical because the Apo is of much higher optical quality, has no central obstruction, and has much less light scatter because of its refractive optics. As we AGREE that both scopes will be limited EQUALLY by the seeing, then the scope with the higher quality optics will prevail as long as the smaller scope has enough aperture to only be limited by the seeing. This is exactly how a smaller but higher quality scope can outperform a larger scope of lesser optical quality especially when object brightness is not an issue – like the moon and planets.

As far as seeing goes. On a night of 1 arc second seeing most of the night is not close to 1 arc second - only the brief seemingly sharp moments are – the mush you see when the object is not sharp is not close to 1 arc second let alone .5 arc seconds.

This quote from previous poster jgraham hits the nail on the head:

I knew Dick Wessling for many years and as a master optician and planetary observer he sought for decades to build The Perfect Planetary Telescope. He went to great lengths (literally) with monster long Newtonians with tiny secondaries and sophisticated unobstructed designs. His conclusion? Visually, seeing rarely supports anything much over 8". Unless you live in an area blessed with exceptional seeing you'll get dimenisioning returns once you get much above 8", the largest benefit being image brightness rather than increased resolution (RESOLUTION BEING SEEING LIMITED).

Bob


Are we sure both scopes are equally limited by the seeing? My understanding, such as it is :p, is that the two scopes are affected differently. Now I'm sure someone will straighten me out if I have any major failures in logic, but the following is how I understand the effects of seeing in different size apertures.

Starlight, which is an incoming plane wave, has two errors introduced by the atmosphere, phase and amplitude errors. Amplitude errors are simply absorption, like that introduced by a fine layer of cirrus. Phase errors are what we refer to as seeing. Phase errors redirect the direction of propogation of the light. The phase error between two points increases slowly as the distance between two points increases. Above a certain size the phase error becomes large enough that the the optic is no longer diffraction limited, but is seeing limited. Different sources list this limit anywhere from 8 cm to about 15 cm; this is where the idea of "seeing cells" of 4-6 inches came from. So under this limit the errors will be similar but not equal, so small apertures will be less subject to blurred diffraction patterns since the spatial variation of the wavefront phase is small over the aperture. Bulk motion will still occur in the smaller aperture because the wavefront phase over the aperture is time dependent due to the seeing. In a larger aperture the phase errors are decorrelated and completely independent, so blurring occurs in addition to the bulk motion. Phase errors are also highly dependent on wavelength. Even huge observatory scopes are diffraction limited at 10 microns.

So in the smaller refractor one has a sharp point of light that moves arond but does not blur much. The first and subsequent diffraction rings are so faint that they are not visible except in very good seeing. In the SCT, a 34-35% central obstruction reduces the Strehl from 100% to 82% simply by adding its own diffraction pattern and blocking the central cone of light. Add 1/4 wave spherical aberration and the Strehl drops to 64% compared to the refractor at 95%+. There are subtle radial zones from "plate through" of the radial holes in the vacuum plate that holds the corrector during polishing. There is also some spherochromatism. In the latest coma-corrected aplanic SCT's the elimination of coma over a relatively large field comes at the cost of doubling the spherochromatism. If one adds all of these errors together including astigmatism, zones, and roughness the scope may be a 1/2 wave instrument but the effect of seeing on the larger aperture mask this from view. The airy disc's brightness is greatly reduced, and the brightness of the first, second, and third diffraction rings combined can be greater than that of the airy disc itself. When the larger scope experiences blurring, these all merge. Under my skies I almost never was able to see an airy disc and first diffraction ring in my C11, just fuzz balls. In areas with superb seeing there is little blurring, so at some point in disparity of aperture the airy disc and the first and second diffraction rings in the SCT will be the same size as the airy disc in the refractor and the SCT will strut its stuff and show its aperture dependent resolution advantage.

Flame suit on! :tonofbricks:


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