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8" SCT versus 5"APO

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#1 azure1961p

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Posted 11 October 2014 - 08:55 PM

This is a variation on a the SCT Vs. Refractor thread that I thought I'd isolate to address this topic specific... 

 

Say you have an acclimated perfectly collimated 8" Schmidt Cassegrain and next to it a 5" premium apochromatic Refractor also acclimated and set to go. 

 

In good seeing is a 5 arc second Mars.  It's 5" in this example to critically compare these two instruments on the two competing criteria:

 

Angular resolution versus Contrast Resolution... 

 

On this 5" disc the subject of study is an elliptical polar cap at  .57 arc seconds in width and about half that in thickness. 

 

Here's the deal: 

 

The 8" SCT will half substantially higher angular resolution to define this ellipse over any 5" aperture Refractor so the polar cap should appear more elliptical in the 8" and more sphericaly spot-like in the 5".  

 

Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary. 

 

That said then,  in this confined example can it be said the higher  angular resolution of the 8" SCT is actually being matched or trumped by the linear contrast resolution of the 5"?  If that's the case then isn't the 5" apo really producing a pseudo angular resolution by virtue of the fact it's contrast throughput is so much higher per inch of aperture than the SCT? 

 

I have never understood in these comparos how small aperture contrast trumps large aperture angular resolution as the physics doesn't bend here. 

 

Isn't the Refractor honestly showing a high contrast image that's so good observers are ignoring the fact the details aren't as small or refined as the larger aperture but it affords an overall more contrasty "feel"? 

 

How could a 5" astrophysics EVER  define a half second wide elliptical polar cap as well as a good 8" SCT? 

 

Pete



#2 jgraham

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Posted 11 October 2014 - 09:10 PM

To be honest, I suspect that the only way to answer this question is with a side by side comparison. I own several achromats up to 6", MCTs to 5", and SCTs to 10" and I don't believe that a good 5" is going to clearly beat a good 8", regardless of the scope types. My 5" and 6" scopes hold their own surprisingly well, but an optically good, aligned, and conditioned 8" still edges them out. An Apo would lift the bar a bit, but at the end of the day a 5" is still a 5". An exceptional 5" may beat a mediocre 8", but not a good 8".



#3 Eddgie

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Posted 11 October 2014 - 11:12 PM

Angular resolution and modulation are somewhat related, but modulation transfer function (MTF) is a far better way to compare how two instruments will deliver contrast.

 

The damage of the obstruction in the 8" SCT will be done to details that are somewhat near the limit of the 5" apertures ability to resolve angularly.

 

And past this limit (the angular resolving power of the 5"), the 8" instrument is still able to deliver contrast on details that are beyond the angular resolution that the 5" is capable of.

 

Another words, if they are made to the same quality level, the 5" APO will at no point be able to either provide higher angular resolution, or better contrast transfer for any size detail.

 

Here is an MTF chart for a 5" perfect aperture vs the 8" 33% obstructed aperture.  The red line is a perfect 8", the dashed red is the same aperture with a 33% obstruction, and the green line is a perfect 5" aperture, and does not deduct anything for the polychromatic Strehl error that is almost always present in all but the most expensive and exotic APO triplets, and which is often much worse in many scopes sold as "APOs".

 

Notice that at no point does the obstructed aperture show more contrast loss than the 5" aperture, and well past the angular resolving limit of the 5" aperture (which is only 62.5% of the 8" aperture, the 8" aperture is showing detail that the 5" aperture cannot show at all.

 

And this is an important point.  People treat refractors as if they are always "Perfect" but it is very rare to have a refractor with perfect polychromatic Strehl.  This chart above assumes that there is zero chromatic aberration, and this condition is almost never present on most refractors people buy.   Most refractors have a polychromatic Strehl that is lower than .95, and many are less than .9.    Achomats often hover around .6.   Even the TEC 140 only has a ploystrehl of .92 I believe.   And yet this is completely ignored when people talk about refractors vs obstructed scopes.  With a .92 Polychromatic Strehl though, a TEC 140 would barely match the contrast transfer of a truly perfect 5" aperture.

 

But the refractor crowd totally ignores polychromatic Strehl, but will quickly point out the evils of an obstruction.  It is to me a very head in the sand kind of comparison.  If you are going to fault the obstruction for lowering contrast (which it does), to be fair, you have to also acknowledge that chromatic aberration/spherochromatism also lowers contrast.

 

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#4 Asbytec

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Posted 12 October 2014 - 01:30 AM

The thing to remember about the right side of the MTF shown above is the fall off of the obstructed 8" contrast at about 0.5 spacial frequency is primarily due to the first bright ring. Twice the Raleigh limit, or about 1.35" arc for the 8", is just above 0.4 spacial frequency. To the right of 0.5 spacial frequency is the realm of high resolution and the Airy and spurious discs, Raleigh (~0.82 spacial frequency) and Dawes (closer to 0.9 spacial frequency) respectively. To the left of that is the realm of bright low contrast planetary detail. Further out to the left edge of the chart, near 5x and 10x the Raleigh limit left of 0.15 spacial frequency, is the realm of the DSO where the diffraction pattern does not matter as it did on the right side of the MTF. Here, aperture and throughput matter.

 

The rise of the obstructed contrast graph above that of the solid red line (perfect unobstructed 8") is due to the slight gain in higher resolution due to the added diffraction caused by the obstruction. The two red lines meet at 1 spacial frequency because they are normalized to the unobstructed aperture. In fact, the obstructed line not normalized to the maximum spacial frequency should fall to zero contrast a bit beyond 1, say out to about 1.1. This is the high resolution realm of bright high contrast detail, such as Dawes and Lambda/D for equal double stars and possibly extended object resolution, where larger aperture and even obstructed apertures rule.

 

To the left of about 0.5 spacial frequency begins the realm of bright low contrast planetary detail. Here, at least briefly, the obstructed scope takes more of a hit largely due to the brighter first ring or two while the unobstructed scope performs as is should with it's dimmer rings. Both are affected by the first bright ring, but the obstructed scope is hit harder causing contrast transfer to fall off somewhat dramatically. This is the short spacial realm where unobstructed apertures rule - near the first ring or so of the obstructed one. Beyond that, moving left from 0.4 spacial frequency (2x Raleigh for the larger aperture) both scope's contrast transfer begins to normalize quickly. By about 3x Raleigh for the larger aperture, or about 2" arc for the 8" scope, both scopes are pretty much giving the same level of contrast transfer. The 5" APO is never better than the 8" scope at any point, especially above 0.62 spacial frequency of the 8" which is maximum spacial frequency for the 5".

 

Bottom line is, keep in mind what scales we're talking about. Aperture rules resolution on the right side of the MTF. Obstructed aperture can add a little resolution (the rise of the chart above perfect aperture) in the scale of the Airy and spurious discs - scales so small they require good seeing to appreciate fully. Unobstructed apertures do not take as much of a hit in terms of contrast transfer (bright low contrast planetary detail) near the brighter first ring of the obstructed larger aperture (near 0.5 spacial frequency...this is the first ring of the obstructed larger scope.) Here is where unobstructed scopes rule. Not far beyond that first ring advantage, getting beyond the realm of the Airy disc and first ring of the larger aperture, both obstructed 8" and unobstructed 5" scopes quickly normalize contrast transfer. Then it becomes a matter of throughput and glare control. The obstructed 8" scope still lags the larger unobstructed 8" scope, but they are pretty much normalized by about 10x Raleigh at about 7" arc at near 0.1 spacial frequency. For scale, that's a very small angular size for Mars.


Edited by Asbytec, 12 October 2014 - 01:53 AM.


#5 eklf

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Posted 12 October 2014 - 08:09 AM

Thanks Edggie and Asbytec for the very detailed explanations!



#6 azure1961p

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Posted 12 October 2014 - 08:03 PM

Thanks for the clarifications. 

 

Edd,  you are right in that the effects of spherochromatism and CA seemingly not getting mention in discussions particularly in these kinds of comparisons.  In fact the only time it gets mention it would seem is when you raise that point! 

 

In looking at the graph now and taking all into consideration particularly the way Norme maps out where given real world contrast frequencies are displayed in frequency,  ie; planetary contrasts versus stellar and such I'm wondering where the 5" Mars polar cap of. 57 arc seconds would rest here.  The reason I raised this example in particular is its minute size would be unforgiving of an aperture design that's insufficient in angular resolution despite the high contrast throughput.

 

Could I be right in guessing the polar cap would be  at  0.55 in the center where the SCT pulls away from the 5" graph line? 

 

It would seem both scopes fare closely on broad festoon and belts but at the 0.5 mark on the bottom where the SCT begins to excel this is exactly the kind of contrast the 0.57 sec polar cap would present.  Or am I making too literal and interpretation? 

 

 

Pete


Edited by azure1961p, 12 October 2014 - 08:07 PM.


#7 Asbytec

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Posted 13 October 2014 - 04:35 AM

Pete, 0.55" arc is basically Raleigh for a 10" aperture. That would be located about 0.82 spacial frequency well beyond the 'hump' formed by the perfect obstructed aperture. I suppose 0.55 is near Dawes for an 8" aperture (the red lines), so the spacial frequency would be 0.9 or so. Both are well above where the 5" (green line) craps out.

 

Now, I am sure a 5" can easily observe the polar cap that small just as it could a star, but it will be more of a point source at about half the Raleigh limit for that aperture. Being at half the Raleigh limit, one might imagine the polar cap might exhibit some elongation in the 5" not unlike observing a tight double at half the Raleigh limit (you and I have done that.) But, the greater resolution of the 8 and 10" apertures have more resolution space to work with being the polar cap in withing Raleigh and Dawes, respectively.

 

For the 5" aperture, 0.55" arc is well below the sparrow limit of 107/127mm = 0.84" arc. So, at that very high spacial frequency there is not enough contrast to see much. Contrast fell to zero in the 5" at about 0.62 spacial frequency which correlates to the maximum spacial frequency of the 5" scope which (correct me if I am wrong) would be near the Sparrow limit of 0.84" arc. This is what the green line falling to zero is telling us....the aperture cannot put any contrast into the image for something that small - that high a spacial frequency.

 

Now, on an object that small, but not quite a point source (at 1/4th Raleigh), it may be possible to observe some brighter, high contrast albedo not unlike observing Osiris on Ganymede (which isn't really resolved, technically, as much as it is just seen.) But, such features that small will just not have the contrast of a Dawes split at around 5% contrast (because contrast transferred to the image is zero) for actual resolution of two separate points within the tiny polar cap. The 8 and 10", however, being the polar cap is at Raleigh and Dawes, respectively, should be able to separate two distinct points on a polar cap 0.55" arc across. The 5" cannot, contrast fell to zero closer to 0.84" arc.

 

That is what the 8" red lines are telling us as they fall to zero well above the 5" green line. Notice at Raleigh and Dawes for the 8", way out at 0.82 and about 0.9 spacial frequency, contrast (left hand scale) is not at zero. Contrast transferred to the focal plane does not get to zero until the red lines hit the maximum spacial frequency at 1. And actually, the obstructed scope's contrast falls to zero out to about 1.1 because in the graph above that "hump" formed by the obstruction is made to - it's normalized - to 1 for the unobstructed aperture 8" aperture. That hump actually extends out to about 1.1, give or take. Still, with low actual object contrast on a bright polar cap, it's possible neither the 8" or 10" could resolve two separate bright points either even though they are separated by enough. But, they stand a better chance of it.

 

That's kind of my take on it.

 

One caveat, though. It's always dangerous to talk about extended object resolution in terms of Raleigh and Dawes because both apply to point sources separated by some angular dimension with black space between them. They are two bright stars sitting in black object space with 100% contrast between them. As the aperture forms an image of these two stars very close together, the image contrast falls off as shown by the line. At Dawes (o.55" arc), or about 0.9 spacial frequency (for the 8" scope - red line) the image looses about 90% of the image's 100% actual contrast. In the case of double stars, there remains enough contrast to see a dark space between two points.

 

In the case of two bright spots on on a very small bright polar cap, the initial contrast is low to begin with. After being imaged and loosing 90% of it's already low contrast, the actual contrast transferred to the image might be too low to be perceived. But, imagine if the polar cap were black, those two points could be resolved in the 8" (Dawes), more so in the 10" (Raleigh), but not at all in the 5" (well below sparrow)...all with a polar cap of 0.55" arc.


Edited by Asbytec, 13 October 2014 - 05:13 AM.


#8 BillP

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Posted 13 October 2014 - 09:55 AM

This is a variation on a the SCT Vs. Refractor thread that I thought I'd isolate to address this topic specific... 

 

Say you have an acclimated perfectly collimated 8" Schmidt Cassegrain and next to it a 5" premium apochromatic Refractor also acclimated and set to go. 

 

...

 

Here's the deal: 

 

The 8" SCT will half substantially higher angular resolution to define this ellipse over any 5" aperture Refractor so the polar cap should appear more elliptical in the 8" and more sphericaly spot-like in the 5".  

 

Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary. 

 

This is an interesting question, but the premise vs the question is apples-oranges and cannot be solved with this approach.  You are asking about what seasoned observers claim to see in the field, but then your model to predict an outcome makes assumptions which will never occur in the field (i.e., perfect collimation, perfect acclimation, the word premium so I am presuming some prescribed notion of optical precision. 

 

If you want to try and understand why seasoned or any observers are seeing what they are describing relative to the two designs, 8SCT & 5APO, being a dead heat for planetary, then you necessarily must model the entire telescopic system and real world conditions.  That would be the only way to determine why what is being seen is, or can be true.  So you need to model:

- Aperture

- Obstruction

- Diffraction

- Wavefront behavior differences due to optical design

- Scatter profiles (very different for mirrored vs refractive glasses)

- Thermal peculiarities of each design since radically different behaviors and impacts

- Light baffling effectiveness (the two designs are not equal in their ability to baffle stray light, especially how typically implemented)

- Collimation sensitivity at differing points from "perfect"

- Environmental considerations (i.e., the heat source that is the body of the observer is in close proximity to the entrance pupil with the SCT)

- And probably a lot more I am missing.

 

Anyway, I think what you are doing is a fun and intellectually stimulating exercise for a theoretical situation (which does not exist in reality since it is only an aperture based exercise with simple CO modelling).  But whatever the conclusion may turn our to be, it cannot be predictive as to why "Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary".

 

Also, the seasoned observer comments need to be a little more detailed.  What does "dead heat" mean and how was that conclusion arrived at?  What I mean is that whether a dot-like polar cap is round or elongated, if both renditions show the same level of edge details (or lack thereof), I would wager that this would really go unnoticed by the seasoned observers so they would judge those two conditions as a dead-heat equal mistakenly.  I think a fun exercise would be to continue out this theoretical model and make a hypothesis based on your model's prediction, then go in the field and see if the equipment tracks with the model.  I like this elongated in an 8sct vs dot in a 5apo hypothesis.  Would be most interesting to see if it actually pans out or not in a real field test.  Heck, you could probably model the real world condition with an appropriate picture of Mars placed far enough down field so you can eliminate much of the seeing component.


Edited by BillP, 13 October 2014 - 10:05 AM.


#9 Jon Isaacs

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Posted 13 October 2014 - 11:53 AM

Just to add to what Bill wrote:

 

When making a comparison based on theory, it's important to look at the underlying assumptions, optical quality, thermal equilibrium, are big but as Bill points out, there are many issues to be considered. The theory can guide one and for some situations is quite accurate, predicting double star splits for example.  And when the differences are significant, say a 5 inch apo and a 12.5 inch thermally equalibrated Newtonian with a high quality mirror, a small CO and excellent seeing.. then the outcome is clear. But when it's close, then small assumptions can play major roles. 

 

At some point, you just has to give credence to what your eye tells you.. 

 

"In theory, there is no difference between theory and practice.  In practice, there is."

 

-Various

 

Jon



#10 Asbytec

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Posted 13 October 2014 - 07:31 PM

The theory can guide one and for some situations is quite accurate, predicting double star splits for example.

 

 

Jon, are you aware of any (other) theory for extended object resolution and contrast transfer we can rely on as a guide just as their appears to be for double stars? Of course conditions will change between observers and even between observers themselves. To get an idea of what we might observe when comparing what two different instruments might show we simply cannot account for all the variables with any success. Otherwise our comparisons will be all over the map and we cannot talk about the different designs at all. It's meaningless to do so.

 

We pretty much have to set all other variables, other than the ones we are considering, to being roughly equal. We have to assume lab-like conditions, average human acuity, and perfect apertures for each and go from there. If we set widely varying environmental variables differently for each, the results are meaningless. We have to assume they are side by side under the same conditions and those conditions are nearly ideal. It's true, that in the real world performance will vary, possibly to a large extent. If we wanted to, we could model some of those variables in the MTF above. We could add 2" seeing, for example. We could toss in a little aberration and some tube currents, too, and see what the graph tells us. We do not know if either observer has 2" arc seeing and the other nearly perfect seeing.

 

I don't think it's wrong to speak only to aperture and obstruction, things we know exist, as long as we realize we're talking about ideal conditions the observer may or may not have, the unknown optical quality of each, or how acute the observer may or may not be. Start with a baseline performance, offered above, then modify it from there to suit. Of course a 60mm refractor will outperform the Keck when the Keck is theoretically placed at the bottom of the Pacific ocean, if those are the Keck's observing conditions. How meaningless is that? 



#11 Bomber Bob

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Posted 13 October 2014 - 08:52 PM

Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary.

 

I wouldn't make that claim.  But after side-by-sides at numerous star parties when I took my D&G 5" f/10, there was often a consensus that it provided more pleasing views on Mars, Jupiter, and Saturn at 300x - 400x than 8" reflectors & SCTs at the same mag or mag per inch.  Purely subjective, and anecdotal, but real world.  I think it is safe to say that a quality refractor will out-perform a similar aperture quality scope of any other design for lunar, planetary, and multiple-star observing -- it's been that way for as long as I've been in the hobby.  During the recent Mars season, I observed & sketched details with my 1962 Royal 76x1200 that were later confirmed by digital images from much larger MCTs & SCTs -- not too shabby!

 

At some point, you just has to give credence to what your eye tells you..

 

Indeed.  But if I thought refractors were the only solution, I wouldn't be putting money & time into restoring a Tinsley Saturn 6" Cassegrain.  Can't wait to see Jupiter through a 3000mm FL reflector, using my 1980s Meade RG Orthos, with an OTA half the length of my long-gone D&G.  Great expectations.



#12 Daniel Mounsey

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Posted 13 October 2014 - 09:26 PM

 

This is a variation on a the SCT Vs. Refractor thread that I thought I'd isolate to address this topic specific... 

 

Say you have an acclimated perfectly collimated 8" Schmidt Cassegrain and next to it a 5" premium apochromatic Refractor also acclimated and set to go. 

 

...

 

Here's the deal: 

 

The 8" SCT will half substantially higher angular resolution to define this ellipse over any 5" aperture Refractor so the polar cap should appear more elliptical in the 8" and more sphericaly spot-like in the 5".  

 

Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary. 

 

This is an interesting question, but the premise vs the question is apples-oranges and cannot be solved with this approach.  You are asking about what seasoned observers claim to see in the field, but then your model to predict an outcome makes assumptions which will never occur in the field (i.e., perfect collimation, perfect acclimation, the word premium so I am presuming some prescribed notion of optical precision. 

 

If you want to try and understand why seasoned or any observers are seeing what they are describing relative to the two designs, 8SCT & 5APO, being a dead heat for planetary, then you necessarily must model the entire telescopic system and real world conditions.  That would be the only way to determine why what is being seen is, or can be true.  So you need to model:

- Aperture

- Obstruction

- Diffraction

- Wavefront behavior differences due to optical design

- Scatter profiles (very different for mirrored vs refractive glasses)

- Thermal peculiarities of each design since radically different behaviors and impacts

- Light baffling effectiveness (the two designs are not equal in their ability to baffle stray light, especially how typically implemented)

- Collimation sensitivity at differing points from "perfect"

- Environmental considerations (i.e., the heat source that is the body of the observer is in close proximity to the entrance pupil with the SCT)

- And probably a lot more I am missing.

 

Anyway, I think what you are doing is a fun and intellectually stimulating exercise for a theoretical situation (which does not exist in reality since it is only an aperture based exercise with simple CO modelling).  But whatever the conclusion may turn our to be, it cannot be predictive as to why "Seasoned observers often claim these two apertures and respective designs to be in a dead heat on planetary".

 

Also, the seasoned observer comments need to be a little more detailed.  What does "dead heat" mean and how was that conclusion arrived at?  What I mean is that whether a dot-like polar cap is round or elongated, if both renditions show the same level of edge details (or lack thereof), I would wager that this would really go unnoticed by the seasoned observers so they would judge those two conditions as a dead-heat equal mistakenly.  I think a fun exercise would be to continue out this theoretical model and make a hypothesis based on your model's prediction, then go in the field and see if the equipment tracks with the model.  I like this elongated in an 8sct vs dot in a 5apo hypothesis.  Would be most interesting to see if it actually pans out or not in a real field test.  Heck, you could probably model the real world condition with an appropriate picture of Mars placed far enough down field so you can eliminate much of the seeing component.

 

 

 

I think Bill hit the nail on the head here. A good way to do this would be to see how the theoretical models compare to real world observations. Everyone expressed valid points, but the key IMO, would be to set them up side by side and see what happens. 



#13 areyoukiddingme

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Posted 13 October 2014 - 10:41 PM

Another saying: Don't let data get in the way of a good theory!  :lol:



#14 sqrlman

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Posted 13 October 2014 - 11:03 PM

 

 

I think Bill hit the nail on the head here. A good way to do this would be to see how the theoretical models compare to real world observations. Everyone expressed valid points, but the key IMO, would be to set them up side by side and see what happens. 

 

 

   I have done the side by side. 130 EDF with perfect optics vs. C8 with very good optics. Target Jupiter. Not even a horse race! C8 wins. Any day, any time, under any conditions.

 

Steve



#15 Asbytec

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Posted 14 October 2014 - 06:22 AM

"Not even a horse race!"

 

This thread will developing into one, at which point I bail from the mighty un-winnable argument. Except to say, if you do a side by side, do so in seeing conditions that offer the 8" a fighting chance to offer a nicely formed spurious disc, cool the thing to ambient, collimate it, and see what happens on a variety of targets, including bright low contrast, DSO, and high resolution. 


Edited by Asbytec, 14 October 2014 - 06:24 AM.


#16 azure1961p

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Posted 14 October 2014 - 08:05 PM

My query here is how?  How does an aperture with a whopping angular res discrepancy match or compete with the 8"?  I'm wondering seriously,  if when a small ago matches or beats  a larger SCT if it's merely aesthetics. Maybe the general non threshold details look so overwhelmingly alike or even better in the smaller refractor that the true threshold achievement over the 5" versus the  is overlooked in all the dazzle?  Is it possible ganymede or following the extent of cassini or some other narrow corridor test  is the truer discerning measure of A versus B? 

 

Bill I agree with all your points,  Norme and Daniel et al as well. 

 

Pete


Edited by azure1961p, 14 October 2014 - 08:06 PM.


#17 Bomber Bob

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Posted 14 October 2014 - 08:23 PM

I bail from the mighty un-winnable argument.

 

I think we debate because observing is personal & subjective -- like Olympic judging, or wine-tasting, or any number of comparisons that boil down to individual perceptions of some very slight differences.  The computer simulation could be validated IF the computer could do the observing.  But get a group of us "carbon-based units" looking through the same scopes, on the same night, at the same place, at the same targets... and there's no telling.  Throw in our emotional attachments to particular scope designs, and it would look like the Dawn Of Man / Monolith scene from 2001: A Space Odyssey.



#18 Asbytec

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Posted 14 October 2014 - 08:26 PM

Pete, the brighter first ring of an obstructed 8"cope will have a radius of about 1.6 Lambda/D or about 181/203 ~ 0.9" arc. That whole pattern will fit nicely inside the Airy disc, with about 1" arc radius, of a 5" refractor. You can see this on the MTF above where the fall off in 8" obstructed contrast is just about where the 5" falls to zero. This means, by the time we reach the edge of a 5" Airy disc, the major damage done to the 8" contrast transfer is already done and the rings are getting dimmer improving contrast beyond that.

 

The 5" will still have it's dimmer first ring outside that radius still taking a hit on contrast while the 8" is gaining contrast near the 5" scopes first ring at 1.4" arc. Even if seeing is somewhat disturbed, but not terrible, the entire boiling blob of the 8" image still is about the same size of the 5" Airy disc. If seeing improves, the 5" cannot give any more that it already has, which is about what the 8" is giving in lesser seeing. The 8" can improve with better seeing.

 

But, this is performance. Not saying the 5" is not more pleasing with other advantages such as the ability to cool and less scatter and relatively better throughput. Smaller apertures can be more pleasing. I have one and it's pleasing when cooled in good seeing and performs to it's aperture. But, no matter how hard it tries, it cannot resolve with an 8" or deliver the better contrast transfer. 



#19 Jon Isaacs

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Posted 15 October 2014 - 07:55 AM

 

But, this is performance. Not saying the 5" is not more pleasing with other advantages such as the ability to cool and less scatter and relatively better throughput. Smaller apertures can be more pleasing. I have one and it's pleasing when cooled in good seeing and performs to it's aperture. But, no matter how hard it tries, it cannot resolve with an 8" or deliver the better contrast transfer.

 

Again, what are the assumptions here?   Equivalent optical quality is certainly a major underlying assumption.  The thermal stability and scatter are important factors/assumptions as well.  So, there are certainly situations, objects where an essentially perfect 5 inch scope can resolve challenging double stars that a larger, less perfect scope cannot situations and/or objects where it can deliver better contrast transfer even when the seeing is perfect. The real world MTF equation is not as simple as just looking at the diffraction effects and concluding that an 8 inch scope with a relatively large central obstruction still has better resolution and contrast transfer than a high quality 5 inch unobstructed scope.

 

Jon 



#20 Steve D.

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Posted 15 October 2014 - 07:56 AM

My query here is how?  How does an aperture with a whopping angular res discrepancy match or compete with the 8"?  I'm wondering seriously,  if when a small ago matches or beats  a larger SCT if it's merely aesthetics. Maybe the general non threshold details look so overwhelmingly alike or even better in the smaller refractor that the true threshold achievement over the 5" versus the  is overlooked in all the dazzle?  Is it possible ganymede or following the extent of cassini or some other narrow corridor test  is the truer discerning measure of A versus B? 

 

Bill I agree with all your points,  Norme and Daniel et al as well. 

 

Pete

 

I've been wondering about this as well and I think the atmosphere just limits the larger scope much of the time.   Also, point source resolution is a factor of aperture while resolution of extended objects is related to the clear aperture (aperture minus obstruction.)  In this example, clear aperture of the two scopes is very similar.  My personal opinion is that the larger scope in this case just doesn't have the opportunity to show off that extra bit of performance as often as we would like.   Here's a website that explains this topic better than I can: http://www.handprint...RO/seeing1.html



#21 Daniel Mounsey

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Posted 15 October 2014 - 08:15 AM

 

 

 

I think Bill hit the nail on the head here. A good way to do this would be to see how the theoretical models compare to real world observations. Everyone expressed valid points, but the key IMO, would be to set them up side by side and see what happens. 

 

 

   I have done the side by side. 130 EDF with perfect optics vs. C8 with very good optics. Target Jupiter. Not even a horse race! C8 wins. Any day, any time, under any conditions.

 

Steve

 

 

Steve, It may be helpful to share a little bit of information about your two 8" SCT's with the forum. BTW I'll contact you today.



#22 Asbytec

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Posted 15 October 2014 - 08:15 AM

Again, what are the assumptions here?

 

 

Hi Jon, the assumptions have to be held somewhat constant, even though they may not be the same every where all the time, otherwise what we talk about will be all over the map. Those assumptions have to be reasonably good optics, good seeing, ambient cooling, and very good collimation in both samples. Let glare control and scatter lay where they may, and talk about things we can measure and speak to like resolution and contrast transfer. No doubt there are conditions where a 5" might do some heavier lifting than the 8", and if we want to talk about the 5" having an advantage we can assume those are "real world" conditions and make our case. And by implication this could true in all cases.

 

At some point, the 5" is giving everything it can possibly give. If the conditions improve,  the 8" will excel. If we talk about the 8" in terms of favorable 5" conditions, the 5" is advantaged. If we talk about the 5" in terms of 8" performance the smaller scope is not advantaged. Both scopes need a reasonable chance to perform well to make a comparison we can talk about that might be more universal. That universal standard might be the ideal performance in theory (and often is), but it should be at least conditions favorable to both, not one or the other. This means we do not assume tube currents, sloppy collimation, or speckling seeing in the larger aperture and not in the smaller. Both must be free of those assumptions because, to be fair, they can be.

 

Anyway...if you have too many variables it's difficult to even make comparisons usually because such discussions are really discussions of thermal equilibrium rather than aperture and obstruction. Or they might be discussions of seeing and collimation, and not of the topic at hand. This is one reason I am beginning to avoid these threads more, we like to assume very different baseline conditions making the argument unending. Even side by side on one night you might have one result, side by side another night with a different result. Take away that variability and speak to what we know can be true on any given night.


Edited by Asbytec, 15 October 2014 - 08:24 AM.


#23 Jon Isaacs

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Posted 15 October 2014 - 12:23 PM

 

Hi Jon, the assumptions have to be held somewhat constant, even though they may not be the same every where all the time, otherwise what we talk about will be all over the map. Those assumptions have to be reasonably good optics, good seeing, ambient cooling, and very good collimation in both samples.

 

Norme:

 

This is how I see it:

 

By trade and training, I am an experimentalist who has worked for many years alongside those who do analysis and modeling. Inherently, I am a skeptic, something my coworkers know well and respect,  they don't get much by me.

 

- It is true that if one does not make certain assumptions that what one talks about will be "all over the map", theoreticians always have to simply problems and make assumptions.  How accurately a theoretical model predicts the real world depends on the validity of those assumptions as well as the accuracy of the modelling itself.  So while assumptions maybe necessary and convenient, their validity must be carefully scrutinized and their importance recognized.  

 

- The real world, this forum for example, does seem to suggest that what we talk about is "all over the map", people have different experiences when comparing scopes.  

 

- In comparison to a high quality 5 inch refractor, the assumption that an commercially made 8 inch SCT will have optics of similar quality is questionable, refractor optics are more expensive to fabricate but the quality can be more consistent. The assumption that the 8 inch SCT will be thermally stable and free from thermal gremlins is also questionable. Other assumptions that are relevant: the importance of scattering, baffling, reflective losses etc...

 

Realize.. I am not the guy with the 5 inch apochromat, I am the guy who thinks 10 inches is barely enough to get the good planetary views and that 12.5 inches or 16 inches are a better place to be.. 

 

Jon

 

 

 

 



#24 Darren Drake

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Posted 15 October 2014 - 01:38 PM

I plan soon plan to do a detailed comparison of one of my very good C8 otas to a 120 ed refractor that I'm borrowing long term.  I will plan to compare lunar and challenging double stars as well as maybe Jupiter.   I may report back here at some point.  I suspect the C8 will dominate but a real world comparison will be interesting.  


Edited by Darren Drake, 15 October 2014 - 01:53 PM.


#25 Lola Bruce

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Posted 15 October 2014 - 03:03 PM

I have to admit some of the great technical information made my eyes glaze over. My back yard is in a white zone so bright objects and planetary objects are my normal fare. Side buy side thermally equalized I compared  my CPC 800XLT and Explore Scientific Carbon 127 Triplet  From my back yard on multiple nights on planets the CPC won hands down every time. I for whatever reason could push the magnification harder for detail on the CAT every time. The refactor had darker backgrounds and snapped to focus but just could not hold up to being pushed.

Bruce




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