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CO vs. All That "Other Stuff"!

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#1 Mike Harvey

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Posted 06 January 2013 - 02:25 PM

I didn't want to 'hijack' (or even divert) the Meade 600 CO thread but, there are so many other factors that affect the performance of a given SCT that need to be considered, it seemed like a new thread was called for.

Hopefully, those who have read the CO posts now have a better understanding of how the obstruction affects the scope's overall performance.
In my experience, there are other factors which are much more important considerations.
*Let me qualify my comments by stating that, over the past 40 years, I've owned or extensively used more than 30 SCT's. These include a variety of Celestron, Meade, Criterion, B&L and Takahashi.
The optical quality of ALL of the Criterion and B&L models (basically the same scopes) was inferior in just about every aspect. The size of the CO would not have mattered!
More than half of the Celestrons and Meades would have been rated 'poor' to 'mediocre' (again, the other shortcomings would have been vastly more important than CO). *Note: these were earlier models. Most of the Celestrons and Meades I've seen in the past few years have been of much higher quality.
The Takahashi TSC-225 proved to be the "gold standard" until the Meade ACF design came along. I don't yet have enough experience with the Celestron Edge to compare them with the ACF.

If you have an older SCT and are dissatisfied with it's performance, you may simply have a scope that came off the assembly line with very little QC exercised. These scopes must exhibit good optical quality on multiple elements: corrector plate, primary mirror, secondary, and diagonal.
PLUS you've got a primary mirror that must remain perfectly aligned even though it is MOVING up and down in order to focus!
This last item is always going to be a potential problem. It is the only weakness I've discovered in the ACF. Fortunately, there is a solution...
LOCK the primary in place at the exact point it should be separated from the secondary, then install a rack & pinion focuser. Trust me, it makes a difference!
There's also the question of collimation. I am constantly amazed at the number of SCT users at star parties who have collimation "issues" and either don't know it and think the soft images are just the price to be paid for having an SCT, or who DO know the images are not what they could be and blame some other aspect of the scope. From what I've seen THIS is the #1 problem for SCT users.
Until the newer secondary-holder design of the Meade ACF, most SCT's were notorious for becoming decollimated when moved around a lot.
Job One, before each observing session, was to collimate the scope!
Today, I'm still impressed every time I use the 10" ACF to discover that the collimation has NOT shifted AT ALL. With Uncle Rod Mollise's help, I precisely collimated this scope when I first received it some three years ago and, despite it being set-up, transported, taken down and set-up again dozens of times...I HAVE ONLY HAD TO TOUCH THE COLLIMATION ONCE...and that was just a near-infinitessimal turn of one screw!

SO - now you've got your SCT precisely collimated, locked the primary in place and installed the R&P. What then happens all-too-often is that the owner will insert a cheap diagonal into the focuser and undo much of the improvement he's made with the earlier tweaks!
You can have a near-perfect corrector, primary and secondary and STILL not achieve great visual results because you've 'bounced' the image off a rough diagonal mirror or misaligned prism.
GET THE BEST QUALITY DIAGONAL YOU CAN AFFORD!
I won't even take this thread into eyepieces because, hopefully, anyone who's taken the earlier steps to improve quality already knows that you need decent eyepieces.

After completing the steps listed above, my 10" ACF displays more planetary detail than any refractor I've ever owned (and that includes a
6" Takahashi FC-150, 6" f/15 AP Triplet and an 8" Alvan Clark). I'm assuming a 10" Zeiss would outperform it but you'd spend at least 10X as much $$$ just for the OTA (not to mention the ginormously expensive and heavy mount and the permanent observatory you'd want to keep it in)!

Mike

#2 Rick Woods

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Posted 06 January 2013 - 03:32 PM

Nice post, Mike!
Not much we can do about the CO; but all the rest of it is in our hands.

#3 DesertRat

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Posted 06 January 2013 - 07:42 PM

There seems to be two well defined camps in the astro community. One thinks any obstruction is bad - and not just bad but unforgiveable. These tend to be refractor owners. The other community thinks it does'nt make any difference. These tend to be people with scopes that have an obstruction. Bless them both.

The truth is of course more complicated. Many amateurs have both kinds of scopes and/or a better understanding of the action of an obstruction. This is the third camp!

You can come back here years from now - and someone will be asking the same question about CO! And the same arguments and discussions will follow.

It sort of reminds me of the StarTrek episodes featuring the Pakleds from the Alpha Quadrant. You know the guys that say "We are smart." They don't want to hear any thoughtful positions, they just want to collect stuff.

There also seems a tendency for people to get diverted away from practical matters as Rick mentioned. Your note on collimation is right on and as far as diagonals go - there are many good ones out there - and they really don't cost that much. A good one will not effect performance in any perceived manner.

Glenn

#4 doug mc

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Posted 06 January 2013 - 08:01 PM

Ive only owned my c6sct for a few months now, and apart from the effects of a larger co., which are noticable when viewing bright double stars,not a problem so far, it clearly outperforms the acro refractors i had. 4,5,and 6inch on lunar and planetary detail. They were all synta scopes, so is the Celestron. My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type. The new Celestron cats would have to the best value for money scope going. Easy to mount (you can have a much bigger cat on that mount than other type of scope) . You have had some extra fine glass and are amazed by a scope thats much cheaper than many you mentioned. Nudge nudge wink wink.

#5 Loren Toole

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Posted 06 January 2013 - 09:21 PM

Fortunately, there is a solution...
LOCK the primary in place at the exact point it should be separated from the secondary, then install a rack & pinion focuser.


Mike
I haven't followed the CO thread really carefully but your statement above seems to be a common belief among many SCT owners... the problem is, how can the correct spacing be determined easily without a heck of alot of testing? I'd guess you're proposing something like a Roddier test? Or, using the intra/extra focal patterns at different backfocus distances? All of this sound very time consuming and frankly may not be conclusive. My SCT images are often mushy, probably due to poor thermals and maybe collimation. I'd need to deal with all of that before even attempting what you're suggesting, yes?

Loren

#6 pstarr

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Posted 06 January 2013 - 09:22 PM

My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type.


A SCT with perfect optics and a 30% central obstruction can never give better that 1/4 wave performance. Not that 1/4 wave performance is bad. That said, you will never find a SCT, or any other design with perfect optics.

#7 Asbytec

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Posted 06 January 2013 - 09:43 PM

Sure, collimation is absolutely important. But, it's easy, as long as the optical axis is true and preferably orthogonal to the focal plane for best results. Getting that done can be a bit more time consuming, but tilting the secondary is easy. I did it each session, whether it needed it or not.

Cooling, same thing. Take precautions, prep the scope, and pray for moderate conditions the scope (all scopes) can stay in tune with. A good diagonal is an easy fix, especially with dielectric coatings, if it is a problem at all. Apparently not, not much anyway. Eyepieces, too. Same thing.

Personally, I am beginning to think the biggest fault, if you will, of a multiple lens/mirror design is veiling glare or scatter. On this, YMMV.

I am not versed on the advanced SCT designs, like ACF or Edge, but don't they use additional lenses to correct for coma? Meade does not say in this ad, but apparently they found a good "hyperbolic" system using the Schmidt corrector and primary.

"Fortunately, Meade engineers developed a radical new Advanced Coma Free design by combining a hyperbolic secondary mirror with a corrector-lens-and-spherical-primary-mirror combination that performs as one hyperbolic element. This Advanced version of the traditional CF design produces a coma-free, flat field of view that rivals traditional CF telescopes "

http://www.optcorp.c...t.aspx?pid=7836

I am wondering about the correction of the wave front on such a hyperbolic system, how far from ideal spherical does a hyperbolic system deviate? Not sure what the wave front looks like, but surely it meets one of the simpler diffraction limited criteria.

BTW, it does seem from anecdotal evidence (and my own experience) the newer CATs are better corrected and smooth these days. They seem to be more consistently capable of 0.95 aberrant Strehl (with a .3D CO means about 0.80 nominal, working Strehl.) Folks are very pleased with some aspect of them, be it sharper images and resulting contrast improvement (I appreciate that, despite a moderate CO) or flatter field.

And, again, this scope is optimized across the, uh hum, MTF with a huge secondary to suit a broader range of applications. It gains a little on the hi frequency end (tight, equal double star pairs), and gives up a little on the mid range (fine planetary and tight, unequal doubles) by changing the intensity structure through the Airy pattern and reducing peak intensity (probably below 80%.) The "aesthetics" of which can be and are hotly debated. But, this does affect contrast at small scales (for visual) down to about 4x the Raleigh criteria, if that scale matters to anyone. It matters somewhat to me, I push my scope hard and deep in better than average seeing. It's a trade off in the design, despite the off axis performance (which seems to be geared to larger imaging chips and not the eye.)

http://www.meade.ru/...00254208048.htm

In any case, get a scope that suits your needs, control the induced aberrations (cooling, collimation, and de-focus) and enjoy it under steady skies. If it pleases you, then there is nothing wrong with that scope. Some folks probably do not collimate their scopes, no surprise. That's on them.

So, if "all other stuff" includes collimation (plus mirrors shift) and a cheap diagonal, then feel lucky. Those are easy fixes. If a fixed focuser is key, find a sufficient back focus and induce ~1/24th wave over correction per inch of back focus. I doubt anyone will notice.

#8 GlennLeDrew

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Posted 06 January 2013 - 11:55 PM

Loren,
If the radii on the two mirrors is reasonably close to the nominal design, then one can simply rely on the manufacturer's specification for the position of focus with respect to the rear opening. No need to laboriously test. Just place an eyepiece field stop at this distance, focus the primary and then lock it down.

Now, if one or both mirrors departs from nominal radius of curvature by some threshold, the resulting optimum position for focus will differ. There is some leeway before the wavefront is affected to any notable degree, and I should *think* these scopes are sufficiently well executed that reliance upon the maker's recommended nominal focus position will not steer you wrong.

#9 Mike Harvey

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Posted 07 January 2013 - 12:05 AM

Fortunately, there is a solution...
LOCK the primary in place at the exact point it should be separated from the secondary, then install a rack & pinion focuser.


Mike
I haven't followed the CO thread really carefully but your statement above seems to be a common belief among many SCT owners... the problem is, how can the correct spacing be determined easily without a heck of alot of testing? I'd guess you're proposing something like a Roddier test? Or, using the intra/extra focal patterns at different backfocus distances? All of this sound very time consuming and frankly may not be conclusive. My SCT images are often mushy, probably due to poor thermals and maybe collimation. I'd need to deal with all of that before even attempting what you're suggesting, yes?

Loren


Hi Loren...assuming your scope does not have poorly figured optics, decollimation is probably the cause of your 'mushy' images.
In my opinion collimation is the FIRST thing to check.
An "artificial star" (point light-source) is invaluable for this purpose.

If you can't get your hands on one, try finding a point-source reflection of the sun off a bright shiny object about 100 feet or more away from the scope. Trying to 'chase' a star around in the dark to do the traditional 'star test' can be extremely frustrating if you don't have much experience (you have to keep re-centering the star after each adjustment) and poor seeing can make it difficult to accurately gauge the diffraction rings.

Yes, you're right about the calculations involved in determining the proper distance between the corrector and the primary. The math hurts my head!
With a LOT of help from smarter people than me, I found that the proper spacing for my 10" was such that the focal plane (at infinity) should fall some 3-1/2" behind the backplate of the scope. Knowing this allows the proper placement of the rack & pinion focuser.

Sometimes (depending on the scope and the R&P involved) you just can't make the two reach acceptable focus at the precise point. But a relatively small difference in the optimal vs. practical placement is not nearly as important to improved images as getting that primary secured in-place and perpendicular to the light path. That primary can, and does, flop around when it's being racked up and down the internal mounting tube. If you reach a given focal point and the primary mirror is even slightly off-axis, you've just UNdone your secondary collimation efforts and induced even more error into the system!

You know we're all crazy, right? I'm just not sure if we actually DO all this stuff because we're crazy...or if doing all this has made us crazy!

:)

Mike

#10 Eddgie

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Posted 07 January 2013 - 10:24 AM

A SCT with perfect optics and a 30% central obstruction can never give better that 1/4 wave performance. Not that 1/4 wave performance is bad. That said, you will never find a SCT, or any other design with perfect optics.


This really isn't quite the case, and should never be offered as a "Baseline" for comparing an obstructed instrument to an unobstructed instrument without at least a nod to the more complete story.

First, the effects of the central obstruction are most severe in the middle and higher part of the MTF curve (large and medium scale detail). Once you get past the elbow, where the smallest detail is to be found on the target, there is littel effect on the image.

The computational Strehl for a 30% obstruction is not .82 as is often written, but more like .88. This is considerably better than the Strehl of .82 that .25% of pure SA would result in.

I have plotted two curves.. One if for a perfect system with a 30% obstruction (left image), and the other is for a perfect system with .25 waves of lower order spherical abberation (right image).

The colored lines are sample points that show how much contrast is lost over the perfect system at each point in the linear resulution (spatial frequency) spectrum.

At the top left, it is easy to see that the obstructed telescope is performing slightly better than the .25 wave scope, and once you get past the elbow, where the most damage is being done, it is easy to see that the obstructed telescope pulls ahead and remains ahead.

This lower right hand portion of the MTF plot represents the finest scale detail the aperture can resolve, and it is ls abundantlhy clear that not only is the obstructed instrument ahead of the .25 wave instrument, it is also ahead of a perfect unobstructed instrument.

Now when the obstruction gets to .33D, the damage at the worst part of the curve is similar to the worst sag in the .25mm instrument, but once you get past the curve, the obstruction is clearly better.

Now the problem is that for visual use, the left half of the graph tends to be more important (often referred to as the important low and mid frequencies) because to get the smallest detail that the scope can resolve large enough for the visual observer to comfortably see, the exit pupil will get small enough that it starts to work against the observer for low contrast detail.

But the camera will show it easily. For imaging the finest scale detail the instrument can resolve, the central obstructoin has no damage at all in the image, and in fact can somewhat enhance the finest detai.

This is why large obstructions are OK for imageing scopes, but not so good for visual observation, but as the chart show, the 30% obstruction is superior to .25 wave over about half of the instruments spatial response frequencies.

I will be fair though and say this... The differences are so subtle that visually, the average observer would struggle to see them.

But the camera will.

That to me makes your statement somewhat innacurate. The CO = 1/4th wave is a good approximation for visual use, but somewhat overstates the damage that a 30% obstruction does.

I do agree with your sentiment on never finding an SCT with perfect optics, but no telescope is truely perfect.

Still, in the sentiment as it was intended, I agree that his is indeed the issue with the commercial SCT. Because it it starts wtih a 33% obstruction, anything other than quite minor optical imperfections will indeed quickly drive the scope to be truely worse than .25 pure LSA, and that is the real problem. To work well, the design has to be made to a very high tolerance. This means that any SA in the instrument combined with the contrast loss from the obstruction quickly pushes the scope into a poor performance envelope. Some SA would reduce the contrast below a perfect unobstructed aperture at both the low and high frequencies, and further deepen the sag we see at the elbow in the plot.

A truely excellent 33% obstructed SCT does little evident damage to the image. A small amount of contrast is lost in the mid frequencies, but the low frequencies are little changed, and the higest frequencies are improved.

But other defects lower the contrast across the entire spectrum, while an obstruction is the only device that can actually raise the contrast at any point in the spectrum over a perfect unobstructed instrument.

Bottom line... .25 SA is worse than a 30% obstructiion... Not much, and for visual use only, it is fair to compare them, but for imageing (and I personally belive for an experienced planetary observer), the 33% obstruction is not that important. The smallest, lowest contrast details available at the target are actually enhanced in the obstructed instrument (assuming good optics).

I routinely see very difficult detail in my 32% and 34% obstructed SCTs. Both have excellent optics, and both are excellent performers.

Side by side with my 6" APO, one has to look hard to see detail in the 6" APO that is not visible in my EdgeHD 8". I say this on the forums and people are skeptical, but people that have been to my house for star parties that have done side by side comparisons are usually shocked at how close the performance is. They have been led to believe that the big CO destroys the image, but the reality is that by itself, it does not have a huge impact.

The difference though is that my EdgeHD has about the best optics I have ever seen in a reflector. Similar to the Russian MNs I have owned. I would estimate a Strehl of over .95. No defect that is easily detectable in star testing, with almost perfect SA control. Not typical of the older SCTs I have owned.

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#11 Asbytec

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Posted 07 January 2013 - 12:43 PM

I routinely see very difficult detail in my 32% and 34% obstructed SCTs. Both have excellent optics, and both are excellent performers.

Makes sense. I imagine that's true because a reasonably good scope can still produce a peak intensity of about 80% (sort of a proxy for diffraction limited performance deemed acceptable) with an obstruction near .3D. Obstruction-less scopes perform with a higher peak intensity equal to their Strehl and the result is dimmer rings.

You are correct, the MTF curves are similar for 1/4 SA and .32D. However, SA affects the wavefront and the CO adds diffraction. With added diffraction, the central disc is smaller and brighter, so their Strehl equivalents are different: .82 and about .88, respectively. The CO has less impact on the image over the entire range, as I read it, especially at the high freq end.

#12 DesertRat

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Posted 07 January 2013 - 09:18 PM

I think the comparison useful as long as you are careful to note that the obstruction does no damage to the resolving power of the scope. SA does some damage, only some of which is recoverable in imaging. That is the right side of the MTF (and yes MTF is a good indicator, phase plays an insignificant part in this situation) shows resolving power even enhanced a little with respect to a perfect unobstructed scope although in practice the difference would be difficult to discern.

Strehl however has more limitations than most people realize. A 0.95 strehl scope is clearly superior to a scope of 0.85. However two scopes with the same aperture and obstruction and identical strehl can yield different results in imaging.

Glenn

#13 freestar8n

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Posted 08 January 2013 - 04:13 AM

Bottom line... .25 SA is worse than a 30% obstructiion..


This is somewhat a side issue - but are your plots based on 0.25 SA without optimal defocus? If the only error in the wavefront is 0.25 SA, you can compensate for it almost entirely by a shift in focus. In Suiter, for example, 0.25 SA actually refers to about +1 wave of SA and -1 wave of defocus. So if you have about 1 wave of SA, you can defocus so the resulting wavefront, combining defocus with SA, has an overall P-V error of about 1/4 wave.

So I think either your plot does not include defocus, in which case it makes 0.25 SA worse than it really is, or it does include defocus, and the actual SA term is closer to 1 wave - which is a big error in the optics.

Either way - it's hard to compare two different systems based on a single number or a single plot. The higher values of MTF at the high end may pass individual spatial frequencies well, but in an actual view of jupiter's bands, for example, they could look ugly and distorted.

Frank

#14 DesertRat

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Posted 08 January 2013 - 04:57 AM

The 1/4 wave in Suiter is the traditional terminology going back to Rayleigh. That is the peak to valley deviation in optical paths across the wavefront, from the ideal gaussian, which reduces intensity at the central peak to a level of 80% compared to a perfect optic.

Perhaps to avoid confusion we should state that 1/4 wave in Suiter is comparable to 0.0745wv rms for low order spherical aberration. Thats approx 0.8 strehl. Because different aberrations are distributed differently, and in real optics not in a perfectly smooth fashion, equivalences between PTV and RMS values are fairly complex.

The focus is shifted yes, and the resultant PSF and MTF should be evaluated at best focus, which it is in Suiter.

Spherical aberration is a true resolution killer, nominal obstruction is not.

Glenn

#15 freestar8n

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Posted 08 January 2013 - 05:29 AM

The 1/4 wave in Suiter is the traditional terminology going back to Rayleigh.



No - it isn't, since the Seidel form of the aberration terms has taken over. Rayleigh includes defocus as a form of aberration, and he describes the impact of defocus by itself on an image. If you always speak in terms of optimal focus, then you can never have aberration due to pure defocus.

The total aberration from combined spherical and defocus is:

W = a040p^4 + a020p^2

The first coefficient, a040, is the amount of spherical aberration and the second, a020, is the amount of defocus aberration. I can point, and already have, to many texts that state that you can tolerate nearly a full wave of spherical aberration when it is compensated by defocus to keep the total wavefront error below 1/4 wave P-V. Suiter is a rare exception that glosses over the difference between the Seidel definition of each third order aberration term and the Zernike form - normally those things are kept distinct to avoid confusion. It is due to this ambiguity and confusion that I asked what exactly is being shown above - since "1/4 wave of spherical" is very different from "1/4 wave total wavefront error due to combined spherical and defocus."

You can have a given amount of spherical aberration and combine it with coma and astigmatism and defocus and end up with any crazed amount of total wavefront aberration. But the amount of spherical inherent in the wavefront remains the same - given by the coefficient on the p^4 term.

If a telescope is guaranteed to have 1/4 P-V wavefront error - the amount of spherical aberration could be much larger than this. Or it could be less and involve some mix of other aberrations. All you know is that at best focus, the wavefront will have 1/4 wave P-V total error from all the terms.

Frank

#16 DesertRat

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Posted 08 January 2013 - 05:39 AM

Yes the 1/4 wave of SA we are talking about is the amount that would lower the strehl to approx. 0.80. The MTF above is using that figure, which does go back to Rayleigh.

Yes a telescope has a mixture of aberrations and all the terms do add, albeit differently. Perhaps since there is some confusion we should stick to rms equivalents, which I like anyway since they are easier to do calculations from.

Since not many here want to plod through Born & Wolf we should try to keep the explanations at a useful level, with emphasis towards practicality.

Glenn

#17 freestar8n

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Posted 08 January 2013 - 08:29 AM


Here is a pretty good write up of aberration theory and Seidel terms, and it shows plots of PSF's due to spherical aberration alone in 8.40. Note that the upper left image of W040 = 0.25lambda is an image of pure spherical, whereas Suiter's non-conventional figure shows SA=0.25 - and in fact corresponds to something different from 8.40 since it is a picture of 1 wave SA combined with -1 wave defocus.

I can answer my original question of "which is being shown in the figure above - 0.25 SA with no defocus, or 1 wave SA with -1 wave defocus?"

This page is not an amateur optics page and is from Wyant at the U. of AZ Op. Sci. Center. If you enter 0.25 lambda spherical alone, you will get a strehl of 0.8 and a corresponding MTF. If you enter 1 wave spherical combined with -1 wave defocus, you will also get a strehl of 0.80, and the MTF is a better match to the plot in this thread. Therefore I conclude that plot actually shows a full wave of spherical aberration combined with defocus to yield 0.25 lambda P-V total wavefront error. At paraxial focus, this same system would have had much lower strehl - around 0.09 - for the same 1 wave of spherical.

Frank

#18 Eddgie

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Posted 08 January 2013 - 09:43 AM

The plots were done at best focus.

As for the rest, I can only say that my experience matches MTF plots farily well.

The EdgeHD 6" Vs the 6" APO, when plotted, suggests that the EdgeHD should maintian about the same contrast as the 6" APO for the finest details, and I find this to be the case.

For example, Cassini is every bit as crisp and as sharp in the EdgeHD as it is in the 6" APO. Likewise, thin band structure on Jupiter appears to be very similar to the 6" APO.

Larger, fainter festoons though, like in the equitorial band on Jupiter, don't show quite as well in the EdgeHD 8". These are larger(mid frequency) details that would be more at the center and left side of the MTF chart, and they are less easily visible in the EdgeHD 8" than in the 6" APO.

And likewise, these large, low contrast details are much more easily seen in the C14 than in the 6" APO.

These large festoons are all within the angular resolution limit of all three scopes, and are a classic (to me) example of "Mid Frequency" performance. While they are angularly large, they start with very low contrast.

The more contrast the aperture looses, the more difficult they become to see.

But my experience has been very consistent. The scopes with more clear aperture tend to show them better than scopes with less clear aperture.

As for refocusing, that might work on a point source, but I have my doubt that it would be effective on a complex extended target. Maybe. I just don't know.

But a CO does damage the image. I do not at all disagree. And maybe it does do as much damage as 1/4th wave of pure SA (I have my doubts, but don't keep a 1/4th wave scope around to compare to).

But in the overall scheme of things, the CO is easily offset by simply adding a bit more aperture.

And this is the benefit of large SCTs. It makes it possible to simply get the performence of a smaller scope using brute force and still having an affordable, compact, (relatively) light, and easy to use instrument.

But CO does damage the image with respect to aperture. I realized how bad it was the first time I compared a C9.25 to a MN61. One would think that the C9 would do better because of the 50% increase in aperture, but there was no contest.. The MN61 was blazingly good in comparison. About as close to my 6" APO as I have seen in a similar or smaller size scope.

Plotted on an MTF chart though, it was easy to see that the C9 would have had to start with perfect optics to be in the game. But the 37% obstruction I think was the major cuplrit in the C9s failure to perform better than the MN61.

#19 freestar8n

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Posted 08 January 2013 - 10:40 AM

I think your plot shows 1 wave of spherical aberration at minimum RMS focus - or -1 wave defocus. Given that amount of spherical aberration, you could focus it in other ways shown in my links above: paraxial focus, minimum rms, least square ray error, and smallest circle of confusion - i.e. there is no single best amount of defocus, and it is good to keep it separate from the problematic aberration itself, which is 1 wave of spherical. The key is that the inherent spherical aberration is fixed and you can adjust focus separate from it to alter all the main metrics: P-V, RMS, Strehl, and MTF.

Since you and other people interested in MTF often point to the high frequency stuff and how it relates to "resolution" - note that if you increase defocus beyond -1 wave (min RMS/max Strehl) you will boost the high frequency part of the MTF in exchange for the mid-frequencies. This may well be what you do when you focus in on the belts of jupiter - it's hard to say. But the amount of spherical aberration (1 wave) is fixed here, and you can alter the PSF and MTF to get the best view of a given object based on the spatial frequencies you are studying. This may end up increasing RMS and reducing Strehl - but it does boost the higher frequencies in the same way an increased CO does - if that is something you want to do.

Frank

#20 DesertRat

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Posted 08 January 2013 - 02:18 PM

Best focus for a scope with low order spherical aberration (LSA) would be where the central maximum of a star is at its highest. This is often referred to as diffraction focus. A primary aberration evaluated at that point is called balanced, the balancing performed by an equal amount of defocus.

Most amateurs chase focus all night at levels that exceed the needed amount of balancing defocus due to seeing effects as well as thermal changes. In typical applications one will find a user chasing somewhere from best focus to smallest blur and back again. Operationally the user has no idea where paraxial focus is, and in any event a scope with 0.25 wv ptv LSA at either paraxial focus or marginal focus at the other end would be simply awful. Knowing the defocus term in backyard astronomy is not a useful parameter.

For some applications a quarter wave ptv of LSA is a lot. For high resolution imaging it is too much, for when the other aberration terms which are surely there are added the usage of the scope will be limited to widefield or low power use. LSA is a resolution killer. Central obstruction is not.

In the case of the SCT it is even more complicated by the fact that in all probability the scope has a minimum amount of LSA at a given wavelength. I've seen a lot of SCT's corrected in red and showing pretty severe overcorrection in blue. One of the reasons the Edge HD series seem to be star testing so well visually is that they tend to be very well corrected in green. Before the SCT users out there protest too much, I hasten to add I've seen a lot of supposedly premium refractors fail on the blue end as well.

There is no doubt from my analysis and from experience that the central obstruction effects on image quality are usually overstated. But they should be understood and the MTF displays the effect very well.

Glenn

#21 Ed Holland

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Posted 08 January 2013 - 06:07 PM

Another excellent thread, with more good theory than I have time to absorb in full at the moment.

Practically speaking, from my experience as a casual observer with MCT, SCT and achromatic refractor, I wish an instrument's intrinsic limitations were the only obstacle to a good view. Usually seeing (high level and local) is the most significant barrier to obtaining a detailed view. I've been careful to accumulate each of my scopes on a modest budget. If one keeps that factor in mind, the views through a sub $300 Maksutov or circa $400 C8 are astounding value, and show me far more than the 6" APO that I don't and won't own. They are also easy to mount for visual use. That these instruments are often considered as a "runner up" is a shame - a well tuned catadioptric scope with good optics can be a powerful tool. Rather like my MGB providing an excellent and fun drive, despite it not being an E Type Jaguar...

Of course, CATs do require attention to detail - for starters, the importance of good collimation is hard to over emphasize. That last "whisker" of adjustment can make all the difference, yet the mechanisms aren't so user-friendly to the newcomer. Furthermore, based on experience with both my CATs I note that mechanical assembly can fall short of what is needed to get the best out of essentially blameless optics. Tilted baffle tubes & primary mirrors, off centre secondarys and the like can really spoil the show. However, many users will be unable to recognise, or be unwilling to investigate & tackle these problems. I'd love to know how many good sets of optics have been compromised by non optimum assembly - could this have contributed to some of the received wisdom regarding CAT performance? As the writer and broadcaster Sir Patrick Moore would have put it "well, we just don't know", but I have my suspicions.

What I do know is that when I got my C8 properly "sorted" to the best of my abilities - ensuring that everything was coaxial, squared-on and collimated - I was amazed by its planetary performance. This was quite different to its "as received" capability, despite seemingly succesful attempts to collimate with the secondary mirror screws. My Orion 127 Maksutov also benefitted from similar treatment, if not to the same degree.

I've rambled on long enough, perhaps too much, but will finish in saying that good execution of a design is as important as a good design.

#22 Asbytec

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Posted 08 January 2013 - 11:29 PM

Yea, this is an interesting thread, especially the discussion between Glen and Frank. A lot of that is difficult to grasp, at least the importance of it. Yes, all aberrations such as seeing, collimation, cooling, and everything have to be absolutely minimal if not perfect to squeeze that bit of performance from a scope. And any amount of de-focus from best diffraction focus counts, too.

What's interesting to me is the idea an obstructed scope has a smaller Airy disc that is a bit brighter than it would normally be if it's intensity were spread over 1.22 Lambda/D. This, in my understanding, is what differentiates CO diffraction and spherical aberration. This is what causes an obstructed scope to exceed a perfect unobstructed aperture on very small scales (near the radius of the Airy disc.) And it is what allows, under very minimal induced aberrations, to actually exceed Raleigh and Dawes limits by about half an arc second.

The peak intensity is a bit higher for an obstructed scope with good correction than with one with 1/4 wave SA. SO, yes, a scope with an aberrant Strehl of 0.95 can be very good while the CO reduces peak intensity (not aberrant Strehl) further to about 80% (a peak intensity comparable the Raleigh limit) or less depending on the combined affects of both the wavefront error and added diffraction.

The scale of these effects seem to be over rated for general viewing because we're talking scales to about 4x the Raleigh limit. But, for critical viewing, they can be important. On Airy disc scales, this is where obstructed scopes rule. On slightly larger scales is where unobstructed scopes rule with peak intensity (nominal, system Strehl) equal to their aberrant Strehl. CATs do not enjoy this trait.

But, we're focused on the CO and not the "other stuff" such as veiling glare and scatter in CAT optics (including reflectors), which happen to be obstructed.

#23 siriusandthepup

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Posted 09 January 2013 - 01:16 AM

I have pretty much the same opinion as Mike on the SCT's.

Optical quality rules and is THE most important factor.

Now I've heard the argument about the 1/4 wave derating for the 30% CO and it's overblown. Let's discuss it. In the case of the 10" SC with 30% obstruction, is it as good as a 10" perfect APO? No. Obstructed scopes will always take a performance hit for the obstruction. Do you really want to compare it to a 10" APO? Get real! A 10" SC with a 30% CO will only perform about as well as a 7" perfect APO. I can live with that...

#24 freestar8n

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Posted 09 January 2013 - 02:21 AM

Hi Norme-

I haven't been talking specifically about CO stuff but more to encourage formal terminology in these discussions, and references to optics textbooks. The nomenclature has been honed over centuries to avoid some of the ambiguities that come up in these discussions. I think there is over confidence in going by a few metrics of a system performance, when things like Strehl and MTF are meant more as guides and figures of merit rather than hard performance metrics that allow systems to be ranked against each other.

When someone looks at an object in an eyepiece, the resulting image involves the object itself, the optics, the perceptual process, and the personal preference for choice of focus. Much of that is not captured in fixed metrics of the optical system alone, but it's all playing a role.

I am always amazed at how different jupiter looks at different powers in the same telescope. If I put a big image of jupiter on the wall and view it from close or far, I don't see much change in detail - but with different eyepieces and magnfications, the colors and spatial frequencies all have different impact - and I always adjust focus to bring out optimal (for me) detail. You could say that at low power I bias the focus so that MTF emphasizes the mid-range, since mid-range is higher frequency at low power; and at high power I bias it toward the high frequency part. But either way - there is a lot going on just in changing powers with a given telescope - let alone comparing two with different CO's. And in terms of the MTF or Strehl of the OTA itself at "best" focus - nothing should be changing at all.

Frank

#25 Jon Isaacs

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Posted 09 January 2013 - 05:35 AM

What's interesting to me is the idea an obstructed scope has a smaller Airy disc that is a bit brighter than it would normally be if it's intensity were spread over 1.22 Lambda/D. This, in my understanding, is what differentiates CO diffraction and spherical aberration. This is what causes an obstructed scope to exceed a perfect unobstructed aperture on very small scales (near the radius of the Airy disc.) And it is what allows, under very minimal induced aberrations, to actually exceed Raleigh and Dawes limits by about half an arc second.



It is my understanding that the central disk is slightly smaller than it would appear in scope without a CO because there is more energy in the rings and less energy in the central disk. One thing to remember is that hidden inside every scope without a central obstruction is a scope with any sized central obstruction one desires.

Jon






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