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do the larger mak casses correct fully for sa?

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#1 Darren Drake

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Posted 09 April 2013 - 09:40 AM

In the book Telescopes, Eyepieces and Astrographs the authors say (on pg 286) that the Gregory type (the silvered corrector)mak casses by design have a bit of zonal spherical aberration (sa)if made with all spherical surfaces. They say that it's pretty negligable under about 4 to 6 inches or so but for bigger apertures the aberrations become significant. I'm wondering if the 7 inch Meade and Q7s ever corrected for this. I would assume that the high end 10 inch AP mak does since its AP. Does anyone know if special attention has been given to these bigger maks to remove the sa from the systems? Thanks.

#2 Eddgie

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Posted 09 April 2013 - 11:14 AM

The Q7, yes.

The Q7 uses a different radius of curvature for the secondary mirror. it is ground into the rear of the Miniscus lens.

For the silvered spot design as is typical of the 6" and under MCTs, if it is faster than about f/15, the high-order spherical abberation (HSA) will limit the scope to performing only to a strehl of .95 (vs one that has taken steps to correct for HSA).

Since these scopes (the mass market ones) appear to use a silvered secondary, we can assume that this component has a curve that matches the rear of corrector.

And since aspherizing the corrector itself would be far to difficult, it is likely that no steps have been taken to fully correct the higher order spherical abberation.

The 6" MCT sold by some vendors appears to deal with this problem simply by cutting the light cone with the front baffle, which "Balances" the HSA, but reduces the apeture.

This my be the case with the 5" MCTs as well. There have been at least two tests on CN where they were found to be working at only 120mm rather than 127mm. Agian, this will balance out the HSA, but only at the cost of angular resolving power and brightness.

If the light cone is cut by the baffle, the effect of the reduced apeture is to make the secondary obstruction correspondingly larger (as a percentage of the remaining diameter) and the larger secondary obstruction (by percentage of the remaining aperture) has the effect of suppressing the HSA.

This is the same reason that Suiter recommends a 30% obstruction for star testing MCTs. The star test is so sensitive to HSA that even a small (meaningless) amount will show up in even an excellent MCT, so he recommends the artifical obstruction to offset minor amounts so as not to get a false reading on the star test (which I can assume is what happened to Ronald Christen because when he did the star test on his own MCT, he slammed the test, but at the same time, it does not appear that he followed Suiter's recommendation to obstruct to 30% for the lower order spherical abberation test).

It is unknown to me whether this was intentional or just the way it worked out, or even if the manufacturer knows that the design is working at less than stated aperture.

I can only relay what has been reported and make a less than fully educated guess.

I have not measured any of these scopes myself, but this has been reported and discussed at lenght in the past.

#3 Eric63

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Posted 09 April 2013 - 11:33 AM

That’s interesting Eddgie. Now I know why my Mak only has a clear aperture of 120mm. This is meant to help clean up the optics and not simply a cost cutting decision by the manufacturer. I guess the aberration is still visible somewhat since my diffraction pattern is softer on one side of focus that on the other (but still well defined).

Eric

#4 Eddgie

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Posted 09 April 2013 - 11:59 AM

Hi, Eric.

Let me be 100% clear. I really don't know why the scope is built the way it is. It could be intentional, it could be done to mask the HSA, or it could just be a design mistake.

The fact is though, that a larger obstruction (by percentage) will help to suppress HSA to get a better reading on LSA.

This is why Suiter is so methodical in his approach to star testing.

People think that star testing is just racking the focuser in and out and looking at the view.

The book on Star Testing though is a very process oriented proceedure, and often there quite specific steps one most take to get a reliable test.

And for HSA, which can affect all spherical instruments, the test uses a lot of defocus with a 30% obstruction.

This is necessary because close to focus, even minor amounts of HSA will cause havoc with the result. I think this is what happened to Christen. The breakout test will fail miserably on an MCT with a small obstruction. That is why the test is done at 10 wavelentghs with a 30% obstruction.

Again, I have absolutly no way of knowing the why of the 127 and 152 MCTs. It is beyond my scope of knowldege as to why they are running at reduced apeture.

The fact is though, that a bigger obstruction does reduce the influence of HSA on the star test.

Does it mean that the manufacturer is playing a game with us, or that they don't know what is going on? Was it intentional? Was it just a case of "Ooops!"

I don't even want to guess.

The only things we know is that these scopes do appear to be working with a slightly smaller apeture and a slightly larger (by percentage) obstruction, and that making the obstruction larger will desenitize the star test to effects of HSA.

But we cannot know if the reduced apeture was a deliberate method for dealing with how the HSA would appear on the star test, or if the manufacturer is even aware that the instruments are working at reduced apeture.

My initial guess when this issue was first reported was that the scopes were not being manufactured to the correct tolerance, and that the primary mirror was mounted to far forward.

I still believe that this is the case.

It is possible that the optimal mirror position was moved forward by reducing the tube lenght to allow for the use of 2" accessories.

Maybe the scope would not reach focus with 2" diagonals if the mirror were mounted further back. Perhaps the mirror would have run out of travel.

And if you move the mirror forward, either the secondary baffle or the primary baffle will cut the light cone first.

In the SCT, it is usually the primary baffle that cuts the light cone first, but with only another millimeter or two of forward travel, it becomse the secondary baffle.

You can see this in Ken Hutchenson's SCT Vignetting analysis.

You see the apeture start to fall (primary baffle), then you see a distinct angle where the falloff curve increases.

This angle is likely the seconary baffle kicking in.

So, it could be that just to allow sufficent back focus to accomdate 2" diagonals, some yo-yo decided that the tube should be cut so that the primary started a bit closer.

If you read Dilbert, you would know that not all important decisions were made by engineers... :roflmao:

#5 Eric63

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Posted 09 April 2013 - 12:10 PM

Thank you again for the explanation Eddgie. My Mak came with a 2” diagonal and after doing much reading I realized that it also came with certain compromises. As a starter level scope it is still great, but I can see myself upgrading down the road as I gain more experience and become fussier about my optics.

Eric

P.S. Dilbert is a favourite here at work :)

#6 freestar8n

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Posted 09 April 2013 - 12:44 PM

I own a 7" mak and the meade docs themselves describe it as having an ellipsoidal primary. So it is certainly not an all-spherical design.

Even the "spherical" spot maks, such as those made and promoted by Gregory, involved a final phase of figuring with an autocollimator. Maksotov's original papers from the 40's also promoted autocollimation. When a surface is being mushed around and "retouched" to create a collimator null - it is effectively adding an arbitrary asphere, although small, to correct for any residual aberration in the system. So even a "spherical" mak has an asphere if the final figuring happens in an autocollimator.

I think some of the very small maks may be made all spherical and work ok - but for larger apertures it is harder to keep the aberrations small enough to remain diffraction limited - and they are either designed to have an asphere, like the meade mak, or they are hand figured at the end.

Frank

#7 Eddgie

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Posted 09 April 2013 - 01:30 PM

Even the "spherical" spot maks, such as those made and promoted by Gregory, involved a final phase of figuring with an autocollimator. Maksotov's original papers from the 40's also promoted autocollimation. When a surface is being mushed around and "retouched" to create a collimator null - it is effectively adding an arbitrary asphere, although small, to correct for any residual aberration in the system. So even a "spherical" mak has an asphere if the final figuring happens in an autocollimator.



Yes, we know that these people would do this.

The question though is do most small mass produced MCTs benefit from this, or are they just ground to spherical surfaces.

The only data I have to work with is the case where a CN member removed the secondary baffle from his 152mm MCT.

He reported that while he got his full aperture back, he could clearly see that the star test now showed additional HSA.

This would indicate that in this case at least, the scope may not have been re-figured as required to reduce the HSA.

I did not see the scope's star test before and after, but the individual has read and I assume used the procedure in Suiter's book to assess the HSA, and felt that it was enough that it could indeed be having an effect.

The dilemma was to replace the baffle to run with a reduced aperture but tame the HSA, or to run wide open and keep the brighter image.

The point though is that we don't really know if the 6" and 5" mass market scopes do indeed have some level of aspherizing done done. We can only guess.

Based on what little evidence we have though, my guess is no. It is time consuming to do, and in most cases, I think they economics of scale preclude doing so.

The Meade was a very expensive telescope.

The Intes Micro scopes no doubt do this, but once again, they are very expensive instruments made on a limited scale.

And note that the Meade unit did not survive in the marketplace.

My guess is that they simply could not sell enough of them at the price needed to make a profit based on the extra attention that was required for every sample.

#8 EddWen

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Posted 09 April 2013 - 04:47 PM

The RuMak configuration of Mak-Cass uses a seperate piece of glass for the secondary mirror. This allows the designer another degree of freedom to correct for abberations. I do not know for certain that is used to correct for HSA.

The "Ru" are for Rutten, co-author of our other favorite book describing telescopes.

The larger Intes-Micro optics are of this type. The secondary mirror is asphere. And, it seems the design allows for a smaller CO for an f10. I think mine measures about 30%.

#9 wh48gs

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Posted 09 April 2013 - 08:44 PM

Aperture size is not nearly as important as the primary f-ratio. What causes residual spherical in a Mak-Cass is higher-order spherical generated by strongly curved surfaces of the meniscus. It cannot be corrected without Schmidt-like profile, only minimized by balancing it with lower-order spherical. Higher-order spherical increases in proportion with the aperture, but inversely to the 5th power of f-ratio (which means that, given aperture, meniscus for an f/2.5 sphere generates 2.5 times more higher-order spherical than meniscus for f/3 sphere). Any Mak-Cass with the primary significantly faster than f/3 will have significant or unacceptable residual spherical, unless the primary is aspherized, allowing for more relaxed meniscus radii.

Vla

#10 Aquatone

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Posted 10 April 2013 - 01:28 AM

I have never seen any SA in my Astro-Physics F/14.6 10" Mak Cass. That the primary mirror is aspheric and the secondary spherical is fairly well known, as is its very small 23% obstruction that also is a featiure of the design. Very expensive and difficult to make though.

Chris






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