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Gregorian vs. Classic Maksutov design

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#1 Rob E

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Posted 21 January 2011 - 09:45 AM

Is the classic design worth the extra cost given the simplicity of the Gregorian design?

#2 Eddgie

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Posted 21 January 2011 - 10:50 AM

As I recall, the all spherical Gregorian Maksutov cannot fully correct spherical abberation on axis, and the field is strongly curved. In addition, there is coma and astigmatism that gets intrusive faster than about f/15.

The practical design limit for a true Gregorian Maksutov is about f/15.

With a seperate secondary as used in Maksutov or with Rumak, the designer has much more freedom to correct off axis performance and control higher order spherical abberation. The field of a Rumak for example, can be much flatter and much better corrected for coma and astigmatism.

For very slow focal ratios, there isn't much benefit (usually the secondary is kept quite small in the Gregory, which results in a small baffle, which limits how far off axis you can go), but for faster focal ratios, it would be I think clearly worth more to move away from the true all spherical Gregorian Maksutov.

So, the answer depends to some degree on what the observer wants. If they are content with a very slow focal ration and a very narrow field instrument, then the answer would be that there isn't a great benefit in going to a different design.

#3 Rob E

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Posted 21 January 2011 - 11:02 AM

I'm looking at purely planetary and possibly lunar. I've narrowed my choices down to one of three scopes. Two are Maks (Gregorian and classic designs) the third is an Apo.

I'm leaning towards the Maks due to cost and longer focal length. The Maks' focal length more closely mirror my dob and lean well to higher magnifications with the eyepieces I use.

I already have a 120ST refractor for wide field use so one of the Maks seems to fit my needs better than the Apo. (and for about half the cost.) The Apo might negate the need for my current refractor but i'm really not looking to replace that right now.

#4 RichD

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Posted 21 January 2011 - 11:06 AM

Don't be too put off by the off-axis coma and astigmatism in a gregorian maksutov. While Rumak types are undoubtedly better in this regard, Gregorian maks are actually still quite sharp off-axis.

At f/12 (common for gregorian maks) with high quality eyepieces, correction across the field with something like a 24mm Panoptic is excellent.

#5 Rob E

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Posted 21 January 2011 - 11:13 AM

I had the Orion 127 Mak and it was a great scope but I sold it some time back. In retrospect, I guess that was a mistake.

#6 wh48gs

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Posted 21 January 2011 - 11:17 AM

You guys got me confused. What you call "Gregorian Maksutov" is Gregory Maksutov, after John Gregory's 1957 designes. These use aluminized spot on the rear of Maksutov corrector, which makes the secondary convex. "Gregorian Maksutov" would have been one using concave secondary mirror.

Vla

#7 Rob E

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Posted 21 January 2011 - 11:52 AM

I guess I'm confusing gregorian with Gregory. My comparison intent is to compare the aluminized spot type with the separate secondary type.

#8 imjeffp

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Posted 21 January 2011 - 12:21 PM

For a lunar/planetary scope, how about a Mak-Newt?

#9 KerryR

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Posted 21 January 2011 - 02:26 PM

The Rumak, with it's separate and often collimate-able secondary, is an overall better design because it gives the optician an additional separate surface with which to optimize the design and execution. They often have smaller obstructions than the common Synta Gregory Mak. It's certainly not a gurantee of optical excellence though.

As an aside, Questars look like Gregory's because of the visible aluminized spot, but they are not: the aluminized area is actually figured to a different curve, like a Rumak, but on the meniscus rather than on a second piece of glass. This must be a very difficult thing to execute well, which probably contributes significantly to the price tag.

For dedicated planetary, get the best optics you can. This will probably mean a Rumak design, and probably not a Synta Gregory, for which you have to win the Synta Optical Quality Lottery-- there are some good Synta Maks out there, but there are also a lot of average ones.

#10 Paul G

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Posted 21 January 2011 - 03:41 PM

As an aside, Questars look like Gregory's because of the visible aluminized spot, but they are not: the aluminized area is actually figured to a different curve, like a Rumak, but on the meniscus rather than on a second piece of glass. This must be a very difficult thing to execute well, which probably contributes significantly to the price tag.


FWIW, the Astro-Physics 10" Mak is an aspherized Gregory, too. The heavy aspherization is done on the primary, not sure what if anything is done to the secondary. Coma-free, very small co, but as you say it comes at a price.

#11 wh48gs

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Posted 22 January 2011 - 07:29 AM

The main difference is off-axis correction. Gregory-Mak has some coma, and for that reason is usually made in slower f-ratios. Typical f-ratios have linear field coma at the level of an f/8 to f/9 paraboloid. Slower f-ratios of the Gregory-Maksutov mean they come with somewhat smaller secondary mirror. Two consequences of it is that it has somewhat more of astigmatism and field curvature, the former relatively unimportant, the latter roughly by a third stronger than in the arrangement with separate secondary (still somewhat better than in a typical SCT). It also results is somewhat higher design spherical aberration (since larger secondary offsets more of primary's spherical aberration, which allows for weaker corrector and less higher order spherical), but there should be no noticeable difference in well executed instruments.

In all, as long as f-ratio stays slow (about f/13 and slower), coma is not an issue visually, and Gregory-Mak has performance level comparable to that of the arrangement with separated secondary. Quality of fabrication is more important than their respective inherent optical qualities.

Vla

#12 teskridg

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Posted 22 January 2011 - 08:46 AM

The ones with the aluminized spot may never require collimation, and if so is generally done by the factory. The Rumak designs with non-integrated secondaries will require collimation. Tim

#13 rmollise

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Posted 22 January 2011 - 10:02 AM

Is the classic design worth the extra cost given the simplicity of the Gregorian design?


What's a "classic Maksutov"? The only two designs I know of that are sold today are the Gregory design and the Rumak design.

#14 junomike

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Posted 22 January 2011 - 10:17 AM

Tim, Although the Gregory Maks (silver spot) are less prone to go out of Collimation, they can and do! Collimation on these is done via 3 screws on the back of the scope which aligns the Primary mirror to the "silver spot". Essentially the reverse of the Rumak design and SCT's as well. Both are usually shipped with near perfect collimation which will hold as long as the scope isn't jarred or dropped.

#15 vahe

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Posted 22 January 2011 - 10:20 AM

As an aside, Questars look like Gregory's because of the visible aluminized spot, but they are not: the aluminized area is actually figured to a different curve, like a Rumak, but on the meniscus rather than on a second piece of glass. This must be a very difficult thing to execute well, which probably contributes significantly to the price tag.


I know of two Maks with “carved in the corrector” secondaries, Questar is one as mentioned the other is Zeiss aus Jena “Meniscas 180” this one is 180/1800 F/10 system, very rare bird, expensive and occasionally shows on in all Zeiss offerings by Markus.

Vahe

#16 tim53

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Posted 22 January 2011 - 07:30 PM

A real gregorian or cassegrain would give you a purley reflective system that would allow you to image Venus cloud detail in the UV, which would be blocked by the corrector on a Mak.

Not that many people do that, mind you...

-Tim.

#17 jrbarnett

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Posted 22 January 2011 - 08:16 PM

In theory that's correct with respect to Rumak vs. Gregory.

In practice, I've had both and the best Synta 180mm I've used was better than the Rumak 7" f/15 I also owned.

Generally I think Synta makes reliably excellent Gregorys.

Regards,

Jim






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