What is the difference between "Schmidt-Cassegrain" and "Maksutov-Cassegrain" telescopes?

Answer 1

Generally speaking, the shape of the corrector plates and secondary mirrors, though both do have convex secondary mirrors but the exact shape differs.

Schmidt Cassegrains have thin, wavey shaped corrector plates and the appropriate shaped secondary mirror.

Maksutov Cassegrains have thick meniscus shaped corrector plates and the appropriate secondary mirror.

As others pointed out, Schmidt Cassegrains can be easily made in larger apertures, while Maksutov Cassegrains can't because the corrector plate is actually more akin to a lens and lenses get heavy quickly and are more difficult and expensive to produce.

Beyond that, the differences between the two get complex and typically deal with how much of a particular type of aberration which one suffers from. These aberrations are typically spherical aberrations, astigmatisms, and non-coma off axis distortions.

It is said that a Maksutov Cassegrain will outperform a Schmidt Cassegrain of the same size, but you don't see many Maksutov Cassegrains over 5" or many Schmidt Cassegrains under 6" so it's a bit of a nil point.

Answer 2

Both are modifications to the Cassegrain design that employs two mirrors, with the secondary mirror directing focus through the middle of the primary mirror. The Schmidt corrector appears flat but just has a small amount of curvature in the radius of the plate. The Maksutov has a highly curved corrector known as a meniscus.

Practically speaking, the Schmidt usually has the secondary mirror attached to the corrector plate via some sort of assembly that puts a hole in the middle of the corrector plate. The Maksutov has a smaller secondary mirror that is usually a reflective surface directly on the back side of the corrector plate (rather than an actual mirror being held in a separate assembly like with the Schmidt). This means the Maksutov never has to be collimated, while the Schmidt does.

Maksutov's generally have a higher focal ratio than a Schmidt of the same size. This means that a Maksutov is generally better as a dedicated, high-power planetary scope while the Schmidt is a more general purpose scope.

Answer 3

The corrector plates are different. The Maksutov has a deeply curved corrector. It is also thick, being 1/2 to 1 inch thick. It is bowl shaped. You could eat cereal out of it.

The Schmidt has a thin, almost flat corrector. It is very thin. Most manufactures make it just barely thick enough to support itself and the secondary mirror without warping.

Answer 4

Aside from the physical differences the others have mentioned, Maksutovs tend to deliver crisp contrast images, almost as good as an apoochromatic refractor, while Schmidts tend to deliver somewhat mushy images compared to other telescope types. Schmidts are lighter in weight and less expensive for a given aperture because their corrector plates are thinner and more suitable for mass production.

gn: Of course I know that Maks are produced commercially (I've owned a Meade 90mm and an Orion 127mm), but only in smaller sizes (mostly 5" aperture or less), not really comparable to SCTs which are mainly larger apertures (8" and up). Maks of 8" aperture and larger are basically custom instrument and extremely expensive. Synta's new 180mm Mak (sold by Orion in the US) is perhaps challenging this.

Answer 5

Both have a large transparent corrector plate, a large main mirror and a smaller mirror near the corrector plate to send light back through a small hole in the middle of the primary mirror. But the two designs used different shapes for the optical surfaces.

Answer 6

Nice long answer GN but there's no need to vote down the other responses. The responses from Daniel and Geoff are absolutely correct because their statements are preceeded with things like "Generally speaking...", "Typically...", "Usually...", "Practically speaking...".

No one said Maksutovs aren't mass produced. No one said Maksutovs always have smaller secondary mirrors. No one said Schmidts can't have longer tubes.

Please save your negative votes for wrong answers; not for answers that you incorrectly interpreted as absolute statements.

Answer 7

There are a number of misconceptions in the previous answers. My esteemed colleague Geoff G errs in thinking that Maks are not suited for mass production: the Meade ETX series and Questars are Maks, and Celestron has several Maks including the famous c90 which has been around for decades. Orion sells a number of Chinese Maks, up to about five inches in diameter, that have been very popular. Another error in previous replies (not Geoff's) is that Maks do not need to be collimated. They do, you have to move either the mirror or the corrector plate, and they can be devilishly difficult.

In commercial designs (celestron and meade) the secondary obstruction of the SCT is larger than in most Maksutovs. This is not intrinsic to the "theory" behind the design but it is the way they are marketed. You could make an SCT with a longer tube and reduce the size of the secondary obstruction, but if you did, you would reduce its portability and increase the demands it places on the mount, which would increase costs.

Another manufacturing weirdness, not dictated by optical theory, is that the majority of Maksutovs marketed have very long focal ratios, even longer than SCTs. They don't have to be this way, and some are not, but most are. When you shorten them up you increase the size of the secondary.

The effect of the central obstruction is grossly over-rated by amateurs who typically are seeing other effects such as dirty eyepieces, lack of collimation, bad atmospheric seeing, or the effect of a hot mirror. In any case the usual rule of thumb is to take an apochromatic refractor as the "performance gold standard" and subtract the diameter of the secondary obstruction of the rival obstructed design to get performance equivalence. So on a 10" Newt you'd subtract 2" you'd have performance on planetary detail about equal to an 8" apochromatic refractor and with a 10" SCT you'd have performance equal to about a 7" refractor.

One of the most accomplished planetary imagers in the world is Damian Peach and he has a full discussion of the secondary obstruction for which I have attached a link. In a nutshell one of the issues to bear in mind is that given the difference in costs, weight, and other factors, one can compensate any putative losses to secondary size by going up to the next size SCT, which is usually a cost effective option (compared to Maks).

The front lens of the Maksutov is very thick and the number one complaint with these instruments is that they take a long time to cool down. While they are "hot" you have heat waves rising off the optic. The corrector of the SCT cools off more quickly, indeed, the problem here is that it cools so fast dew can form on it so that prevention measures must be taken. I have read only this morning that some people think SCTs take a long time to cool down; in my experience that's not the case, and there are a number of reasons, including the conical mirror, and the fact that the center mass, which holds the most heat in other designs, is hollow in an SCT, and the heat gets drawn to the rear casting which is an effective radiator. But there are a number of users out there who complain about SCT cool down and even more who complain about Mak-Cass cool down.

In any case the technical differences in construction and design are one way of talking about the "differences," the in-use differences are another. A fourteen inch Maksutov Cassegrain would rapidly hit the engineering limits of an amateur scope: very expensive, very heavy especially up front, very difficult to mount.

As one who looked into the two designs, what struck me is that the SCT provides a larger aperture at a lower cost, for example, the 9.25 is in my personal experience a better telescope for general purposes than a 6" Intes Mak, and a good deal less expensive. And as it happened I had the opportunity to compare the 9.25 and a 6" Intes Mak side-by-side some years ago.

And you might be tempted by an 8" Mak but then again you could get a c11 or a c14 for less out lay, less cool down time, and less demands on the mount; and put any money saved towards a used apochromatic refractor if one is inclined to own a "precision optic."

To sum up:

1. both need collimation
2. both are made "at commercial scales"
3. The SCT has a secondary housing with an independently mounted mirror on a thin corrector plate, with adjustment screws in the back of the secondary. In commercial models the primary is not adjustable for collimation.
4. The Mak-Cass will have a thick front lens with a secondary called a meniscus applied directly to the back of the front lens. The Mak-Cass in production typically has collimation screws for the primary mirror.
5. The SCTs are significantly cheaper.
6. The SCTs are marketed at much larger apertures, up to 14 and 16" vs. about 8" topping out for the Mak-Cass.
7. The SCTs are shorter and lighter.
8. The Mak Cass will have a smaller central obstruction.
9. The Mak-Cass is difficult to cool, the SCT requires some effort to keep the corrector clear of dew--and the Mak might too (as do refractors).
10. I have heard bad things about the "fit and finish" of manufacturing of almost every telescope imaginable so I don't think any particular design has an advantage on that.

If you're wondering "which one you should get" the answer is that you'll get a good deal of enjoyment out of either. No one choice is correct.

Hope that helps,
GN