Difference between an SCT and a Maksutov ?

[gfletcher]

Hi Guys

What is the basic difference between an SCT and a Maksutov ?

Is one type superior to the other ?

The Maks appear to be more expensive "inch for inch"

Graham

[JamesF]

I don't think there's an enormous amount of difference. There are a number of variations on the Mak design but I believe the common form has the secondary mirror actually silvered directly onto the back of the corrector lens, whereas in an SCT the secondary is a completely separate mirror.

It may be that a Mak primary is spherical by definition and that an SCT primary, whilst often spherical, doesn't have to be, but I'm really not sure about that.

James

[Chiron]

Google and Wikipedia throw up some good answers to this question.

The way I read it is that both have a refractor element to correct aberations. In the Mak's case, this is a meniscus lens, so a lens where one side is concave, the other convex. The Schmitt has an element known as a Schmitt corrector plate. One side of the plate is flat, but the other side has a complex profile where the curves aren't a section of a cone or cylinder.

Schmitt correctors are much thinner than meniscus lenses and in the 70's Celestron worked out how to mass produce them by using a vacuum to hold the plate over a master block during manufacture. As a guess the meniscus lens would have to a ground each time hence this is why at larger Maks are more expensive than Schmitts of the same size.

Hope this helps.

XPN

[E621Keith]

It depends on quality. Mass market Mak is usually around the same price as their SCT counterpart.

Skymax 127 OTA £255, C5 £365 (FLO)

Skymax 150 £575, C6 £579 (Greenwitch)

Skymax 180 £780, C8 £790 (FLO)

However, if you compare a premium Mak (Intes Micro, Astrophysics, OOUK) to a mass market SCT (Meade, Celestron) then it's only natural for the mak to be more expensive. Unlike Mak, no one makes premium grade SCT so there are no direct comparison in the premium catagory.

Advantage of mak (in theory)

1. Smaller central obstruction - better contrast

2. All spherical surfaces - easier to make and test to high accuracy. SCT uses a complex asperical corrector which is harder to make to a high degree of accuracy.

3. Factory collimated, so less fuss and less maintenance. However I heard it's harder to collimate a mak when it gets knocked out of collimation.

Advantage of SCT (in theory)

1. Thinner corrector - quicker cool down.

2. Lighter weight

3. Slightly 'Wider' field

4. Some models can be converted to ultra fast f2 astrograph.

5. Available in larger aperture

I think the process Celestron developed to make Schmidt corrector produces highly variable results. Each corrector needs to be match to a primary mirror with the right distortion characteristic. It's the reason why you can't get replacement corrector if the one on your SCT is damaged, and if you ever disassemble a SCT the corrector plate must be reassemble in exactly the same orientation. The latter suggest both the primary mirror and corrector have astigmatism that's cancelled out by each other.

A mass market manufacturer can produces lots of correctors and mirrors such that they can always find a reasonably good match. Celestron and Meade advertised their scope as diffraction limited (1/4 wave), while premium scope makers often guarantee their scope to (1/6 wave or better). It's highly likely that higher accuracy cannot be achieved economically through the rather clever but crude corrector plate manufacturing process.

[Whippy]

I don't think there's an enormous amount of difference. There are a number of variations on the Mak design but I believe the common form has the secondary mirror actually silvered directly onto the back of the corrector lens, whereas in an SCT the secondary is a completely separate mirror.

There are two main types of Mak-Cass, the gregorian version with the mirrors silvered directly onto the meniscus while the Rumak version has a seperate secondary like an SCT. IIRC, the rumak has the advantages of the being easier to collimate due to the seperate seconday and a flat field.

In terms of any advantages between a Mak-Cass and SCT, there's probably not a great deal in it.

tony..