Optical Experts : Differences from an SCT to a Maksutov-Cassegrain

[toryglen-boy]

As title, just looking for some clarification, whats the major optical differences between an SCT and a Mak-Cassegrain?

Whats got a better image, and whats better for imaging, and observing?

sorry to be a jerk and ask such a question.


thanks
:)

[M_Lewis]

http://astro-observer.com/telescopes/catadioptric.html

Shows a 2d visual view of the main differences.

Answering which is better, is like asking how long a piece of string is. Some are of the opinion that SCT's are optically better than Mak's, others (like myself) are of the opinion they are not. It's really more about what do you want to do with the telescope, not about which is "better". So I prefer to stand by the statement, they are similar, but different. Lens and surface quality will be a major factor in the overall quality of the tube.

You'll be hard pressed to find large Mak's because of the lense weight at the front of the scope, so you'll see a lot of small sizes. Hence why you'll commonly see a SCT up to 14" or bigger.

[toryglen-boy]

Thanks for that Mark, but i kinda knew all that :thumbsup:

i was really after what are the differences to use, and what the slight optical differences do to performance, eg. does one have a flatter field than the other, or does one have a wider field of view than the other

that kinda thing

thanks anyway !!

[Geoff45]

It's almost impossible to generalise. There are many designs that all go under the name of either Schmidt-Cass (SC) or Maksutov-Cass (MC). Are all surfaces of the MC spherical or not? Is the secondary of the MC a silvered part of the meniscus corrector or a separate mirror placed between meniscus and objective mirror? etc etc.
The original Schmidt camera was designed to work with a spherical primary and was coma free, but this makes the focal point is inaccessible for visual use, so compromises have to be made, which introduce some coma and astigmatism. Different designs make different compromises.

If you can lay your hands on it, there is a great book by Rutten and Venrooij called "Telescope Optics-a comprehensive manual for amateur astronomers" which describes just about every type of telescope design. ($69 at Bintel)
Geoff

[Satchmo]

The important difference is that Maksutov cass. use all spherical surfaces whislt the SCT has an aspheric corrector plate and hyperboloid secondary. More likely to have a smoother more accurate wavefront in the Mak, and being all spherical the smaller ones can be manufactured more cheaply. On the downside the Maks are usually F12 or slower which is a bit of a narrower field than the SCT. You may be able to get a visual tele-compressor for both.

[Wavytone]

From a practical standpoint:

1. Focal ratio. Almost without exception, the commercial SCT's are all F/10. if you are happy with that, fine. If not, look at the commercial maks on the market - they range from f/9 to f/20 depending on manufacturer and aperture. If you include mak-newts, down to f/5.

if you are interested in high magnification for moon and planets, few scopes can beat a good f/15 Mak with a small secondary.

2. Optical quality. SCT's are a bunch of compromises. So are the cheap chinese maks. But then there are Maks from Intes Micro and above all, Questar which optically are about as perfect as theoretically possible. You simply won't get that kind of image from a Celestron or Meade SCT.

In this area you do get what you pay for.

You can get above average optics in a cheap chinese mak, but count yourself lucky if you do because there are some fairly awful ones too.

3. Dew and tube currents.

SCT's have a thin flat corrector with lower thermal inertia, whereas Maks have a steeply curved and thick corrector which takes far longer to cool down. Each poses problems - the SCT will cool down faster but also dews over faster; heating the corrector is a must and on a bad night the dew will win no matter how much juice you pour into the heater.

A large mak corrector is much more resistant to dew - mine usually goes for several hours without heating, but Maks seem to be more prone to tube currents inside the OTA and the way to beat that is to use a tube fan to cool the interior quickly.

4. Size and weight. For a given aperture, Maks are a LOT heavier than the corresponding SCT due to all the extra glass up front, and tend to be longer as well.

5. Aberrations. There are several variants of Maksutov (Gregory, Rumak and others) not to mention the ones with sub-aperture correctors. Similarly there are some variations among SCT's - the bog standard original design, Meade's ACF or the Celestron Edge version.

Among maks, the Gregory design was all-spherical but there are recent designs where the primary mirror is no longer spherical.

The variants have different characteristics and the best one for you depends to some extent on whether you are a visual observer or want to do "imaging" (dreadful word IMHO). For the former, image quality close to the axis is paramount, field curvature is not a big deal. For the latter, image curvature and sharpness well off axis are the priorities and some softness on axis is a necessary compromise - a design optimised for photography is not necessarily diffraction limited on-axis.

6. The back on the OTA and focusser. Some maks have the same back commonly seen on 8" Meade and Celestron SCT's. Some don't - so be careful to check this as the ability to screw things on the back may be quite limited. Some have mirror-shift focussing like the SCT's, some have a fix mirror and a Crayford focusser on the back of the OTA. Again the best choice depends on how you intend to use the scope.

[Paul Haese]

The new Edge HD SCT's and ACF's are very good for imaging. Virtually no coma and nice flat field, but do suffer from dew issues.
SCT's generally are great for planetary imaging. Noted greats are the C14 and C9.25.

Maks do tend to give very pin sharp stars on the other hand and some of the more expensive ones are quite nice for imaging. I think Eddie Trimarchi (spelling?) is using a Mak but of the more expensive variety.

Large Maks are untested for planetary imaging in general but I would say they could be quite good. Cost factor is certainly an issue with large Maks.

Visually both scopes are dim but the SCT's are the way to go.

If I was going down this path I would suggest either of the new SCT's over the more costly Maks.

[AlexN]

I've had a C11 SCT and currently have an Intes Micro 7" Mak, I've not tested the Mak yet, but the SCT's views once properly cooled and collimated rival any of the beautiful APO's i've owned. For planetary work, the SCT has stomped all over any of the other scopes I've owned.

Both the SCT and Mak suffer greatly from dew issues, due to the fact that they have a large plate of glass right at the front of the tube that cools below ambient much faster than...

The higher quality Mak's have views that rival large, but the price is hellish... The 7" Intes Micro Mak is $2500USD. Around $2800AU before shipping from the US, and import tax etc. Call it $3200AU by the time its at your doorstep... About $200 shy of the cost of an 11" SCT in Australia...

What would I do, well, I had an 11" SCT and I now have a 7" Mak... For deep sky imaging, the Mak's usually have a flatter field, and the more expensive ones generally have better optics, but dollar for dollar, the SCT's are way out in front I think. The fact that you can get an 11" SCT for the same (close enough) price is reason enough to go for the SCT most of the time.

[AlexN]

Well - Sort of... Roland at Astro-Physics has done a bit of planetary imaging with an Astro-Physics 12" Mak. Results were good, however he is hardly as accomplished as others in the field of planetary imaging (capture and processing) I think in your hands the 12" AP Mak could be insane! I would imagine, knowing what Rolands optics are like, the 12" AP Mak would decimate any mass produced SCT for sharpness and optical quality.

[Paul Haese]

So long as I cooled the monster I am sure I could obtain some really nice data at some point. The massive cost though is gonna put a dampner on it though. Unless he would like me to test one for planetary imaging. Say over a year or so time frame. :lol: Maybe in my wildest dreams.

[AlexN]

If I had the money I'd sponsor it just to see the results!! :) Imagine that, a peltier cooled 12" AP MCT with a Luminera camera firmly planted in the rear and a pro at the helm.... droool..

[Wavytone]

There are plenty of comparisons made over the past 40 years that show a well-made f/15 Mak easily outperforms an f/10 SCT of similar aperture when it comes to image quality (Strehl or MTF) for planetary imaging - when side by side in the same seeing.

In terms of bang-per buck, the SCT's win as noted previously; an 11" SCT will slaughter a good 7" Mak costing much the same.

But if cost is irrelevant, and the comparison is based on equal aperture, the Maks will usually prevail.

There remains a very valid question, however: would I prefer to be lugging my 7" Mak or an 11" SCT next Saturday night.
I don't know the answer, to be perfectly honest.