Difference between a Mac and SCT scope

[nickd]

Hi everyone,

Just getting started and using binoculars at the moment but exploring the possibility of acquiring a telescope.  This forum has been a great help already.  

I have read a few threads discussing Mak's and SCT scopes.  I am aware of how a Schmidt Cassegrain works in theory but how does it differ from a Mak, assuming it does of course.

Basic stuff I know but have to start somewhere.

Thanks for any help,

Nick.

[michael.h.f.wilkinson]

These scopes differ mainly in the way they address the same problem: the aberrations caused by spherical mirrors. Bot hfeature a corrector: spherical meniscus lens for the Mak, Schmidt corrector plate in the SCT. Most SCTs are built at F/10, with F/2 primaries, most Maks are slower, with slower primaries (but all sorts of exceptions exist). Slower primaries can yield smaller secondary obstruction, and less field curvature, at the expense of a longer, heavier build, and smaller field of view (FOV). Maksutovs are generally heavier than SCTs anyway (the Skymax 180mm is 9kg, the C8 (203mm) is 4.7kg). This is why I chose the SCT route: ease of transport. Light weight often also corresponds to faster cool down. 

Note that optical figure is more important than the precise optical design. A well-made Maksutov (Intes, APM) will take an SCT of poor quality to the cleaners, and vice-versa. Both designs yield compact scopes compared to their aperture, with comparatively large central obstruction (compared to say a Newtonian of the same focal ratio). I do not mind the central obstruction too much, others do.

[rowan46]

Its the design of the front corrector thats the main thing I am not sure of the exact maths but basically it works out that the front lens is thicker on a mak than on an sct so that  with the larger ones particularly it takes longer to cool down. However because the mak has a longer focal length  and smaller central obstruction it gives better contrast on planets. both types of scope If they have enough apparture are good on dso's but they will have a very narrow field of view. which really makes them prime candidates for needing tracking mounts.

[nickd]

Thank you for taking the time with such in depth answers.  I deduce then that a Mak and SCT of comparable quality will perform similarly.  Regarding cool down time, is this a big issue for you guys using these scopes?  What length of time from house to typical evening temp around 5 degrees C?

Michael, you mention the larger central obstruction.  Other than reducing aperture, does this have any other visual side effects? 

Thanks,

Nick.

[John]

Just to confuse things a little further, there are Maksutov-Cassegrains and Maksutov-Newtonians. The former is the design discussed above. So it's better to refer to them as Mak-cass or Mak-newt to avoid confusion !

The cooldown down time for  Mak-cass is usually a bit longer than for an SCT which in turn is longer than a newtonain or refractor. Big mak-casses (7" or more) can take 2 hours or more to fully cool.

[michael.h.f.wilkinson]

I can typically start observing within about half an hour after setting the scope up, but my SCT is stored in garage which is not heated very much. If there is a lot of difference between inside and outside temperatures, an 8" SCT will be usable after 45 minutes to an hour.

The central obstruction reduces contrast. It is not the bugbear many make it out to be, but a slow Newtonian with e.g. 20% central obstruction by diameter (4% reduction in light throughput, or about 0.044 magnitudes) is almost indistinguishable from a scope without central obstruction. With the 34% C.O. by diameter my SCT loses 11.6% light or 0.133 magnitudes, but clearly has less contrast than my 80mm triplet refractor. An 8" triplet refractor would be better, but costs an arm and a leg (and be unwieldy, and it loses light through transmission losses in glass itself).

[nickd]

Believe me John, I could not be more confused!  Having said that the basic differences make sense.

Michael, given the various compromises involved with SCT and your impressive array of kit, what do you like to use the C8 for? I.e. what does it give you the other equipment does not?

Thanks,

Nick.

[ChrisLX200]

Tha Maksutov was a design aimed at eliminating the multi-curved corrector plate used in an SCT, the double-spherical meniscus (with silvered patch on the inner suface) in the Mak is much easier to manufacture. These days SCT corrector plates are made on automatic machines and don't need to be hand-figured like they used to, so the manufacturing difficulties is a moot point and you can choose the optical system most appropriate to your needs. Look at the relative focal ratio of each to decide which you need - Maks generally good for planets, Mak-Newts for wide-field, SCT is a do-all design reasonably good at most things but not specialised for anything.

ChrisH

[cantab]

An SCT uses a corrector plate that appears flat but actually has a complex curve, a Mak-Cass uses a deeply curved corrector plate.

One key consequence of the Mak design is that the secondary mirror can be simply a silvered spot on the inside of the corrector, while SCTs need a separate secondary. This saves complexity and presumably cost, and makes the Mak design good for smaller scopes. (You do get some Maks with a separate secondary, like an SCT.) Celestron do a 2 inch Mak-Cass spotter, I don't know of any SCT that tiny.

On the other hand the heavier corrector plate and longer cooling time of the Mak design may account for why you don't tend to see very large Mak-Casses.

[nickd]

Thanks for all the info.  An education as always.

Cheers,

Nick.

[John]

 Mak-Newts for wide-field

Actually, despite the relatively fast focal ratio, mak-newts are superb planetary and lunar scopes, second only to fine apochromats. They do low power / wide field pretty well too but their very small secondary mirrors can mean a loss of light around the edges of the field of view when used with eyepieces with large field stops. I'm talking about visual use here as I don't image.

Sorry Nick for a further snippet of information 

[michael.h.f.wilkinson]

I use my C8 for most of my nighttime observing, except for really wide-field work (bins and the APM 80mm are stars there). Most of my 400+ galaxies were found with the C8, along with most of the planetaries. It is also a good planetary scope. The key reason I chose it was its portability. Mak-Newts are optically somewhat better, but they were not affordable (or indeed readily available) when I bought my C8 18 years ago. Besides, a comparable Mak-Newt is much heavier and larger, requiring a much heavier mount.

[John]

Virtually all 8" scopes are heavier and larger than an 8" SCT. For a brief time I had an Celestron Nexstar 8SE last year and though the single arm fork mount is probably on the limit at times I was pleasantly surprised how light and portable the whole scope was, even more so than an 8" dob     

[nickd]

Thanks guys.  Very enlightening.  Will try and get a look through one in the near future.  

Nick.