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CC, SC and MC ... what is the main difference?

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#1 BromoGNU

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Posted 17 April 2021 - 01:05 PM

I was curious what the Schmidt-, Classical-, and Maksutov- Cassegrain telescopes brought to the table?  The strengths, weaknesses, and the type of observations they best support.

Looking at what's commercially available, seems like things tend from f/10 to f/15 or so so all are somewhat longer than typical current Newtonians & Refractors.

But specs only tell half the story, what do you all know about this?



#2 havasman

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Posted 17 April 2021 - 01:17 PM

Looking at what's commercially available, seems like things tend from f/10 to f/15 or so so all are somewhat longer than typical current Newtonians & Refractors.
 

Longer in focal length but with generally shorter package sizes.

 

https://www.telescop...BLE_OF_CONTENTS

Sections 8, 9 and 10 give the most complete explanations of scope types.


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#3 Mr. Pepap

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Posted 17 April 2021 - 07:04 PM

telescope-optics.net is your go-to friend for any technical telescope-related questions. It's basically a big dictionary of everything you could possibly know, or want to know, about telescopes. It goes into excruciating detail about nearly every conceivable topic with fancy diagrams and the like, the kind of thing us engineers drool over. Check it out! 


Edited by Mr. Pepap, 17 April 2021 - 07:08 PM.


#4 Tony Flanders

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Posted 17 April 2021 - 08:59 PM

I was curious what the Schmidt-, Classical-, and Maksutov- Cassegrain telescopes brought to the table?  The strengths, weaknesses, and the type of observations they best support.

Looking at what's commercially available, seems like things tend from f/10 to f/15 or so so all are somewhat longer than typical current Newtonians & Refractors.

But specs only tell half the story, what do you all know about this?

The Cassegrain design and its variants have inherently long effective focal ratios. That's because in a Cassegrain the secondary mirror has negative curvature, and therefore acts very much like a Barlow lens -- a lens with negative curvature. The net effect is to take a fairly fast primary mirror (typically f/2 for commercial SCTs) and turn it into a system with a much longer effective focal ratio. All things being equal, that's a bad thing, because it reduces the telescope's maximum field of view.

 

The main virtue of the Classic Cassegrain is that you end up viewing from the back of the tube rather than the front, as with a Newtonian. That's not necessarily beneficial for small scopes, but when you get to very large apertures it eliminates the need for a ladder.

 

SCTs and Mak-Cas telescopes are compromise designs. They (usually) have spherical mirrors, which are much easier to figure than parabaloid mirrors, and use a strangely shaped corrector plate (for the Schmidt) or thick meniscus lens (for the Mak) to compensate for spherical aberration. The Maksutov variant in particular has much less coma than a Newtonian with an equally fast main mirror -- which is to say, with an equally short tube. The main virtue of SCTs and Maks for most amateurs is their very short tubes.


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#5 sevenofnine

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Posted 17 April 2021 - 09:06 PM

Ed Ting (a guru for me) of scopereviews.com has many reviews and videos of these and other scopes. It's an excellent website! Perhaps a good counter point to Mr. Pepap's more technical recommendation. In general, what I like about my 5" Maksutov is it's compactness for it's focal length of 1540mm. That's equivalent to a 5' Newtonian tube! So lots of power in a very small package. Downside is it's slow f/12. That means it's best on bright objects like the Sun (filtered), Moon, planets, stars (doubles) and brighter DSO's like the Orion Nebula etc. On it's tracking Go-To mount it makes a perfect patio scope. waytogo.gif



#6 aeajr

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Posted 18 April 2021 - 10:21 AM

Different types of Telescopes
https://telescopicwa...-of-telescopes/

 

The MCT, SCT and similar scopes have inherently longer focal lengths, but it smaller packages so they are easier to transport and place less stress on the mounts.

 

They are more optimized for higher magnifications.  Not well suited to low power wide views.  But all telescopes have the same function, gather light and present it to the eyepiece or to the sensor.  The various designs just do it in different ways. 

 

The F10 to F15 focal ratios make them easier on eyepieces in terms of edge distortion, but they bring their own challenges when it comes to image quality.

 

All optical systems are compromises.   I have two refractors, two Newtonians and a Mak, and I use them all.   I have another refractor on order.


Edited by aeajr, 18 April 2021 - 10:25 AM.

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#7 Dave Mitsky

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Posted 18 April 2021 - 11:33 AM

While other variations exist, commercial catadioptric or compound telescopes typically employ either the Schmidt-Cassegrain or Maksutov-Cassegrain design.

https://optcorp.com/...s-of-telescopes

https://www.highpoin...AiAAEgKVAvD_BwE

https://starizona.co...68092e59f&_ss=r

There are also Schmidt-Newtonians and Maksutov-Newtonians.

https://starizona.co...hmidt-newtonian

https://telescopegui...ate-astrograph/

 

The classical or classic Cassegrain design consists of a concave parabolic primary mirror and a convex hyperbolic secondary mirror.  There are a number of variations of the classical Cassegrain.  These include the Ritchey–Chrétien, Dall–Kirkham, and Pressman-Camichel Cassegrains and off-axis designs such as Schiefspieglers and Yolos.  Modern observatory telescopes employ the Ritchey–Chrétien design.

 

https://www.britanni...grain-reflector

https://starizona.co...f283d039b&_ss=r



#8 Hesiod

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Posted 18 April 2021 - 11:37 AM

The current commercial Cassegrains are for the most designed to fit a somewhat long focal into a compact packaging.

The Maksutov and Schmidt "variants" resort to a full aperture lens, placed at the front end of the tube to address spherical aberration caused by the fact that, unlike the Cassegrain reflector,  use a set of spherical mirrors.

In the years the so-called "sub-aperture correctors" have been also employed: here the correcting lens is placed somewhere inside the tube (e.g. in the VMCs or Klevtsov lies near the smaller secondary mirror), which is therefore open as those of Cassegrain reflector.

Beside the spherical aberration the correctors have also been employed to get rid of field curvature and other aberration, especially across a wide field required by modern cameras: a notable case is that of Celestron's EdgeHD design, which adds another lens assembly in a sort of Schmidt-Cassegrain layout (so you have a system made by two mirrors and two lens assemblies, one being the single Schmidt lens, the other the multi-lens corrector).

The main selling point is that the short tube allows to use a more compact mount compared to a refractor or Newtonian reflector with similar specs, but this does not mean that can always use a small mount.

Larger ones (IMHO from 11" upward) need pretty substantial mount and if are in the search of the best portability a truss travel Dob ends being way more portable; but if want to use a conventional mount a 12" SCT is much more manageable than a 12" Newtonian.

Another advantage is that the Cassegrain and its variant has the eyepiece at the back and can use a conventional stardiagonal, which is pretty handy if are using a German or fork equatorial mount.

 

As for the kind of observations, IME the smaller ones (e.g. C8) are quite versatile but of course were never meant as "rich field telescopes".

The long focal makes easy to attain good magnifications and therefore these telescopes are widely used to observe small objects, be them either planets or deep sky stuff; on the other hand the available eyepieces hampers somewhat the possibility to get very large exit pupils*, so may struggle on targets with very low surface brightness and with nebular filters.

Personally have a C8 as my largest aperture, two small MCTs (IM M500, SW 3.5") and the VMC95: I use the smaller ones mostly for deep sky objects, and the C8 for basically everything

 

*most Cassegrain variatns focus by sliding the primary mirror and therefore their focal length is not constant. Namely, if the stated focal is attained w/o stardiagonal, you may figure an increase of ca 200mm with a 2" "SCT-type" stardiagonal



#9 Dave Mitsky

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Posted 18 April 2021 - 11:46 AM

The main virtue of the Classic Cassegrain is that you end up viewing from the back of the tube rather than the front, as with a Newtonian. That's not necessarily beneficial for small scopes, but when you get to very large apertures it eliminates the need for a ladder.

That virtue certainly comes in handy at times.

Here's a photo of the Naylor Observatory's 17" f/15 classical Cassegrain.

 

Attached Thumbnails

  • Naylor 4-13-21 17-inch IMG_3309 Processed Resized 1000.jpg

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