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Casses vs. RC

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#1 symbiosis

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Posted 24 March 2019 - 09:01 AM

A general question: If SCTs and Maks take a long time to cool down because they are closed systems, why couldn't they be open, like a Ritchey-Chretien? Or alternately, why aren't RCs also closed?


Edited by symbiosis, 24 March 2019 - 09:07 AM.


#2 Darren Drake

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Posted 24 March 2019 - 09:12 AM

The design of SCTs and mak casses require a front optical element.  Therefore it's not possible to have an open air system.   RCs on the other hand do not have a front optical element and are open by design.   



#3 RalphMeisterTigerMan

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Posted 24 March 2019 - 09:29 AM

Vixen has an open tube design because their Modified SCT uses a spider assembly and a 2 corrector system in front of the secondary. This is tricky to do. An RC does not require a corrector lens accept for any field flatteners in-front of the focuser and behind the primary.

 

Good-luck.

RalphMeisterTigerMan



#4 Mark Costello

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Posted 24 March 2019 - 09:44 AM

A general question: If SCTs and Maks take a long time to cool down because they are closed systems, why couldn't they be open, like a Ritchey-Chretien? Or alternately, why aren't RCs also closed?

 

Hi, symbiosis: I'll respond to your second question, why aren't RCs closed.  First, a SCT and Mak has a lens up front to slightly bend incoming light rays.  This compensates for "errors" produced by the two mirrors so that ideally, the light comes to focus at the eyepiece.  This design allows the SCT and Mak builders to make simpler spherical mirrors.  It also closes the tube assemblies.  On the other hand, the mirrors of a RC have more complicated shapes (hyperboloids) so that a correcting lens up front isn't needed.  That way, the RC tube is open.  A similar approach is taken in making Cassegrain telescopes such as those being rolled out by Astronomics.  In that case, the primary mirror is a paraboloid while the secondary mirror is a hyperboloid.  Hope this helps.


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#5 macdonjh

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Posted 24 March 2019 - 11:13 AM

It is possible to make an open SCT or Mak, simply replace the OTA tube with truss poles.

 

However, the question would be why?  The biggest commercially available Maks are 8" or smaller (yes, I know, there are larger exceptions).  A scope that small doesn't leave much room for truss poles.  The same applies to many SCTs, which can be had as small as 4".  Perhaps it would be worthwhile to try with a 10" or larger SCT, but with the tooling Meade and Celestron already have it's so much easier for them to cut a piece of aluminum tubing square.

 

I think there's a thread here where a Cloudy Nights member took a C14 optical set and made a Dobsonian SCT out of it.  But perhaps he put the glass in a wooden closed OTA.



#6 carolinaskies

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Posted 24 March 2019 - 04:39 PM

I have to laugh... there are advantages and disadvantages to open and closed tube systems.  The main advantage of open systems is air currents are not trapped to create unwanted effects. As aperture increases a secondary benefit can-but-not-always-be decrease in weight.  However most truss RCs are typically heavier than the same size SCT.  

There are two major disadvantages to an open tube system, 1- stray light interference from off axis light sources... 2- foreign contaminates on optical surfaces.  Anyone with a truss tube system whether RC or Dobsonian is going to be subject to dust and other things possibly finding their way onto surfaces.   A fabric shroud can help with the first disadvantage and somewhat with the second.    

The closed tube system also can have advantages if designed well.  A built in fan (like on the 7" Meade Mak or 16" Meade SCTs) and baffling to reduce stray light within the tube.  

A corrector up front has no bearing on having an open or closed tube from a performance perspective, you could literally perforate the OTA of any SCT or Mak if you had the desire without effectively changing the optics whatsoever.    



#7 symbiosis

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Posted 24 March 2019 - 05:39 PM

These explanations make sense, but couldn't the front corrector be in a "secondary cage" with an otherwise open tube, ala a standard truss Dobsonian?

The design of SCTs and mak casses require a front optical element.  Therefore it's not possible to have an open air system.   RCs on the other hand do not have a front optical element and are open by design.   



#8 Hesiod

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Posted 25 March 2019 - 06:30 AM

It could, but the scope would be unnecessary more expensive to build.
SCTs and MCTs are usually the result from compromises, cost being one of the most important parameter.
In the Newtonian design, the flat is a rather "trivial" component; in a SCT or MCT, both the front lens and the secondary mirror are instead "active" elements


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