These posts have focused only on focal length and aperture reduction.
Not mentioned has been the change in spherical aberration that occurs in an SCT when you add back focus.
Assuming that the scope is perfectly corrected for SA when the mirrors are spaced for a back focus of 100mm, then every 25mm of additional back focus will increase the SA by 1/23rd of a wave.
This is not important for changes of 50mm or even 100mm, but binoviewers are an exception in that unless great care is taken to keep the light path short, the SA can indeed become sufficient enough to degrade the contrast transfer.
This is my recommendation for anyone that wants to do high resolution observing with a bibnoviewer in and SCT.
Learn to star test.
When you have your binoviwer configured, use just enough amplifications (GPC or Barlow) to perfectly offset any SA induced by the binoviewer.
I do not know this to be the case, but logic tells me that it is: Roland Christen says that a GPC sould ALWAYS be used with binoviewers (and for plantery use, I concur).
Now I do not think it is at all just one of those random chances that Christen ships the Mark V with a mirror diagonal and that he has designed a 2.6x GPC.
The question for most people that took the time to think about it would be "Why does he make the 2.6x GPC, when most telescopes will reach focus using a 1.25" diagonal and the 1.7x GPC?
Well, I think the answer to that question is how much the effective light path of the T2 mirror and the Mark V binoviewer results in.
If you look at the Maxbright Owner's Guide, it shows that the effective light path for the Mark V and do the math, what you will find is that using the 2.6x GPC with the Mark V and the T2 mirror diagonal, the effective light path works out very close to 100mm.
In other words, when you use a Maxbright or Mark 5 with the T2 mirror diagonal in an SCT, the light path lengthb places the mirror spacing pretty close to what it would be if you use the factory visual back and 1.25" prism diagonal.
Anyone that thinks this is pure serendipity has not played with path lenght and SCTs.
Christen is a brilliant optical designer and since he designed the GPCs and he specifies the mirror in place of the Ziess prism, one has to seriously consider what his rational for these decisions was.
For general use, most of this stuff does not matter.
If one wants to get the highest possible contrast from their SCT, then one should pay very close attention to these kinds of details.
It is not just about aperture and focal length. Contrast performance of an SCT can also be affected.
My advice for years now has been to always Barlow or GPC up an SCT for planets and always use a Baader GPC for fast refectors if one wants the absolute best performnace possible for high resuliion observing.
For everything else, none of this is all that critical and you should just use what makes you happy.
If though you want the best possible contrast from your SCT, use it with a back focus that produced the best possible SA correction and assuming the scope has good optics to start with, then the system that will do this in a standard SCT is the T2 mirror, the 2.6x GPC, and a good binoviewer direct attached to the top of the T2 mirror diagonal.
Or... Just use what you like. Often these small differences will be to small for a non critical observer to be concerned over. No one should agonize about 1/8th wave of SA change or a couple of millimeters of aperture loss if you re not extremely demanding in your system expectation. For all but planets, I learned to make a lot of compromises.
Edited by Eddgie, 12 December 2016 - 02:04 PM.