Oh duh. I see now. I'm tired.
Ok so these are good numbers to use.
Thanks again to all. However.....
I do have another question.
It seems from the info Ed gave earlier that for optimal performance out of my C8 I should use a 1.25" prism diagonal. Am I reading that right?
If your SCT has no sperhical aberration with a 1.25" prism diagonal, then the best performance at the center of the field will be when you use a 1.25" diagonal (though you can do much better than the factory 1.25" diagonal).
But the best performance at the center of the field would come at the expense of being limited to a 1.25" format, and this means narrow fields of view.
The SCT diagonal you mentioned does in fact provide one of the shorter light path lenghts for a 2" diagonal, and if your system has perfect SA correction with a 1.25" prism diagonal, then this 2" would keep you closer to perfect than a 2" refractor style diagonal with a long visual back.
And the change in SA would not be enough to matter.
My point was simply that if the system starts with excellent SA correction as it was shipped, then the further you get away from the light path lenght of the 1.25" visual back and 1.25" prism diagonal, the more SA is induced. A little SA makes no difference, but get much longer than a standard 2" SCT diagonal and the SA will increase enough that the scope my no longer be able to match a good 5" APO in performance (which an excellent C5 should be able to do).
If you want your cake and want to be able to eat it too, the shorest possible light path lengh for the C8 is going to be with the Baader 2" Clicklock diagonal.
This diagonal body thread directly on to the C8 rear port. It has a light path lenght that is almost the same as the 1.25" prism and standard visual back.
This keeps the focal lenght very close to optimal, and if the system started with zero SA, will keep the SA very close to ideal.
But again, the difference between the 1.25" digaonal and SCT 2" diagonal is very small, and most people would not see it.
When excellent C8s loose out to 5" APOs though, there has to be a reason.
To really know the truth though, you need to test the system before you do anything. If the optics are perfect with the 1.25" diagonal, then increaseing back focus lowers performance. Only a tiny bit, but lower is lower. If on the other hand, there is some undercorrection, adding a 2" diagonal might correct it.
Here is the Baader 2". For a system that comes out of the box with perfect SA correction using the 1.25" factory prism, this is the 2" diagonal that will keep it closest to perfect and still give you 2" capability.
It will also give you the widest, brightest field possible with a 2" eyepeice becasue the focal lengh will be shorter.