I might add this weird fact. Assuming that the SCT starts with perfect SA correction, if you use back focus past the nominal back focus (100mm for all but the C14), the SA of the system increases. Again, between a 1.25" diagonal and a 2" SCT diagonal, the difference is too small to be easily seen.
However I think the increased s/a is probably more noticeable in the smaller SCT's than it is in the larger ones.
That's why I don't understand why add on crayford focusers are so popular.
OK maybe to cut down on the mirror shift while doing photography, but I have seen plenty of add on focusers with 2 inch diagonals in them...
While some might think that this would be the case, it is is not. The 1/23rd wave per inch is uniform for any apeture size that uses the same forumla (f/2 primary and -f/5 secondary). It is not affected in any meaningful way by apeture change. A C11 will have a 1/23rd wave SA change for every inch of back focus change.
Here is a quote from the website Telescope-Optics:
"The relation implies that, for nominal primary mirror shift ∆, error induced by extending back focus is independent of aperture. It only depends on secondary magnification and primary's focal ratio. For F1=2, m=5, and K2=0, every mm of reduction in mirror separation (∆=1), or nearly for every inch of focus extension, induces ~1/23 wave P-V wavefront error of over-correction, and as much of under-correction for widening the separation."
You can find this qoute in the full context at this link, forth paragraph down:
Again, I get why people do all of the things they do. My goal here is not to tell people how to use their gear.
My goal is to help them understand why their C8 may not have compared well to their 5" APO. If you want to get the best performance possible out of an excellent C8, you have to use it with a light path lenght that results in the best SA correction. The SCT is starting with a big handicap in the form of a large central obstruction, but if it is an otherwise excellent example, it should outperform a 5" APO on planets. But if you mix in a 1/6th wave of SA, it does not stand a chance against a good 5" APO. Maybe that SA is ground into the glass, but because almsot eveyone uses 2" diagonals and long visual backs, they could be inducing SA that is not present when the scope is configured for optimal SA correction.
And again, by using the star test, even if the scope is a bit undercorrected, it is possible to find the setting that completly nulls any SA that is present.
That is actually a very powerful thing.. You can actually fix SA if it is present in an SCT simply by changing the focus point.
It is a good bet though that many people seeing a bit of SA in their systems may be seeing it because they are not set up with optimal mirror spacing.
I read how bad SCTs are all the time, but my experince does not really mirror that. I have had great planetary results using many SCTs (not all were excellent though). I have been advising for a long time that the best thing that somone can do with an SCT to get the best performance out of it is to keep the light path as short as possible. While it is true that optical qualiity does vary, and other optical issues are almost always present, SA is perhaps the most damaging optical aberration, and a little SA does more damage than a small central zone for example. Most SCTs have a combination of small defects, but SA is usually decent, and again, if you now what you are doing, you can tune SA out....
But people want a wide field form the scope, and that is a good tradeoff (though to me, you are far better off getting a different kind of telescope if wide field is your goal). Testing an excellent C8 against a 5" APO though, and calling it a looser without ensureing that it is working at best SA correction is pretty unfair to the design. Not exactlly "Snow tires on a Ferrari", but more like "Bald tires on a Camry." Push it hard around corners and it is going to squeal.
To respond to your post simply though, the above page will explain why this is a function of mirror speeds and does not change in any meaningful way based on apeture. f/2 and -f/5 gets you 1/23rd wave SA change for every inch of back focus change. You do as much damage by putting a 2" diagonal and crayford focuser on the C11 as you do on the C5. Just the way it works out, becuase both use an f/2 primary and - f/5 seconary.
Edited by Eddgie, 13 October 2014 - 09:12 AM.