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Originally Posted by Dennis
Hi Graeme
Thanks - that is very interesting information regarding SA, as I have just added a flip mirror unit to the JMI motofocus on the C9.25, which further extends the back plane to a total of approx 6 to 7 inches, or 15 to 18 cms, from the bare position.
Hmm, one night I must capture some star test avi's with the SCT in the OEM bare configuration, then add the JMI motofocus and finally add the Meade flip mirror and see what results I get.
Cheers
Dennis
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The contemporary SCT design is only diffraction limited with one set distance between the corrector and primary. Moving the primary mirror from this perfect position influences more than spherical aberration--it also changes the f/ratio. Essentially, as the back-focus distance (the distance the focal plane is moved back from the back of the scope), the longer the f/ratio, and the greater the spherical aberration.
For the purely visual observer, paying attention to that would result in seeking out the shortest visual back available (some 1-1/4" ones are up to 1/2" shorter than others), and using a 1-1/4" star diagonal to shorten the back focus. I would note that this will be of greater importance to the lunar and planetary observer than the deep-sky observer.
For the deep-sky observer, the elimination of vignetting would be equally as important, and for this, the use of a 2" adapter tube with a refractor-style 2" diagonal would help (or the equivalent of the Peterson Eye-Opener), but not the thread-on SCT diagonals, which all have smaller openings. The best I've seen in answering both back-focus and vignetting issues is the TeleVue Shorty adapter and Shorty EverBright Diagonal. The use of both together results in the smallest back-focus distance available in a 2" diagonal, and the least amount of vignetting as well.
Don Pensack