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Old 14-08-2006, 02:45 AM
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Don Pensack
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Changes with back-focus

Quote:
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
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
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