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Hello,
I haven't stumbled across a formula, but it seems that on nights of very good seeing, working anywhere from F20 to F40 can produce excellent results. I tend to operate my C9.25 F10 at F25 using the TeleVue x2.5 PowerMate when the seeing is better than 6 or 7. This might happen say, 6-12 times per year.
I can operate my Vixen 4" F9 refractor at F45 on nights of exceptional seeing using the TeleVue x5 PowerMate. This might happen say, 3-5 times per year.
I have used the TeleVue x5 PowerMate on the C9.25 to give F50 for imaging the Trapezium using my ST7E CCD camera but this was really pushing the C9.25 way too far. I’ve only tried this a couple of times and got away with it due to the sensitivity of the ST7 and the good tracking of the Tak mount, plus a lot of help from deconvolution and stacking tools in image processing software.
Cheers
Dennis
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