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If you halve the F number you can use 1/4th the exposure time
eg an F10 tha reaches the sky background limit in 1 hour, an F5 will do so in 15 minutes.
However, go faster then about F6 and coma starts to appear as elongated stars at the edges. faster f ratio = more coma. Thankfully Baader Planatarium boght out an excellent coma corrector that unlike the even more expensive televue parracoor, virtually eliminates all coma in systems down to F4
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