Quote:
Originally Posted by citivolus
I've read 24x focal length as the minimum, but am no authority on that. I'll have to look up that book.
Eric
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I just did a quick check on Table 5-2 for you.
Artificial star distance is very dependent on the aperture and F ratio. Distance of light source ( in multiples of focal length ) can range from 10X for a 2.4" F5 refractor to 126X for a 24" F4 reflector.
This is the minmum distance to cause 1/4 wave overcorrection at the eyepiece. If you were wanting to evaluate the wavefront probably 1/10 wave or less would be a better figure with coreesponingly longer distances.
If you are just wanting to collimate your scope, or solve problems with say , astigmatism, then distance is not so important.