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In an f/6 Newt the longest useful f.l. eyepiece will be 25-30mm.
With a 40mm... the exit pupil will be too big for your eye, cutting off light equivalent to losing the outer inch (25mm) of your mirror, making it work like a 6" f/8 with an oversize secondary.
it is for this reason I shelled out for a wide field 30mm, and not a 42mm.
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