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Large eyepieces...
Well we've had the thread about 120 degree field of view eypieces... here's another extreme.
I recall using an f/15 cassegrain a long time ago which had a massive old war-surplus 60mm Erfle eyepiece. The eye lens must have been in excess of 60mm and the focusser barrel was of the order of 75mm diameter.
When selecting eyepieces for a scope the lowest power eyepiece is governed by the exit pupil, this sets the maximum focal length at 5mm x the scopes focal ratio.
So if you had say a nice old-style f/15 refractor or a big f/15 Cassegrain, a 75mm eyepiece would be fine... for f/20, 100mm. Obviously there's a problem with the apparent field of view vs the size of the field stop, which is in some respects dictated by the size of the focusser barrel, so it could be time to give the focusser a thorough reaming, too, 75 or 100 mm would be about right.
I'm just curious to know if any other mass produced beasties of this scale are around, beyond the ones I'm aware of:
a) the old war-surplus Erfles, rare these days;
b) The Unitron 60mm Kellner from the 1970's, and
c) The Masayuma 100mm Kellner made for observatory telescopes.
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