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Old 18-09-2019, 06:16 AM
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Don Pensack
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Join Date: Aug 2006
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Quote:
Originally Posted by Stonius View Post
Its a pity all manufacturers dont publish these. *Do any other manufacturers publish this data?

It's also interesting to note, that
1) if it is indeed true that different types of telescopes have different focal plane curvature, that...
2) must be matched to the curvature of the eyepiece used, then...
3) given that the XW's were originally designed for the pentax spotting scopes,
4) half of the range must have been not fit for purpose, by design.

Which doesn't seem right to me. Given the luxury of custom designing high quality eyepieces to suit a particular telescope, why would they design half their range 'wrong' for that purpose? It doesn't make sense.

Markus
Even Pentax no longer publishes this data. My link used the Wayback Machine to find a link several years old and long deleted from the web.

As for field curvature and sign--shorter focal length eyepieces cover smaller portions of a curved focal plane in a telescope. The Earth is curved, but 1m of ground can be quite flat. So it is more important to match the field curvature of the average scope with long focal length eyepieces than it is with short focal length eyepieces because the long ones see a larger portion of the scope's focal plane, and see more curvature.

Note that the longer focal lengths of XWs also have fewer elements than the short ones, so there is a greater freedom of design in the shorter eyepieces.

I can't speak for compound catadioptric scopes, but other forms of commercial amateur scopes have positive field curvature at the focal planes, so one would normally have expected the longer focal lengths to fare well in those scopes. Note, though, that short focal length refractors have a lot of field curvature, with a radius of curvature about the same as 1/3 the focal length, whereas newtonians have considerably flatter fields, both because of the longer focal lengths, and because their radii of curvature matches the focal length. An eyepiece that appears to have a flat field in one might not appear flat in the other, even if the field curvature in the eyepiece matches the scope's sign.
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