Re: 2" 90° Apparent Field Eyepieces Announced

From: Tom Davis (tdavis11_at_carolina.rr.com)
Date: 07/02/04 

Date: Fri, 02 Jul 2004 15:27:58 GMT

"> Don Dilworth demonstrated that its possible to build a 90deg. eyepiece
> with extraordinary performance. See U.S. Patent 4,720,183. The
> designs published in this patent are among the highest performance
> eyepieces I've ever seen. Much better than even the published Nagler
> designs. No ED or high-index lanthanum type glasses are needed. The
> only drawback is size. A 29mm focal length would have some elements
> as large as 4" in diameter. Below is the patent prescription for a
> 15.9mm Dilworth:

I remember Don's eyepiece. I got a chance to look through it
at Stellafane some years ago. The issue with it was not one
of performance. It performed quite well. It was that it would
cost to much to produce commercially. I felt sorry for Don
because he had done a tremendous job in designing the
eyepiece and producing the prototype. It just would have
been too expensive to produce. What you would save in
not using expensive glass would be lost in the difficulty of
producing the glass curves commercially, at least at that time.

As to size, that was not an issue at the time. The longest FL
Nagler was the 13mm Type I at that point. The Dilworth
design allowed for a longer focal length to be produced
within a reasonable size. After this Al came out with designs
that finally allowed for a 31mm FL to be practical. The Type
II Naglers were produced right after the Dilworth, and
allowed for somewhat longer focal lengths (the 16mm and
I believe the 20mm). The bar has been raised a bit since that
time.

Thanks, Tom Davis

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    ... > eyepiece sounds very attractive. ... with extraordinary performance. ... See U.S. Patent 4,720,183. ... designs published in this patent are among the highest performance ...
    (sci.astro.amateur)