Re: Keystone achromats

[P3D Alan Lewis <dlewis@xxxxxxxxxxxx>]

Peter Abrahams asks:

>shows that the doublet is .88" thick.  Can any reader inform me as to an
>optical basis for the use of such thick glass?  A color corrected >doublet
>is not usually so thick.  This is a short focus lens, but in general the
>thickness of the glass apart from the steep curvature of the surface >does
>not serve an optical purpose.  The picture also shows a third element, >to
>add prism for divergence, similar to a simple stereo viewer.

George adds:
>I think the answer is that this is cut in half out of one huge achromat
>(mother of all achromats?).  But I'll let Alan Lewis fill in the details


I can't add to that, he's right.
The Keystone achromat is made from the equivalent of a 70mm dia. single
achromat lens.  That's a big achromat, and may explain the thickness.

The single lenses shown in the web photograph are standard Holmes
prismatic lenses for comparison, they are not part of the achromat lens
system.  The thick achromats pictured are in fact prismatic by
themselves.
-- 
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Alan Lewis
mailto:dlewis@xxxxxxxxxxxxx
Home of the stereo wedding package, and weekend stereo projects
http://www.arlington.net/homes/dlewis


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