Re: ATM More Spheroidalage

[Bob Goff <goffaxe@azstarnet.com>]

I have found in the past that even class 25 ball bearings have way too
rough a surface to be useful in high contrast observations. I have found
that polished glass and quartz balls that are appropriately coated give
excellent images even under close scrutiny.

Bob


At 07:16 AM 08/24/2001 -0700, you wrote:
>
>>I have been considering the case of the
>>1/10th wave parabolic primary combined with a spherical
>>versus hyperbolic secondary in a classical Cassegrain design.
>
>Dominic,
>
>A year or two ago there was an article in S&T which presented an ultra-high
>magnification cass.  It used an f/typical paraboloid and a ball bearing
>sized secondary.  As I recall there was some controversy over a patent
>application.
>
>This over-corrected Dall-Kirkham design works as a slow system but suffers
>tremendously, as you would expect, from off-axis stig and field curvature.
>As a result, tiny tiny tiny fields are useful.  Like, 3 arc minutes and
>less with magnification ratio's in the 100s and barely more that this with
>magnification in the 10s.  Still in a Nasmithe configuration, a ball
>bearing convertible might be interesting.
>
>As for more conventional magnification ratios, I afraid you're stuck with
>either a true cass or a true Dall-Kirkham.
>
>
>Anthony
>
>
>
>
>
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