Re: [ATM] aperture mask /astigmatism

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

Jeff,

at "best focus", you see the mirror as deviating from a perfect 
paraboloid. The aperture mask will pick out a part of this (perhaps 1/6 
or so of the total area), and at *its* best focus, not necessarily the 
exact best focus of the full mirror, the deviation from the best-fit 
paraboloid is in general much smaller, and the Strehl ratio is likely 
very good. Likewise, in the case of a turned-down edge, the edge will 
represent a much smaller part of the masked aperture and be less 
important.

It is often said that with a really good mirror, there is no 
resolution to gain from masking. It may well be true, but with a 
mediocre mirror masked to a small but much closer to perfect aperture, 
the opposite could well be true, as you have found.

Using two such masked-down apertures (there i plenty of room), one for 
each eye, would be even better!

At least this sounds reasonable to me,

Nils Olof

>>>My first mirror (a 14.5" porthole) ended up with a reasonable case 
of astigmatism. Not a big problem for deep sky viewing but an obvious 
problem for planetary viewing.The use of an aperture mask virtually 
eliminated this problem. I noticed on David Harbours page he talks 
about using an aperture mask for folks who  themselves suffer from 
astigmatism. It seems I've stumbled onto a nice solution, thoughI'm not 
sure what the mechanisms are for reducing the astigmatism. Any 
explanations?? Is it simply a reduction of that area that is 
astigmatic?
  Thanks--Jeff Newsom
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