[ATM] Re: Weird Foucault readings

[Vladimir Sacek <vladis.2@juno.com>]

Jeff,

>If the target is to work towards perfect parabolic mirrors, then at 
least the error should diminish the closer you get to a "perfect" 
mirror.  It might be a useful exercise though to try and estimate an 
error bound for say quarter wave and eighth wave mirrors, so you know 
how much farther you need to go than advertised to achieve your desired 
goal.<

As long as mirror has good conical figure, it will null at its respective
best zonal foci similar to a perfect parabola, and the reading will
indicate the deviation. The problem are smaller-scale figure
irregularities, as well as surface rotational asymmetry. They do affect
null readings, but can't be anticipated simply because there is too many
different scenarios possible.

Star test - or test under the stars - is definitely what matters in the
end.

Vlad

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