[ATM] Cass mirror separation

["John Sherman" <atm@johnspics.com>]

Hi List,

I know that the distance separating the mirrors in a Cass-type scope is fairly critical for high-res views, unlike in a Newt. So what if you have a pair of mirrors and are unsure of the designed separation? Sure, one could measure the curves and put them in OSLO and let it optimize. But since they're aluminized a spherometer won't be good to use on the convex. 

I can already hear a mathemagician saying to dunk it in water and measure the displacement, calculating the sag from there. But is the convex a sphere?

So I suppose that, since I already know the approximate separation, the best thing is to star test a finished scope extensively. System over- or under-correction should indicate a direction to move. Assuming both mirrors are well-made, trial and error should get me there eventually. 

Is there a better way?

Thanks,

John





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