[ATM] how accurate is the foucault test

["David Williams" <dwilliams1128@earthlink.net>]

Hello

I wondered how accurate the foucault test is because once a person knows that the light or the knife is close to where it should be (inside or outside of focus) how will you know what the focus is.  Won't you have to rely on something crude like a tape measure to roughly measure the focal length?  

Can someone talk about the Couder mask.  This sounds like something interesting.  

Thank you for your help.
David Williams
dwilliams1128@earthlink.net
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