Fwd: Re: [ATM] Foucault

["Matthew L. Brown" <mlbrown@everstrive.com>]

I found it easier to understand by first setting up the knife edge to be 
well (say 0.25 inch = 6mm) inside the general focus of the mirror.  So set 
up your tester and retract the knife edge and use a card to find the mirror 
focus point.  Physically move the entire tester so the focus point is about 
the where the knife edge would be in mid stroke of your micrometer.  Now 
move the knife well (that 0.25 inch) toward the mirror.

Get your eye behind the knife edge, really close, so your eyebrows are 
almost touching the knife edge, and slowly bring the edge across.  Watch 
what the shadow does on the mirror -- it will appear to move in the same 
direction as the knife.  Now move the knife edge about the same distance 
beyond the focus and cut the knife thru.  The shadow will now move the 
other direction.  Your measurement points will be somewhere between the two.

If you try putting the knife edge at that distance from the mirror, at 
first, to an untrained eye, it will appear that the entire mirror winks out 
in an instant.  Move the knife edge slower and get much more critical.

This is where a couder mask is helpful.  Make one up (I like to use the 
couder.ps postscript program -- search the internet on "couder.ps") and 
carefully prop it in front of the mirror.  Now, looking at a pair of 
windows, your task is to get those two windows to go grey 
simultaneously.  When they do, note the micrometer measurement.

Now what I do is spin the micrometer, alternately in or out, and attempt 
the same measurement.  At first you should expect successive measurements 
to vary by 0.020 inch (0.5mm).  Your goal should be to get them much closer 
together, say, 0.005 inch.  This takes a lot of practice.  Take several 
(minimum 3) readings at each zone and average.

Now, if you watch carefully, the two windows won't go grey evenly inside 
the window.  One side will be darker than the other, especially for shorter 
f/ratio mirrors.   This is enough to get you started.


=Matt


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