From: Brad Davy 
To: wayne hilliard 
Cc: atm list 
Reply-To: Brad Davy 



On Tue, 10 Dec 2002, wayne hilliard wrote:

> wouldn't half the mirror be blocked by the knife and only half be visable.

The entire basis of the Foucault test is that all the light from a point
will focus back to a point.  When that point is blocked, the entire mirrors
surface will block out.  When you actually try it, you will see. A good
sphere will gray out uniformly and almost instantaneously.  With the
slitless tester what you are actually looking at is not the light, but the
shadow of the knife edge.  When the knife edge passes across the focal
point, the surface goes dark.  The light is simply to make the knife edge
easier to define.

Brad
bdavy{at}fred.net

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