Re: ATM Focault Tester Setup

["Bob May" <bobmay@nethere.com>]

You've already got a fair bit of info from the others but I'll address
another area that most newbies don't realize with the test - how far you can
get your eye behind the tester!
The full illumination of the mirror requires that your eyeball's pupil be
larger than the cone of light coming from the edge of the knife-edge.  What
this means is that if you have a 10" F5 mirror that you are testing you will
have a ROC of 100" and an angle of cone of tan(5/100) or 2.86 deg.  If your
pupil is open 1.5mm then the eye's pupil can be no more than 30mm behind the
KE.  For practical purposes, you should be at least half that or 15mm which
isn't much more than 1/2"!  This means that you should have the KE so close
to your face that you start getting worried about slicing your nose with the
blade!
Longer FRatio mirrors do relax this requirement for getting close but not
all that much.  This is part of the reason I love using a cheap security
camera instead of my face at the KE.  The only problem with the security
cameras is that they need to have their wide angle lens replaced with
something that is a lot longer for a FL.  Due to the long FL and the single
color of light used on most Foucault testers, a simple acromat can be used
and something in the range of 20-30mm is a good size.  I might note that
only an approximate focus is really needed so you don't need to spend a lot
of time making a good focusing system to make things work.
Bob May
http://nav.to/bobmay
bobmay@nethere.com