ATM Focault Tester Setup

[Mike Warne <mwarne@pihana.com>]

Hi,

I seem to be having trouble setting up for a focault test reading.  I made
my couder mask, and am ready to take my first readings on my first mirror, a
10 inch. 

The knife edge is mounted on the left, over and touching against a jumbo LED
with the dome cut off, and sanded flat.  I adjust for ROC by watching the
shadow come from the left when inside roc, or right on the outside of ROC.

I'm sure it's just inexperience, but I just can't seem to get this thing set
up right. I spent a few hours just trying to get some kind of image from the
tester.

My tester stage rides on 2 V blocks on the left, with an adjustable foot on
the right to tilt the stage up or down.  When I try to make the knife edge
cut through the returning light by adjusting the tilt,  the illumination on
the mirror moves too, so I have to move my head to find it again.  I'm
guessing this is normal, because this is a moving source tester, and the
knife and the LED move together.  But it seems like the image stays the same
no matter how I tilt the stage, it just moves away from me. 

It seems that when I'm at ROC, the mirror doesn't fully illuminate. I can
see the half moon shape of the half blocked LED, but it doesn't cover the
whole mirror.  When I do a ronchi test, I also have problems outside ROC
getting the whole mirror to illuminate. Is my light source too small, or am
I just not set up correctly, not really at ROC? 

The mirror stand is on an end-table, the tester is higher, and on a desk, is
this a problem, or should I try with everything level?

When I look past the knife edge, how close should the mirrors edge be in
relation to the knife edge? 

Maybe my big nose is getting in the way? :)

I guess I'm so confused, maybe I just need to know the steps to set up the
tester in the first place, and then how to use it after that.  The books
I've read all speak of fixed source testers, where the light doesn't tilt
with the knife edge. 

Sorry for rambling on, but I spent so much time with set up, and I never did
get my first measurements.  

Thanks,
Mike.