first time Foucault attempts kind of long

Hi guys

Since you've all been so helpful in the past, here's another question:

I have been attempting to use a homemade Foucault tester. It's been a
lot of work, because of the quite critical adjustability needed to get
stuff set up properly.

I use a little laser pen that also has a bright blue LED, as a light
source. You know, turn on the red laser and search the room behind me
for the blip reflected from the mirror. Then get it close to the knife
edge and switch to the blue LED.

I think I'm close. I was beginning to see some "patterns" this
evening, suggesting I have a pretty decent parabola rather than a
spherical surface, which suits me just fine!

The mirror image, when filled with light as it is at 2F, is not very
big, being an 8" with a focal length of approx 70".

It's hard to hold my head still though, and the image shimmers and
wobbles and the patterns change. Do I need a chin rest like the
optometrist uses!?

THe LED has a lens with a surface that looks sorta crystalline when
magnified. I will have to add a diffuser.

Is it correct that when the tester is set up right re the focal
length, without a diffuser, I should see a sharp image of the LED lens
surface visible behind the pinhole, at the knife edge? I mean will
seeing this image of the LED surface tell me the
LED=to-mirror=to=knife edge distances are good?

thanks, anybody and everybody! Texereau's book is good but does not
say much about the _experience_ of doing a Foucault test-.

James