Re: [ATM] Foucault imaging fast mirrors?

["James Sturtevant" <jhsturte@hotmail.com>]

>From: Dominic-Luc Webb <dlwebb@canit.se>

>I set up my Foucault rig (classic/generic razor blade construct). Test
>subject is a 310 mm diameter F/1.55 partially polished plate glass
>sphere (slumped). I thought this was going to be quick and easy, but
>I encountered a couple of difficulties. First, light source had to be
>changed because it was only illuminating part of the mirror, and when
>that was fixed I realized my rather small Canon A-85 CCD camera
>cannot get close enough to capture the fully illuminated surface with
>the light source within 5 cm of COC: lens hits razor blade before
>getting close enough to obtain a fully illuminated surface.
>
>Anyone have experience with actually getting CCD image of Foucault
>pattern for such fast system? Is this worthwhile test?

F/1.55 is awfully fast for Foucault. I think you would be much better off 
with caustic testing.  If you don't have, or want to build a super precise 
two axis testing rig for caustic testing, you can use your camera to record 
multiple images of a slit, measure the separation of the slit images in 
pixels and, if you know the size of the pixels, convert pixels to mm.

You will need a mask with a center hole and 5 to 8 holes on either side of 
the center. I make the center hole larger than the others so it is always 
easy to identify the center slit image because it is much brighter.  You may 
need to experiment with the size and spacing of the mask holes to avoid 
diffraction/interference effects that muddy the image and make it difficult 
to identify the true slit images.

Jim Sturtevant

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