Re: ATM New mask for Foucault

["Bill T." <twentiethwave@hotmail.com>]

> Last night I decided to try putting the mask on the TV rather than the

> Has anybody else done this before? I haven't seen mention of it


John,

I have a tester which benefits greatly from evaluating mirrors using
captured images rather than the retinal image:

http://www.kupercontrols.com/tiltafouc.htm

I give this technique rave reviews.  Not only does it not require extreme
bodily control or holding one's breath, it can be done in full room light
sitting back in a plush chair.

However, a video image can speak with forked tongue!

http://www.kupercontrols.com/misc/FoucOgram.jpg

No, this is not a picture of Martian volcanoes, but an Foucault image of a
crater strewn 8" mirror.   Look closely -- that's not a circle, but an
egg-shape!  The exact shape is dependent on how bright the CRT image is!
It's an artifact of the TV trying to keep the image under control.
Professional monitors don't do this, but almost all TV's do.  You have to
take this into account when taking your readings.  One way around this is to
cut two pieces of cardboard with one slit each, and position them
independently based on % of radius to edge, independently calculated for
each side of the mirror.

I am developing a frame-grab system which doesn't use a mask at all.  The
motion control slowly moves the slit over the range of the caustic, grabbing
an image every mil or so.  Software then crunches the images to evaluate the
mirror.  I'm still tweaking the software, and hope to publish it here in a
few months.

Bill Tondreau