Re: [ATM] RE: [atm_free] The zonal Foucault test is free ofinherentcorrection bias - some supporting graphs

[Jarvis Krumbein <jkoptic@juno.com>]

John, if the light source and knife edge are displaced vertically instead
of laterally there is no parallax error and the error due to the mask
being flat versus the curved mirror surface is insignificant as Bob May
wrote.

Jarvis Krumbein

On Tue, 15 Mar 2005 20:07:33 -0500 "john sherman"
<atm@mail.johnspics.com> writes:
> 
> 
> >He [John Strong]points out that Foucault tests 
> >are sensitive to parallax, 
> 
> Not being a mathemagician, still I calculate that I've jumped off of 
> the deep end before, and lived, so why not do it again??
> 
> Here's a reason why the parallax might be:
> 
> Every good program has to compensate for the error of having a flat 
> mask in front of a curved mirror. In the center zone, you are 
> looking flat on to the mask, so there is no real problem. The 
> outside zones are physically close to the mask, so there is only a 
> little problem. But at the 60% zone and thereabouts, the light rays 
> go in and out of the mask at an angle, and hit the mirror at a 
> different radius than the actual mask openings. 
> 
> Now enter a lateral spacing between the source and knife (creating 
> the parallax error/demonstrable astigmatism). Are the zones you are 
> actually measuring on the mirror still at the same radius on each 
> side?? Apparently not! 
> 
> It seems the rays would go in and out, and show a slightly different 
> zone on the mirror in the left mask opening than in the right. This 
> is easy to see in a diagram I just made (hopefully, I made it 
> accurately enuf to be worth something). Far be it from me to do real 
> calculations (as I said in a recent post). But I can imagine that 
> this is indeed a problem, and it might explain some of the posts 
> that were made.
> 
> The little diagram that I made also indicates that lateral 
> separation of source and knife means you are actually now seeing a 
> smaller zone of the mirror than before. Almost like your zone's 
> aperture is now actually a smaller effective aperture. Perhaps this 
> allows diffraction effects to have an even greater effect than 
> normal? Should the zones at ~60% use wider openings to compensate? 
> 
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