Re: ATM Autres follies de Foucault (More Foucault Follies)

[jerry@cts4.europe.dg.com (j.royston)]

In addition to the points below, I've also found it useful to make several
measurements, first from the outermost zone to the central zone, then move the
knife edge a long way (0.5 mm or more) towards the mirror, then measure the
zones going outwards. Repeat this so that you have an even number of 
measurements of each zone. It's important to adjust the knife edge in one
direction ONLY. Reject the maximum and minimum measurement for each zone, then
average the remaining ones, and plug them in your favourite reduction 
program.

Best wishes, Jerry
> Over a number of years of testing, I've gradually discovered more and more
> how error-prone the knife edge test is:  1) you have to allign the knife
> edge and slit; 2) you have to allign the tester with the optical axis of
> the mirror; 3) you have to cut a Couder mask and measure its radii; 4) you
> have to position the mask correctly in front of the mirror; 5) you have to
> move the lamp house until the corresponding zones on opposite sides of the
> mirror look equally bright (very subjective); 6) you have to make the
> readings by balancing the greyness on the corresponding zones (very
> subjective).
> 
> I often find that I can make a good-faith reading, look away, turn the
> flashlight on, write the result down, look back again without moving the
> micrometer, and the zones don't look balanced anymore.  The brain/eye
> system just ain't good at absolute measurements: physiology/psychology
> affect the results in varying ways.  And forget making measurements if
> you're tired! 
> 
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