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[ATM] Re: [ATM) David Williams
Jim and all who have tried to help,
David is not ready for testing yet. It is only partly polished out and very
lumpy. Not surprising since it is so thin and is resting on an irregular
surface. I spoke to him for over an hour today and we should realize that
not only is he tackling an exquisitely thin mirror 20" in diameter, but, he
has never made a mirror, does not have the books, experience nor does he
know our specialized language.
Did I tell him give up? NO WAY! He has now been instructed as to how to
mate his work surface to the back of the mirror and finish polishing. If we
can help an absolute newbie, with lots of determination and smarts, make a
20 incher less than 3/4" thick into at least a useable telescope it will be
worthwhile. Tutorials on optics and literature citations won't help him. Can
we lead him step by step with common everyday language. Perhaps.
Let us try.
Bill Kelley
Subject: Re: [ATM] zones
> At 2004-12-09 18:55 -0500, David Williams wrote:
>
>>But when I try to do this (my knife is on the right and moves to the left
>>by the way), and move the knife to the left, the right opening in the mask
>>starts to go black before the left one. And the measurement on the
>>micrometer that this happens is .61" I hope this helps both you and me.
>
> That says for this zone, the KE is too close to the mirror. What you want
> is the (longitudinal) position of the KE that makes the two holes for the
> zone equally gray.
>
> W. J. Smith, "Modern Optical Engineering", p. 108, has several comments
> about using the eye as a comparison photometer in the Foucault test.
> 1) The fields that we're trying to match should be bright. Smith says >
> 10 millilamberts (whatever that is).
> 2) The background (the mask) should be lighted, i.e., turn on the room
> lights.
> 3) "... the precision of the setting is increased by making a series of
> readings. In half the readings, the brightness of the variable area is
> raised until an apparent match is obtained; in the other half of the
> readings, the brightness is lowered to obtain the apparent match. The
> average is then much more accurate than either set."
>
> -- Jim Burrows
> -- mailto://burrjaw@earthlink.net
> -- http://home.earthlink.net/~burrjaw
> -- Seattle N47.4723 W122.3662 (WGS84)
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> ATM mailing list http://www.atmlist.net/
>
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