Re: [ATM] I need some info on the hot wire foucault test.

[Dominic-Luc Webb <dlwebb@canit.se>]

On Sun, 2 Dec 2007, Thomas Janstrom wrote:

> Hi All,
>
> I seen passing references to this test, but I haven't been able to get too
> much "real" info on it. Having said that I have only really been looking for
> about 30mins so its not all that surprising, is it?
>
> I do recall reading that a "length of platinum wire" is used, but other than
> that I'm guessing at things.
>
> I would prefer online resources as I have trouble getting to proper
> libraries...


This is described with citation to the inventor in:

Standard Handbook for Telescope Making by Neale E. Howard

You can get this book for as little as 1.57 USD through Amazon.com:

http://www.amazon.com/gp/offer-listing/006181394X/ref=sr_1_olp_1?ie=UTF8&s=books&qid=1196669552&sr=1-1


I have this book. I bought it many years ago and like having it handy.
Regarding this test, there is a platinum wire situated directly in
front of the knife edge. If you are facing the mirror from behind the
tester, as usual for the foucault test, the platinum wire would be
just in front of the knife edge. Whereas the slit for the light
source might normally be on the left and your knife edge on the right,
the platinum wire would be offset by 1-2 mm to the right of the knife
edge. With this arragement, the light source can easily be extremely
close to the knife edge. The knife edge itself prevents your eye from
directly seeing the platinum wire. Because the wire radiates more or
less evenly across 360 degrees, there is a nice even distribution of
the light onto the entire mirror.

One modification I have made to this test is to spray paint or
otherwise treat the surface of the knife edge if a razor blade or
similar is used. The very shiny surface acts like a mirror and
can make focusing a bit challenging. Overall, however, I would
say it is a nice arrangement, especially if you happen to have
platinum wire. The 0.01 mm diameter platinum wire you have
should work great. Actually, I am curious where to buy this?

Dominic




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