Re: [ATM] LWT setup questions

[Bernard Maugoust <maugoust@free.fr>]

Hi John,

In message <200505171730.AA2645098710@mail.johnspics.com>, 
 "john sherman" <atm@mail.johnspics.com> wrote:

> 
> Hi Bernard,
> 
> >thinking about modifying my existing Foucault tester to allow
> >it to perform the lateral wire test as well. 
> 
> I made a tester from scratch, instead of messing with an existing tester.

Of course, but for personal practical reasons it's still easier for me to
just make an add-on.
 
> The inventor of the LWT, John Francis, once had several pages of website,
> two of which got moved to http://hometown.aol.com/julesfran/ which answers
> some of your technical questions. Also, there are others on this list who
> are much better equipped to answer the hard questions than I am. 

Yep, I've given a careful look at those. Thanks (even though the maths page
goes way over my head).
 
[indicator precision]
> >current one limited to 0.01mm (0.0004")?
> 
> On his website, Francis points out that since there is such a small
> movement of the wire, he recommends an indicator that can read 0.0001". But
> you should be able to use yours to play with. With my set-up, using a
> strong magnifying glass, I found I could start and stop the needle's
> movement ten times between each division marked on the scale. With similar
> care, you could probably use your current indicator to make a fine mirror.

That's what I'll try to achieve at first. I've searched the net for such
precision indicators, and wow, they don't come cheap (at least the more
recent digital ones) :-/
 
> Good luck!

Thanks! BTW, I've failed to make a satisfying 'spot' test device, even though
the idea was very appealing. That's a good indication of my crudeness level I
guess! I still believe I'm clever enough to polish a good mirror though.

My refined testing needs arise from wanting to check with care the shape of
an 8" F/5.25 and if need be polish out any microsurface roughness while
retaining the good shape, and figuring another 8" F/4.4 with a conic factor
of 1.346 as a R-C primary. I suspect that the LWT can come out as more
reliable than other ways.

Others will no doubt say: Foucault rules! I like experimenting though... and
sure like to assess a mirror general shape by looking at the Foucault shadows
too!

Regards

Bernard

-- 
Bernard Maugoust 3°59'E 44°28'N

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