From: "Aditya Munshi" 
To: 
Reply-To: "Aditya Munshi" 


Hi Jim

I already have a Foucault tester I can trust but the table for the testing
arrangement is not stable at the joints, i.e. I figured it would be a great
risk to trust zonal measurements taken with this setup even if I manage to
remove the motion in the joints. This was the main reason for asking this
question


Thanks A lot for the help


Aditya Munshi

-----Original Message-----
From: jsgrmfg{at}gracemfg.com [mailto:jsgrmfg{at}gracemfg.com]
Sent: Thursday, June 26, 2003 9:41 PM To: Aditya Munshi
Subject: Re: ATM How much to test ?


Hi Aditya,

    Foucault testing with a Couder mask is the time honored way of
amateur testing  - perhaps because it requires less equipment than any
other method.  I prefer a pin stick to a mask because I like being able to
see the shadow patterns on the whole mirror instead of just a couple of
peepholes in the Couder mask.
    Full Foucault correction for an 8" f/7 mirror is about 1.8mm (moving
light source) or 3.6mm (stationary source).  You just need a way to measure
the movement of the knife edge toward and away from the mirror with an
accuracy of about .25mm.  You can do that with a simple visual scale and a
pointer of some kind attached to the knife edge, perhaps aided by a
magnifying glass.  Or, you can arrange to move the knife back and forth by
turning a bolt.  Just about any old bolt will do.  If you
know the pitch of the bolt threads, you can easily calculate how far the
knife moves for one complete turn of the bolt.  Then make a simple,
circular scale that divides the full turn into smaller increments and you
are all set.
    There are better, more reliable tests than the Foucault.  I like the
lateral wire test myself.  But, the better tests all require a high precision
measuring device on at least one axis.  That's why the Foucault test is
still used more than any other.  Besides, a lot of very fine mirrors have
been made with  simple Foucault rigs made out of scraps and strings.

Good luck,
Jim Sturtevant





"Aditya Munshi" {at}shore.net on 06/25/2003 12:02:10 PM

Please respond to "Aditya Munshi" 

Sent by:    owner-atm{at}shore.net


To:    
cc:

Subject:    ATM How much to test ?


Hello all

I just finished correcting my 8" f.7 mirror. The surface is smooth and
spherical except for a central hole which is not greater than the
obstruction diameter so I will mask it.

Now coming to the point, After the parabolizing should I go for full
fledged couder mask testing or go for some other method? The reason I ask
is simple, I do not have a good and stable testing setup suitable for the
sensitivity of zonal measurements and hence I feel if anything goes wrong,
the damage caused would be much greater than in the case of being
conservative.

However if such testing may be mandatory I feel I can find another place
where the conditions are better.

Please advise me what to do further and suggest any alternative testins
methods if any


Any help is highly appreciated

Thanks


Aditya A Munshi

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