From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 



It is possible to test in the scope with a knife edge at the focus.  Use an
artificial star at least 20 focal lengths away.  Air stability and a small
"star" is important.  Sun glinting on a small shiny ball is good.
Use a black background, and good light tight scope parts.

When you first put the sphere in the scope you will see an oblate spheroid
shadow shape (with the hole in the central dome).  Correct using the usual
methods until you see your old friend the sphere appear again and you will
be at full correction.

Sounds easier than it is, but it is a null test and requires no
interpretation.  I haven't used this method entirely, but have used
versions of it.

If you use the best Foucault setup you have and use the star test to
confirm, you may get very accurate results.


The Dall null test, the Ross null test are also possibilities but you must
be careful in setup of the null lens and the light source.  A good lens is
essential.

Just some ideas

Colin



-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of
Aditya Munshi
Sent: Wednesday, June 25, 2003 1:02 PM To: atm{at}shore.net
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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