[ATM] parabolizing

[foreilly at bestweb.net]

Surinder,
An eighteen inch plate glass blank 3/4 of an inch thick
is a very challenging project. I understand that 19mm thickness is a
standard size throughout Europe and Asia (and probably the rest of the
world outside North America)  St. Gobain is an excellent manufacturer with
plenty of experience in telescope optics over the last 100 years. The
project is not impossible, but difficult. Was the blank designerd for
telescope purposes or converted from another use such as a ships
porthole?
In 1862 Henry Draper made a three fifteen and a half inch
mirrors an inch thick from porthole glass. Some friends and I had the
opportunity to test it recently. My friend who led the testing came to the
conclusion based upon reports of Draper's observations that the mirror we
tested was not the one he used for research purposes. The figure was lousy
showing astigmatism roughness and a severe turned edge. The good news is
that you can probably do much better.
Some tips that I have learned
from people along the way about very thin large diameter mirrors. I have
never attempted such a mirror and I probably never will. It is just too
challenging for me. Based upon where you are located, it may be the only
glass available.
Proper support is critical at every stage of
manufacture. One friend strongly suggests using carpet under the mirror at
all phases for support.
If the glass is not properly annealed, you
will have nothing but frustration. Try to ascertain through the use of
polarized light if there is any strain in the glass. A Google search
should help to show the proper tools and methodology for the test.
I
am forwarding this email to some friends for their comments. They may also
have some suggestions.
I am aware of the political situation between
India and Pakistan, however I believe that there is a small group of
telescope makers in Lahore. If you are interested, I can reach out for
them and try to arange for some further hands on assistance for your
project.
Best,
Francis

> Dear sir, Iam
pleasently surprised at so prompt a reply .Thank you very 
>
much.I am sure you will consider it foolhardy on reading the vitals of the

> mirror.It is Saint Gobain plate glass of 19 mm thickness,fratio
is f5, 
> pich is what i get here in India;it is soft to bite
in,though it has not 
> flowed to close the channels.As it has
given a good 
> sphere on the normal recommended strokes,I feel it
is ok. Thichness of the 
> pich is 1cm and the polishing compound
is cerium oxide 
> II am going to work on the mirror as per your
recommendations and will 
> report back on the progress 
>
I live in Mohali near chandigarh.Interes in astronomy is non exitant in

> this part of the country. Most of the enthusiast are from the
south of the 
> country like Banglore. S chander shekhar of the
chandershekhar limit and C 
> V raman were from the south 
> Thank you again   Surinder Singh 
> 
> 
>