Re: [ATM] Fw: telescope

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi David,
Since you don't know how to do Foucault testing, I am assuming you made a
20" x 1" thick mirror as your first mirror?  I don't envy the journey you
have undertaken.  I did a 16.5" x 3/4" F4.5 mirror. It took me 15 months.
And the figuring of it took 1 year of that.  I reached a sphere in 3 months
from the start of hogging the mirror out.  Then the nightmare began.  1 year
and 150 hours of figuring.  And it still isn't perfect, but it is decent and
will do what I want it to, which is deep sky observation.  My 10" has nearly
the same focal length and is excellent for planatary, moon, and the brighter
deep sky objects.
A 20" F5 is a incredible challenge for a first mirror.  Add to that the fact
that it is a thin mirror, 20 to 1 ratio and you have multiplied the
challenge.  You will want to learn the matching Ronchi test.  On a fast
mirror, it will help you reach your goal. If I can be of assistance, please
let me know.  There are a few things about making a thin mirror that only
someone else who has done a thin mirror will be able to tell you.  There are
a number of people on this list who have done thin mirrors.  They are all
willing to help.
When you design you floating mirror cell, you will want to use Plop.  It is
a necessary tool to design a mirror cell to support a thin mirror.  I can
give you some suggestions on what will and what won't work for support.
As for Foucault testing.  If you can get some 'in person' type help from
someone who has done it, by all means do.  Foucault testing a fast mirror
was impossible for me. Either due to my eyesight or for whatever reason, I
could not get accurate numbers on my 16.5".  My 10" wasn't a problem.  I
resorted to the matching ronchi test on the 16.5" and finally got a good
figure but don't ask me the Strelh ratio.  I don't know it.  I do know it
works.
Clear skies, Jerry


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