Re: [ATM] 40" mirror

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Jerry...

Before you indulge your illness (aperture fever),
please take a  moment to analyze what you are getting
into IF YOU SUCCEED. Mounting a 40 inch piece of glass
is going to be a Herculean task (even if you make the
cell out of plywood and AstroTurf), the cost of
aluminizing a 40 inch mirror is bound to be well over
$1500.00 plus shipping, and the number of places that
can do this work is fewer than you might imagine so
you likely won't get an ATM discount. Hell... Just the
cost of a 20 foot stepladder these days is outrageous
and you're gonna need 2 to put it together. (I know
you already have them from your business) ;0)

You mentioned earlier a figure for THE weight of the
glass. I don't remember what you figured it to be but,
it seemed awful low at the time. I think my 26.5 inch
X 1-5/8 inch blank weighs more than you quoted for
your proposed 40 inch disk.

I'm not trying to inhibit or discourage you in any
way, just suggesting that you consider the
possibilities of what you are in for once you have a
40 inch blank. I was offered a 1 meter cast blank (at
no cost) by Lynn Hepburn shortly before he passed away
and wanted it to work out but after figuring out the
amount of work and finances needed to put it into
service, I declined the offer and took the glass I cut
the 26.5 inch blank out of instead.

Good luck! For one man it's a lifetime committment.

Ken Hunter

ps...

Today I received a quote for a 2 foot square of clear
plate glass 3/8 inch thick to be used for a
Schmidt-Cass corrector. Over $100.00 plus tax, plus
shipping and this did NOT include any kind of
guarantee that the glass was annealed to a usable
amount of stress or usable for ANY purpose. I cannot
even order the glass and refuse it if the stress test
is not good. Spend your money, take your chances. I'm
hoping to find someone with the glass in stock that
will let me test it on the rack before I purchase it.

Wish me luck!




--- "Jerry B. Hillman" <JBHillman@ev1.net> wrote:

> Hello everyone,
> 
> Since I am going to invest a good deal of money and
> a considerable amount of
> time in the venture of melting the glass and casting
> a 40" mirror, I would
> like to hear anyones input about very large mirrors.
> I am interested in hearing about F ratios compared
> to useability and
> difficulty in manufacture.  I don't want to invest a
> lot of time making an
> F2 only to find out that uncorrectable coma is the
> inevitable result.
> Don't misunderstand me.  I have no idea if an F2
> would have uncorrectable
> coma.  I just pulled an example out of my hat to
> help explain what I need.
> The first piece of glass I am going to cast is for
> the secondary.  I need to
> know what the F ratio of the telescope is going to
> be before I take that
> step so that the glass will be large enough.  I know
> that starting with the
> secondary is sort of putting the cart before the
> horse, but casting and
> annealing the secondary will tell me if my methods
> are feasible for a larger
> mirror.
> Clear skies, Jerry
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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