Re: [ATM] newbie question

["Bill Kelley" <flex@commspeed.net>]

Nigel and List friends,

I remember undertaking to make an aluminum mirror many years ago when I was 
equally uninformed, and did not consider the problem with a look at the 
physical properties of aluminum first. As many on the list will know, and 
chuckle, glass is removed  when grinding glass on glass by bits of glass 
being removed by the rolling carborundum grains fracturing the glass. The 
carbo grains rolling against the soft aluminum dented the surface but 
removed very little aluminum. I seem to be a slow learner and after giving 
up on aluminum I tried 2 - 6 x 1 inch disks of  acrylic- with some of the 
same problem. I won't bore you  with that long story- but I did finally 
complete a very poor mirror. Surface under foucault looked like spider 
webbing. Inhomogeneities in the plastic were at least a part of the problem.

Yes- I am the same uninformed ATM that introduced "flex" mirror technology. 
As developed with help from others that has stood the test of time. So even 
we "ignoratti" who just like to try stuff can get lucky. Nigel if you'll 
provide a phone number I'll call and tell you a bit more. Mirrors are made 
of aluminum.

Bill Kelley
----- Original Message ----- 
From: "Nigel Weeks" <nweeks@prism9.com>
To: <atm@atmlist.net>
Sent: Tuesday, September 11, 2007 9:40 PM
Subject: [ATM] newbie question


>
>
> I've had a bit of a search through the archives for this list, but
> can't find any threads or comments, but I apologise if it's been asked
> before.
> What are the pros and cons with making a primary paraboloid mirror
> purely out of aluminium?
>
> I can see it's softness making it easy to machine, shape, scratch,
> and repair.
>
> I can see it's thermal conductivity enabling it to reach ambient
> temperature very quickly, as well has heating from your hands when
> shaping it, possibly causing errors during grinding and finishing,
> requring a jug to prevent hand contact transferring heat.
> I can see it's light weight being beneficial to the portability of
> scopes, especially if additional material is removed from the back of
> the mirror prior to grinding and shaping, so the back of the mirror
> has a honey-comb appearance.
>
> Being an alloy, you could tap threads straight into the back for
> mounting and adjustment if needed.
>
> Aluminium is a popular optic  mirror coating, so all the
> alloy
> mirror needs is the protective coating after polishing
>
> It's tendency to oxidise may be a drawback, but the protective
> coating will handle this.
> Over to the gurus! I'm about to go buy 2x 18"x2" chunks of alloy,
> one for tool, one for mirror, and start grinding.
>
> Someone talk me out of it...
> Nige
> Nigel Weeks
>
> Software Engineering
> Prism9 Industries
> email: nweeks@prism9.com
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> ATM mailing list http://www.atmlist.net/
> 

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