RE: [ATM] Why not metal mirrors? (was Beryllium)

[vorblesnak@peak.org]

On 14 Sep 2004 at 23:10, Ellen Mackenzie wrote:

The real trick here is to polish out the aluminum.  NASA did this by diamond 
turning the surface and then polishing it out with india ink.   I think the best 
idea is to cast in the rough curve,  as I don't have a precision lathe.  The old 
timers talk about using blue stone, cut to small squares and pitched up on a 
positive to grind out the metal mirror.  Any soft coarse stone should work.  Or 
perhaps a lap of silicon carbide cut off disks of decreasing grit size.  The key 
seems to be no loose grit like one would use on glass.  Also shape the lap 
well and maintain it.  Polish on hard pitch, cerium oxide then india ink.

Once I get the last two pie plates done and figure the 16, (almost done), I will 
try a metal mirror and let you all know what I find.

What word lately from the speculum caster back east?

David Davis
Toledo, OR

You know Mel,  that nine inch may never get finished.

> Hate to tell you Mel but nickel and aluminium are a really bad cobination -
> - it makes a nice thermet bomb. It' really hard to find an inexpensive over
> coating for aluminium. The surface prep is done with fluorides, very toxic,
> and only a few materials are suitable. Copper is a good intermediate for
> subsequent finish reflecting surfaces like chromium.
> 
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
> Mel Bartels
> Sent: Tuesday, September 14, 2004 9:53 PM
> To: ATM Mailing List
> Subject: Re: [ATM] Why not metal mirrors? (was Beryllium)
> 
> 
> The late Tom Waineo sent me an aluminum substrate and grinding/polishing 
> instructions.  Basically, as I survey the literature, the biggest issue is 
> extreme warpage thanks to the differential expansion between the coated 
> nickel and the aluminum backing.
> 
> Mel Bartels 
> 
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