Re: Are Aluminum Mirrors in reach of the ATMs?

["Sam Michael" <samm@idcomm.com>]

Aluminum is cheap, but it does not grind well and I disagree that it has bad
thermal characteristics.  On the contrary, it has excellent thermal
characteristics for telescopes.  It is more than 100 times more thermally
conductive.  This to me, means that it will reach equilibrium far more
quickly.  Thermal problems are by FAR the most damaging to reflecting
telescopes.

Yes, it is much more expansive than glass but this will be irrelavent in a
few minutes.

In fact, thermal characteristics are the only reason, I can see for making
an Aluminum mirror.  Its thermal characteristics would be very desirable.

The biggest challenges with aluminum is to make sure that it does not become
astigmatic during grinding and polishing.  The metal is soft and weak.  It
is easy to deflect it.

I attempted an aluminum mirror sometime ago.  I found grinding to be very
difficult.  I should have put the general shape in on a lathe.  This would
have made the process MUCH easier.  As it was, a 4.5 inch mirror took a day
of grinding on my machine.  In the same amount of time, I could have easily
ground a 16" mirror and polished it.

Eventually, I ground it down to 25 micron.  It easily scratched beyond this.
I then attempted to go to polishing and hit a brick wall.  While the surface
did get semi-polished.  It never achieved a full polish.

It was clear to me that I would have to coat it with nickle and polish that.
Perhaps india-ink would allow me to go further with the aluminum.  I don't
know.  I do know this, that for a 4.5" mirror, it wasn't worth it.  I have
several other aluminum alloy mirrors that I was going to attempt, but never
got around to them.  I did get a piece of 7075 that I was going to try and
believe will work.

I recommend that anyone doing this anneal their aluminum after rough
grinding.

Anyone thinking that an aluminum mirror is going to grind faster is in for a
big surprise.  The softness of the metal means that the cutting action of
the grit is strictly limited to "cutting".  The metal will not fracture as
the glass does, allowing for quick grinding.  They are a BEAR to grind using
standard techniques.  Get the initial shape cut in first on a lathe, then
finish the surface.

It is not necessary by the way to rig up a complex jig to get the shape cut.
Just step the tool.  In the end you will be off only by .001 steps and that
should quickly go away with rough grinding.

Sam.
----- Original Message -----
From: "Mike Coslo" <mjc5@psu.edu>
To: <atm@shore.net>
Sent: Friday, March 23, 2001 6:31 AM
Subject: Re: Are Aluminum Mirrors in reach of the ATMs?


>
> Alumininum is the holy grail of some mirror makers. It is cheap, easy
> to machine, and as far as I can see, just doesn't make a very good
> mirror. There have been experiments lately (NASA) with polishing AL with
> India ink, which is interesting, because I recall reading about India
> ink as polish in an old book somewhere. I believe a link was posted here
> with the india ink polishing a day or so ago.
>
> But there are some big issues with it. There is the grain of the
> material. There is also the thermal characteristics which aren't all
> that great. Then there is a tarnish issue. Assuming you are trying to
> have just an aluminum surface on the mirror, you will need to
> polish/refigure every so often.
>
> IIRC, some people have made the basic mirror, then plated it with a
> harder metal, then figured, then applied an aluminum overcoat. Seems
> like too much trouble for a mirror with not-so-good thermal
characteristics.
>
> - Mike -
>
> Malcolm wrote:
> >
> > Hi Everyone,
> >
> > Is there anyone out there thinking about making an all Aluminum mirror?
> >
> > I'm just a little confused if you would or could (how) parabolize.
> >
> > My first thought would be that its almost identical to using glass, but
the
> > final figuring would be a little more involved
> >
> > Regards
>
>