Re: [ATM] newbie question

[David Weinshenker <daze39@earthlink.net>]

Nigel Weeks wrote:
> 
>  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.

Hmmm... I'd worry about getting a good optical surface
with conventional grit grinding. You might end up with
abrasive particles embedded in the surface.

Your best bet might be to form the parabola with a fine cut and a 
rounded tool on a CNC lathe programmed as close as possible to the
exact curve... polishing would be interesting: I'm not sure if pitch
and rouge would work - you might end up with something like a buffing
wheel on a spindle on your polishing machine, suspended with controlled 
pressure and swinging in and out over the slowly rotating disk, spinning 
over the surface in blended strokes - with relative dwell time adjusted
according to a variable profile as needed to control the figure while
proceeding to use finer and finer polishes.

The above is purely a speculative suggestion - I'm new to this telescope
stuff myself but I have a background with machinery, so take that for
what it's worth. I just don't know how well you can polish the stuff,
or if the high-speed buffing techniques usually used for metals can even
be modulated with the finesse needed to polish an optically-correct
surface (or, for that matter, whether the low-velocity pitch laps used
for optical glass could be adapted to metal surfaces...)!

You might be able to tell something about the polishability of your 
intended alloy by going at it with a rag and some Simichrome (a metal 
polish sold at motorcycle shops; it seems to be a fine abrasive 
in a paste base). 

>From a purely mechanical point of view, the best blank material may 
be the cast plate known as "Mic 6" (and similar brands) - it's a very
machinable aluminum with excellent dimensional stability, so that
it doesn't warp when machined down to shape. I don't know how well
it would take an optical polish...

-dave w
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