RE: [ATM] Astronomium Metal Mirror

["Cary Chleborad" <Cary@Chleborads.com>]

> I used to have a collection of forty years of Sky and Telescope magazine;
> over the years there appeared several articles about very large (70"
> aperture or better) Cassegrain reflectors equipped with aluminum mirrors.
> I
> am not sure where all of them are (I think I know) but I remember that in
> most cases, the astronomers (and the opticians who figured the mirrors)
> were

It's also interesting to note that I've participated a bid to refurbish one
scope with a metal mirror.  The bid called for discarding the "old" metal
mirror and replacing it with modern optics..

I have also manufactured metal mirrors for IR laser applications.  It is NOT
easy.  The mirror was cryo-annealed 6061-T6 Aluminum.  First we machined it
to rough shape and then ground it traditionally to final curve and 15 micron
grit.  Aluminum is far to soft to work up an optical surface past this grit
without serious scratching problems.  Next we electroless nickel plated it
and then ground and polished the hard nickel to an optical surface.  As hard
as the nickel is, it still was a pain to polish to an optical surface
without serious scratching.  We ended up building a plastic tent over the
polishing machine to keep dust away.  The final mirrors were gold plated to
operate at 10.6 micron laser light.

All in all, I could have probably made 20 duplicate mirrors from glass in
the same time span.  Metal mirrors are a great fantasy, but just not
practical for astronomical optical work.

This application required metal mirrors because of the extreme amount of
energy that was being propagated with the laser pulses.  These mirrors had
to be able to rapidly conduct away the heat of the laser that was not
reflected - remember 98% reflectance = 2% absorbance.  The way this system
is used that 2% of the laser pulse would have overheated a glass mirror.

The CTE of metal in astronomical terms is also terrible!  We can get away
with this with IR laser applications because the wavelength is so long.  In
the case of my laser optics above, 1 whole wave as visual (550nm) is only
1/18 wave at CO2 laser light.  To top it off, those optical systems don't
form images, they just collect the light for measurement.

Recently there has been work on a Super-De-Duper-Alloy in which many types
of metals are mixed together.  The solution is basically so confusing on a
molecular level that crystallization cannot occur.  The resulting alloy
remains amorphous and displays glass like properties.  So far the best one
of these is largely Aluminum based.  I find this pretty funny because we are
approaching MR. Scott's transparent aluminum concept (Star Trek).

At any rate... that's my rant.  Metal mirrors SOUND like a good idea, but
they are just not practical at this point.

There seems to be a tendency to go off dreaming about how wonderful and life
changing future technology will be when there is so much yet to be done with
what we have in hand now.  Make a mirror NOW, make a telescope NOW.  Do
something NOW instead of talking about what you are going to do at some
point.

-Cary




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