On Oct 5, 5:26 pm, Steve Hix wrote:
In article . com,
Fred the Red Shirt wrote:

On Oct 4, 4:52 am, Darrel Toepfer wrote:
Ron Wanttaja wrote
:
...

Can "transparent aluminum" be far behind?

It's here now. Aluminum Oxide is transparent which is why telescope
mirrors which have a very thin layer to minimize defects, have to be
realuminized every few years ago.

Um, sort of, not quite.

Aluminum oxide forms quickly on pure aluminum surfaces, which is a good
thing, corrosion-wise; it's very hard, has little effect on
reflectivity, and thus makes for a good mirror coating.

The thing is, it's not so much something you do, specifically, as
something that happens when aluminum is exposed to oxygen. And if you
scratch the surface, the scratch gets an immediate re-coat. Not that
you'd want to scratch your mirror at all, if you could possibly avoid it.

Eventually, though, dust and other sources of wear damage it and the
aluminum under it, and you've got to redo the aluminum coating; it's
only a few millionths to a few thousandths thick, from glass to air, so
it's easy to damage.

Fortunately, it's an easy process, and last a *lot* longer than
silvering the mirror, even though the silvered mirror, for the first few
minutes anyway, is a couple percent better than aluminum.


Obviously you have not seen how a telescope mirror ages over the
years. The coating gradually becomes more and more transparent
over the years.

This happens even if you never clean, and therefor never damage
your mirror. It is certainly not due to wear! Clearly (no pun
intended)
the typical coating is thin enough that as the oxide layer slowly
thickens
the transparency rises.

Were wear alone responsible, the change would be episodic, not
gradual, and would not happen at all between cleanings.

Could be worse, we could still be using electrum or speculum metal to
make mirrors.

Yeah.

--

FF