Re: Are Aluminum Mirrors in reach of the ATMs?

[Doug Miller <mildg@sk.sympatico.ca>]

I thought that I should add my $0.02 worth to the aluminum mirror discussion.

Aluminum mirrors are made for some specific (mainly infrared and/or military) applications.
The commercial technlogy is very well developed but the resulting mirrors are very expensive
and not suitable for ATM use.

My WEB site has a list of the mechanical and thermal properties and some figures of merit
for  many optical materials including aluminum.  The page for aluminum is:

http://www.minerals.sk.ca/atm_design/aluminum.html

If you want to check out other materials, the main page for the materials properties is at:

http://www.minerals.sk.ca/atm_design/mirror_substrate.html

Although aluminum mirrors have good thermal conductivity, the large coefficient of expansion
will result in noticeable change in radii and figure.  It is also difficult to achieve a
good surface finish, even at professional optical shops. If you look at the 4 thermal
figures that I list, aluminum beats zerodur only for thermal difussivity (a measure of its
ability to dissipate thrmal gradients).  The distortions produced by steady-state and
transient thermal changes are far smaller for zerodur than for aluminum.

There are good reasons why many of the modern era large telescopes use zerodur, fused
silica, ULE, CerVit and Astrositall and not plate glass or aluminum.  The reason is the low
coefficient of expansion of these materials, their availablity in large stress free blanks
and their ease of fabrication (easy compared to aluminum).

I am sure that an aluminum mirror can be successfully produced by some ATM's, but it will
not  likely perform as well as a good pyrex mirror.  If you decide to try an aluminum
mirror, I suggest that before you start, that you do some research in the professional
literature to see what the pros are doing.  There is a lot of information available, but it
is not  necessaarily available outside of the university libraries.

Regards,

Doug Miller
Northern Lights Optics
http://www.minerals.sk.ca/atm_design/optical_design.html