ATM Aluminium optical substrates.

[Clive Milne <milnecd@yahoo.com>]

   Wilson has some interesting references on the
subject of Aluminium (& Stainless Steel) mirror
substrates. (Yes, I know the thread died some days
ago, but I don't get much time to participate in the
ATM list anymore).
   In fact, Aluminium was seriously considered for the
8.2M VLT primaries as an entirely feasible alternative
to Schott, ULE ceramic.  
   There are two significant points to keep in mind
though; 1). Active temperature control of the mirror
substrate was considerd a given.
        2). Active control of low frequency (large
area) surface profile errors, was also assumed a
given.
     Point one is entirely within the scope of the ATM
domain. Point 2 is perhaps not, but this may change in
time. I think currently, the break even point is
around the 1.2m aperture mark. (I may be in error
though).
    Wilson also shows an Aluminium blank in the
process of manufacture. Molten aluminium is deposited
onto the rim of a slowly rotating mandril. (Imagine
the rim of a snow ball accumulating material as it
rolls down a slope). The resulting blank is a tight
spiral of Aluminium pad welds, in effect. 
    Perhaps someone with a Godzilla, MIG welder might
like to tilt their lance at the technical challenges
there-in. ?
     Good Luck Mr Gorsky !
         Clive.  ;)


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