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A bit more
I failed to mention a couple of important points.
1. When doing any welding on the assembly, ensure that it is all clamped together as best you can. Aluminium can warp terribly with heat stresses, and once bent, is difficult to re-straighten.
2. The mirror support pieces are made of 4mm aluminium plate. ( something which I have a stock of) The support points were calculated using the program mirror plop obtainable from the Bintel website. Rather than 'point' supports, I am using an 'area' support centred on the 'point'. ie I have calculated the support points, but am using thin o-rings filled with RTV silastic as the adhesion mechanism. A relief hole is drilled in one of the plates, the o-ring is slightly over-filled with RTV. As the support plate is pressed into position, the excess RTV extrudes through the relief hole. After the RTV is cured, ensure that the excess/extruded RTV is not in contact anywhere it shouldn't be.
I have used this method successfully on my 12.5" F5 Parkes mirror, and I believe Peter Bobroff has used it on his 16" F4 Suchting. The RTV requires about a week to obtain full setting strength. Thickness of RTV (which means o-rings) should be about 1-2mm. As the support layer mounting points decrease, the area (diameter) of the o-ring adhesion points should increase to maintain about the same contact area at all layers.
More as it comes to hand.
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