Hi Steve, I did FEA analysis when we were working out the mounting of the 20"RC's secondary and determined that for our needs, the backing for the secondary mirror which was Sital needed to be carbon fiber.

In addition we determined that 3 disc of 3M silicon aquarium adhesive .6-.7 inches in diameter placed at 120 degree intervals at the 72% of radius offered the least distortion. The pads of silicon were 0.055" thick.

The secondary mirror was 7 inches in diameter and 1.1 inches thick.

The next best thing was Stainless steel and the worst was aluminum

I do not however have the calculations with me here in Mexico and will not be returning to Houston for likely about a year.

Hope that helps a bit.

Best Regards,

Preston