Hi Mark,

In using either aluminum or wood you are correct, the thicker thickness of the silicone allows better for the expansion/contaction to the base substrate.

In our case with the Zero expansion Sital and the Zero expansion Carbon fiber mounting base, using anything thicker than we did caused a noticable collimation shift from the vertical to 30 degrees above the horizon...due to the added flexture of the silicone.

The secondary mirror weighted about 5-7 pounds if memory serves correctly and this also caused a lateral shift of the secondary of about 0.001+ inches in thicknesses greater than 0.075" when lowered from vertical to 30 degrees.

All of these calculations were confirmed by Algor FEA software with thier software engineers doing the final confirmation...it was just a bit beyond my level of trusting that I fully understood all of the criteria...

I was however able to get very close in determining that aluminum would have cause about 1/4 wave of an error and the thin silicone with the CF back plate was just about perfect.

Mike Jones defined the error envelope for movement of the secondary and then we became very concerned to get it right the first time and not be chasing after trying to find the errors when using the CCD, for which was the scopes total purpose.

Best Regards,

Preston