I would use 3/4" plywood for the side, front and back boards; (2) 3/4" ply
layers for the bottom board.
>
> 2) What thickness of aluminium tubing to use for the
> serrurier truss tubes
I use aluminum 1" square tubing 1/16" walls.
> 3) What thickness to use for the ground board
I use (3) 3/4" ply layers with feet spread 3 feet apart. I do this because it results in a very stable vibration free environment at the eyepiece. A solid knock results in instant damping.
> 4) What thickness to use for the semi-circles that
> will be used as altitude axis
A single ply layer is sufficient.
> 5) We would like to make the secondary mirror cage
> rotatable ( for later ). Are teflon pads
> between two rings of plywood ( one covered
> with formica ) enough to provide a smooth
> rotation of the cage ?
Yes.