[ATM] steel tubing for mirror cell

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

The K/B mirror cell support is okay as is but I've found it's much 
preferable to separate the collimation bolts from the mirror support. 
The collimation bolts can add yet another unknown into the your 
finely-tuned Plop cell. Now you don't want stray light sneaking in from 
behind the mirror anyway, so thicken up the baffle behind the mirror 
cell to 1/2" or so and attach the K/B steelworks directly to the baffle. 
Choose any 3 corners of the baffle (or if it's round plot the widest 
right angle triangle you can) and mount the mirror cell and it's 
baffle/backboard to the mirror box using a 3 separate pairs of  
push/pull adjustment boilts. I used pairs of  3/8" on my 20". Separating 
the adjustment bolts from the mirror support makes for a better mirror 
support, the wider stance of the push/pull (thankyou Robert Cox) is 
extremely sturdy, and the wide stance right angle triangle (again 
thankyou Robert Cox) both really simplifies and fines collimation. I 
shimmed very thin teflon underneath the entire bottom of the backboard  
and about two inches up the sides from the bottom to reduce friction and 
thus ease in collimating. I also put a single non-collimating support 
bolt in the 4th corner to help support the weight of a heavy mirror and 
an even heavier 27 pt cell.

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