[ATM] steel tubing for mirror cell

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

Daniel:
    Let it bind. The other three corners are all that need to move, and 
no more than about 1/8" at that. The way Robert Cox designed his, he 
used only three support points. Upper left and lower right, say, were 
pairs of push/pull bolts. Lower left was a pivot, as you could collimate 
a mirror perfectly using only two points instead of the traditional 
three. Upper right was left totally unsupported. Because of the weight 
of my mirror and cell (I wanted to bring the balance point on the scope 
as far back as I could so I opted for a heavy combination rather than 
counterweights), about 60 lbs, I chose to suspend the pair from 4 points 
instead of 3. It is still collimated using only two pairs, and the pair 
where Cox's pivot is, is really just a (probably unnecessary) 
collimation lock. I used 1/4" stainless bolts throughout the mirror cell 
and 3/8" for the push/pulls and pivot, in a K/B grid of 1" square steel 
tubing. Note that stainless does not have the strength of regular steel. 
If I were to suspend this weight from just three points, and single 
bolts, I'd either go high strength steel bolts or 1/2" stainless. 
Probably overkill, but when has that ever hurt when it comes to the 
safety of a mirror?

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