[ATM] steel tubing for mirror cell

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

Richard:
    Sadly, I wonder if I'd notice. But there have been a couple threads 
on the list in the last year or so in which concern was raised re: 
making the three primary cell pivot points also the collimation points. 
The concern there has been primarily friction/stiction of various parts 
of the mirror cell. S&T recently did an article on micro-flexing 
spherical mirrors into exquisitely corrected parabolas. Given that the 
surface of a mirror has the most precise tolerances of anything ever 
created by man or machine in the history of the world, it seems highly 
likely that it's possible to microflex a truly wonderful mirror into 
something ordiinary by the very act of using the presssure of the three 
collimation bolts, depending on the design and accuracy of the cell 
manufacture, and especially on the edge support. If the edge support 
isn't well-nigh frictionless, if it grabs the edge of the mirror or an 
unbalance portion of the wieght of the mirror and holds it back while 
the collimation points push  one third of the mirror forward or pull it 
back, you might be micro-flexing the mirror.
    In the S&T article, the writer judged the adjustment and accuracy of 
his micro-flexing by watching how the image of Jupiter changed. 
Unflexed, it looked okay, possibly the way it looks through most scopes 
to most observers. He finished flexing when the image was really stunning.
    And that's what I mean by, sadly, I wonder if I'd notice. I've 
primarily been a low power observer. Galaxies, etc, have always looked 
fine to me.  But last year with a couple of my re-builds, I've been 
startled by  some breathtaking views of the planets, limited, for the 
first time in my life, by a lack of really high magnification eyepieces, 
which is something I never thought would happen. I always assumed I've 
been limited by atmospheric conditions. Now, I think a lot of time, the 
atmosphere is getting a really bum rap.
    A long while ago, Dick Suiter did a lot of testing and came to the 
conclusion that any home-made mirror, made with care, was of shockingly 
better quality than the vast majority of commercial optics. I don't know 
if in the last decade or so commercial optical quality has improved by 
an order of magnitude, but possibly the reason that most observers don't 
notice this is (we don't, do we?) is because we're microflexing 
beautiful optics into ordinary oprics.
    Anyway, I think it's a possibility. No direct evidence, just 
anecdotal. It's simple to build a mirror cell using the three support 
points as adjustment points, but it adds an unnecessary level of 
complexity to the cell itself. I must say those three points have always 
worked just fine for me with my small scopes. Big and thin mirrors more 
prone to friction/stiction unknowns... I just don't believe in it.
    And moving to an orthogonal system makes collimation both easier 
(you only have to move the mirror in right angles - up/down and left to 
right) and more precise (the wider stance between adjustment points). By 
themselves, I think they makes changing over worthwhile.

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