[ATM] ATM: Thoughts about mirror cell design from a stubbornRTV-head

[mikell at optonline.net (Michael Lindner)]

On Sunday 15 February 2004 06:09 pm, Bruce Sayre wrote:
> ...How could they work at all if
> none of them (except for Fr?d?ric G?a's magnificent 32" Dobsonian,
> http://astrosurf.com/altaz/index_e.htm) use astatic supports? My guess
> is that even today's "thin" mirrors have so much inherent stiffness that
> any distortion introduced by bending or overbending, even with a perfect
> mirror, is inconsequential because the wavefront error is dominated by
> seeing on those scales even in the best conditions. Bubble wrap,
> carpeting, whiffletrees: all seem to work.

I'll note that Mel Bartels claims there is noticeable distortion with even one 
ounce of weight on a thin mirror, and if we believe this (and I do have faith 
in Mel's skills as an observer), then even though seeing dominates the 
aberrations most of the time, it is human nature to strive for unattainable 
perfection.

Now, let me expose some of my ignorance on this topic. I don't "get" astatic 
supports, and I hope another list member (perhaps even Fr?d?ric G?a) can 
educate me.

1. Given the supports are at the same points, how does the force on the back 
of the mirror differ if the supports are part of a whiffle tree or part of a 
passive astatic system? In my mind, the forces on the whiffle tree points of 
support should all be equal (barring friction and poor manufacturing). 
Likewise for the astatic system (with the same caveats)

2. When the mirror is collimated (as is inevitable in a truss scope), the 
"hardpoints" which define the plane of the mirror move. The counterweights 
are no longer horizontal when the scope is vertical and vice versa. Does that 
mean the counterweights need to be "tuned" each time the scope is set up?

-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net