Flex mirrors (was Re: ATM Travel Telescope Design)

[Archer Sully <archer@meer.net>]

"William Kelley" <wekelleylwk@sedona.net> spake:

> also a
> meniscus, in which the back has a rough ground radius equal to the surface.
> ...
> These mirrors become
> progressively thicker from the edge to the center, where they rise abruptly
> to a short, cylindrical stub which is smaller than the shadow of the
> secondary mirror. A  tensioning bolt is epoxied to the stub, and a spring
> maintains the proper tension.

I've been plugging some numbers into flex.exe recently, specifically
working with large, fast, thin blanks with a back radius equal to the
front radius plus the thickness of the glass.  Yes, a slumped blank.
Results are interesting:

16.0/f4.5, thickness .5", back radius: 144.5
spot ratio .009  tension 8.99lbs, uniformity: .92%
wedge: .0002".

The worst figure is the wedge, which leaves little room for
argument.  The error budget is only slightly worse than for
an 8" f/6 flat back, and the tension is less than 1/15th of 
that needed for the 8".  

Slumping seems easier to me than diamond grinding a fairly 
precise shape out of a much thicker blank.  The cell would
of course be more complicated than a simple bolt epoxied to
the center, but it may turn out to be cheaper to make a cell
than to grind that much glass. 

If someone were to go commercial with blanks specifically 
made for flexing they could be molded in the proper shape,
like Molestron's SCT main mirrors (which have a similar
shape except with a hole instead of a stud). 

Archer Sully
Boulder, CO