Re: [ATM] Ultra-Thin Edge Support Idea - RE: [atm_free] ULTRA-THIN30" mirror from Hong Kong

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Mark Holm wrote:
> I am going to be skeptical about nearly everything on this web
> page.

Me too.

But I will be the first to congratulate them on polishing out a thin 
30" mirror blank!  Even if it has some issues, it will still show 
bright low-power images.  Hope they had a flat support, used carpet 
underneath it, and rotated it often.

> No time to explain in detail why for each item, but I see too much
> that is way too far from fairly firmly established knowledge and
> practice.  Not that I am saying innovations are impossible, but,
> there are a lot of rather far out things here with little or no
> justification given for them.

As someone else said in a later message, the test for astigmatism was 
nowhere near sensitive enough.  I suspect that will be confirmed later.

Also, a tentioned metal band running entirely around the mirror will 
compress the mirror (fairly uniformly at its edge) when it is 
"hanging" from its support, causing it to "cup" and change shape.  The 
tension is relieved when the mirror is horizontal and resting on its 
supports, so the mirror shape will change as the tension is reduced. 
The mirror supports we amateurs use shouldn't change the mirror's 
shape as a function of altitude.

With a metal band, there will be changes in dimension with 
temperature, that is, the band will expand and contract with 
temperature, possibly altering the "squeeze" on the mirror.  Adding 
some padding to the inside of the band would help reduce this effect, 
but may introduce non-uniform compression around the edge of the 
mirror, which seems to be what they were trying to avoid.

I'm not thrilled with the plywood support triangles either, but those 
are easily replaced.  The top support bolts need to be enlarged to 
avoid bending/sagging.  I know how much a 30" x 2.125" mirror weights, 
and it bent the sling support pins my friend had purchased (obtained 
from a certain commercial manufacturer who assured him they would hold 
it).  Those pins were a LOT bigger than the ones shown in the photo.

	Mike Lockwood

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