Re: [ATM] Crazy simple mirror cell idea

["Robert Houdart" <robert.houdart@cruxis.be>]

Hello Bob,

> Lets start with the misconception that larger mirrors need mour
> support points.
> The need for more support points is a function of the thickness
> to diameter ratio.  At around 12:1 ratio, the mirror starts
> becoming glexible enough to where the error budget starts
> affecting the real shape of the mirror to the point where the
> mirror isn't working right.  It's all in how much flex will you
> allow.
> Note that I didn't mention any particular size when I mentioned
> the ratio?  That is because the ratio sets the amount of flex in
> the structure, not the actual size of the structure for anything
> of a real size that we'd make.

I'm afraid that's a misconception.

It's quite easy to demonstrate that the mirror surface deformations are
proportional to the square of the diameter, all other proportions being
the same.
 
In other words: a 24"x2" mirror will produce four (4) times larger
surface deformations than a 12"x1" mirror if they are supported in a
similar way (say an 18-point cell).

It follows that a 24"x2" definitely requires more support points than a
12"x1" mirror, although they both have the same 12:1 thickness ratio.

Best wishes,

Robert Houdart
www.cruxis.com/scope/scope1100.htm

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