Re: [ATM] Updated web site: mirror testing and Draper machine

["Bill Kelley" <flex@commspeed.net>]

Matt wrote:
> How does it work out for small central obstructions ? It appears the 
> center
> is where the worst figure errors are , and if the area is not shaded by a
> significant enough secondary, things deteriorate rapidly.

Flex as offered by Alan Adler's FEM program Nov. 2002 S&T stresses spheres 
for Newtonian's to paraboloids. Small obstructions by secondaries are common 
to high f ratios where the sphere approaches the paraboloid as the optimized 
shape and you will note that the calculated error near the center is 
miniscule. It can be improved usually by "massaging" the beta 1 value with 
slightly different values than calculated by the program.

> Does the mirror hold its figure with temp changes ? I played a little with
> the flex program and it looks like the back tension has a real sharp 
> optimum
> peak, not a lot of leeway to maintain a good figure. With temperature
> changes the elastic elements that are used to support the mirror change 
> and
> presumably the back tension changes too , or not ? I'm very interested to
> learn more .

Yes, but not because of elasticity of structural elements.The figure will 
change slightly as a mirror shape follows a temperature gradient. But, since 
gradients are usually pretty steady through the observing evening, once you 
have tweaked to slightly under corrected, the mirror will hold its figure as 
it cools. The many people who have made successful flex scopes have not 
reported the problem you pose.If you decide to do a flex mirror, and will 
contact me off list, I may be able to suggest some  recent approaches that 
will simplify your efforts.

Bill Kelley

> matt tudor
>
> -----Original Message-----
> From: Bill Kelley <flex@commspeed.net>
> To: Mike Spooner <spoon@PageAmerica.net>; Ric Rokosz 
> <radicell2@yahoo.com>;
> scott <srychnov@chem.ps.uci.edu>; atm@atmlist.net <atm@atmlist.net>
> Date: Tuesday, November 30, 2004 10:23 PM
> Subject: Re: [ATM] Updated web site: mirror testing and Draper machine
>
>
>>List,
>>Some reasonable argument can be made here for flex mirrors. We make them 
>>of
>>plate glass and field tweak them for the conditions. Exquisitely smooth
>>spheres are easy, and a Foucault null beats parabolizing and zonal 
>>testing.
>>Before I get flamed, yes, I also parabolize mirrors occasionally- usually 
>>a
>>replacement mirror for someone.
>>Bill Kelley
>>
>
>
>
> 


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