C I am about to build a mirror cell for my 8" mirror and I had planned to C follow Texereau's advice on placing the three suport points near the edge C instead of at 0.707 from the center. Then I read this in usenet: C C " With regards to mirror cells, he says that it is best to do a C threepoint support at the extreme edges of the glass, because it only C effects the F.L. of the mirror and will not distort the radial symmetry of C the glass. An article on mirror cells a while back in Sky&Telescope proved C this to be totally wrong. Three point supports should be located approx 70% C outward from the central radius for the minimum distortion of the glass." C C Could someone confirm whether Texereau is wrong here and in what position C it is best to place the supports? Thanks.
How thick is your mirror ? If it is "standard" 1-3/8 inch, you can support it anywhere you want it, it won't flex significantly. If it is thin (say less than 3/4"), it is better to use 6 or 9 point support. As to comment the above, I will just repeat again - the article in S&T sounds OK. Modern computers and finite element analysis programs are powerfull tools. But Texerau's advice (originally advocated by Couder and followed by Maksutov as well) has at least these three ultra-heavyweights behind it. Those guys have experience in their small finger more than all of us here together. I'll let you draw your own conclussions.
Bratislav
PS I have NEVER seen in my life effects of flexing - even on 8" mirrors as thin as 20mm (3/4"), or 6" only 12mm thick. All of them were supported at the edge. Anyone with more experience on this ?