Autocostruttori

sabato, ottobre 27, 2007

Sling support for a 30" mirror


Here is the result of a finite element static three-dimensional analysis for a 30" diameter mirror. The mirror is 2" thick and supported by a sling. It is assumed that the sling applies uniform pressure across the edge. This means that the centre of sling forces is not exactly on the plane of the centre of gravity, but slightly forward. One could reduce deformations by accurately centering the sling. This analysis is representative of what might happen if the sling is not exacly centered. In a certain sense it shows what is likely to happen. No other defects are there. The deformed hape is magnified 1 million times. The material is Pyrex 7740.
The deformed shape is ugly: the mirror centre goes back 40 nm, while the whole lower edge moves forward 140 nm. The total surface error is thus 1/3 wave. 0.66 wave on wavefront. This is much worse than for a 16 inch diameter, same thickness mirror described in a past post. Of course the above numbers include tilt and defocus. The real abberations are somewhat less.

1 Comments:

  • Concordo con la veridicita dei tuoi calcoli , avendo un 30" spesso 1,5" quindi con figure ratio di 1/20 le deformazioni in verticale dovute al peso proprio sono notevoli e si notano ad occhio alle quali si sommano quelle dovute a cattiva lavorazione!!
    mentre in orizzontale sono quasi assenti , quelle dovute al peso ,tranne la componente dovuta alla cattiva lavorazione. Ruotando lo specchio nella propria sede si riescono a compensare in minima parte tra loro i defetti.

    Franco salvati

    By Anonymous Anonimo, at 30 ottobre 2007 07:28  

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