Quote:
Originally Posted by multiweb
That's very interesting. I would never have thought that cooling fans would cause astigmatism by blowing air locally on a glass surface. I guess it makes sense, the bigger the volume of glass the more amplitude in change.
So the challenge is to mount a large chunk of glass so it keeps its position mechanically but still has room to expand and breathe. How do you do that? Cement it with a compound that has the same or very close properties as the glass you're using. Like a crumple zone?
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In this case, with the cross blowing fans, it is simply a matter of uneven cooling that sets up thermal gradients within the mirror, and that leads to asymmetrical figure.
Fans behind the mirror are not as problematic because there is enough turbulence within the small space to even out the temperatures in a symmetrical fashion.