Quote:
Originally Posted by rmcconachy
Three points define a plane in space so if you pretend for a second that the mirror doesn't sag at all then you can determine where the mirror sits and faces with just three supports. Any extra supports make it likely that our non-sagging mirror will not be in proper contact with all of its support points and will rock just like a chair or table with one leg that is too short or long. A perfectly flat support plate doesn't exist (just like my imaginary non-sagging mirror), what would really happen is that the mirror would rest on a few higher points on the plate and you wouldn't know where these higher points were located. If we start with three support points then we get to choose where they touch the mirror rather than their position being left to chance.
Happy observing!
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thanks. bit clearer now.
but just as there is no perfectly flat plate, the different points in a mirror cell can't also be guaranteed to form a perfectly flat plane, in addition, the bottom surface of the mirror would also be less than perfectly flat. so do mirror cells compensate for these differences by lowering or raising individual or group of points?
an imperfectly flat plate supporting a mirror, could for example cause the mirror to tilt from the horizontal say by 0.1 degrees, but we're then collimating the three support points on the plate to re-orient the mirror back to zero degrees so our collimating light beam returns correctly.
how different is this from a cell where we still adjust the 9 or 12 point cell at three collimating points to re-orient the mirror to zero degrees?
one major issue with the plate I imagine would be unpredictable surface deformation due to expansion or contraction. this would be minimal in a cell as contact surface area is minimal.
but then a temperature controlled plate could minimize these deflections? it might be more complex than a mirror cell, but woudl reduce mirror sag between cell support points a fair bit.