Our large telescope is a professional instrument.
On the one hand, the optics are “active”; adjusting weights at the primary mirror ensure the correct mirror position at any viewing direction and distance rods made of Invar (smallest coefficient of expansion) between primary and secondary mirror guarantee the exact focusing at any temperature. Secondly, the entire back of the main mirror rests on an air cushion with cut-outs for three load cells that monitor and control the air pressure. This effectively prevents the main mirror from distorting under its own weight.
We do have two air cushions, but because of their aging, we need to put in a new one. Unfortunately, an expensive telescope needs expensive spare parts – a cushion that resembles a swimming ring costs around 750 $US!
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