It is temperature controlled and made of a low expansion ceramic (zerodur?) [...]
Borosilicate glass. I also don't think they control the temperatures of the mirrors but I am not sure about that.
[...] but still bent by active optics to make the focus of "guide star" perfect (and to deal with any high altitude atmospheric "seeing" artifacts).
For the GMT atmospheric compensation happens at the secondary mirrors. The primary mirrors are still active to maintain the optical alignment but slowly - less than 1 Hz - as compared to atmospheric compensation in the kHz range and I guess this also compensates for thermal expansion.
> I also don't think they control the temperatures of the mirrors but I am not sure about that.
I know they typically air-condition telescopes during the day to keep the mirror at the anticipated nighttime air temperature, to avoid convection currents caused by a warm mirror. That requirement would seem to preclude controlling the temperature to a specific value, I think.
Borosilicate glass. I also don't think they control the temperatures of the mirrors but I am not sure about that.
[...] but still bent by active optics to make the focus of "guide star" perfect (and to deal with any high altitude atmospheric "seeing" artifacts).
For the GMT atmospheric compensation happens at the secondary mirrors. The primary mirrors are still active to maintain the optical alignment but slowly - less than 1 Hz - as compared to atmospheric compensation in the kHz range and I guess this also compensates for thermal expansion.