3 ms·
1nm precision of the curve and 20nm for surface irregularities. That's incredible. Wouldn't just thermal expansion cause it to deviate from the 1nm precision? W
by rdtsc 9y ago
1nm precision of the curve and 20nm for surface irregularities. That's incredible. Wouldn't just thermal expansion cause it to deviate from the 1nm precision? What about dust, precipitation just bacteria creating a film on the surface.
- kurthr 9y agoYes, the "nanometer level" is local flatness not the position. The mirror itself is only polished ~20nm flat. It is temperature controlled and made of a low expansion ceramic (zerodur?), but still bent by active optics to make the focus of "guide star" perfect (and to deal with any high altitude atmospheric "seeing" artifacts).
- danbruc 9y agoIt 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.
- tesseract 9y ago> 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.
- deleted 9y ago[deleted]