The LFAST 0.76m mirrors: mass production, active control and on-sky performance

The Large Fiber Array Spectroscopic Telescope (LFAST) pursues large collecting aperture at low cost. Arrays of 0.76m, f/3.5 mirrors will focus light into fibers that are combined at a high-resolution spectrograph. The mirror substrates are fabricated from 25.4mm thick Schott Borofloat® discs in a one week slump and polish process that leaves less than 80nm rms wavefront error in medium and high spatial frequency modes. Low-order figure errors are corrected with a perimeter ring of thermoelectric controllers that induce expansion or contraction with top-to-bottom thermal gradients. In operation, temperature variations from nighttime cooling cause time-varying aberration modes. Using feedback from a stellar wavefront sensor, these aberrations can be compensated to focus starlight energy into a 1.4 arcsec fiber.