Mass production of the LFAST 0.76m primary mirrors: fabrication and on-sky control
Abstract
The Large Fiber Array Spectroscopic Telescope is ramping to high volume manufacturing of 0.76m, f/3.46 spherical mirrors that focus light into fibers combined at a central spectrograph. Separating the collection aperture into individual mirrors allows for significantly reduced cost and mandates a fabrication regime where cycle time and “cost per photon” become driving factors. Circular discs are cut from 25.4mm-thick borosilicate float glass and slumped on a quartz mandrel (17mm sagittal depth) in a 60-hour thermal cycle. Surface patterns from an interstitial release agent are removed by grinding on Trizact pads, then the mirror is polished and figured on pitch using metrology from a Twyman-Green interferometer. A large 32:1 glass aspect ratio was chosen to reduce material costs and permit float glass, but the lack of stiffness causes characteristic print-through errors and fabrication tradeoffs. An optimized process combining these tradeoffs is being used to make 20 mirrors for the first telescope unit. The current benchmark of 70% encircled energy coupling within the 1.4arcsec fiber subtense has required bespoke polishing and long cycle time. Mirror support during fabrication has been overhauled to emphasize repeatability in mass production. The finished mirrors are supported using an 18-point passive whiffle tree and will be ventilated to minimize mirror seeing and thermal distortion. Figure changes from the gravity vector and thermal variations will be measured with focal plane wavefront sensing and corrected using 24 thermoelectric coolers installed on the mirror perimeter that induce bending through temperature differentials across the glass. The twenty mirrors are to be installed on a common tracking mount and will steer light into their respective fibers using tip/tilt stepper motors. This prototype 20x telescope achieves a collecting aperture equivalent to a 3.4m primary mirror and establishes a roadmap towards orders of magnitude increase in aperture size.