LFAST, the Large Fiber Array Spectroscopic Telescope

The LFAST concept is to use thousands of small telescopes combined by fibers for high resolution (R=150,000) spectroscopy, in a way that will realize large cost savings and lead to an affordable aperture as large as 20,000 m2. Such large aperture is needed, for example, to make a comprehensive search for biosignatures in the atmospheres of transiting exoplanets. Each unit telescope of 0.76 m aperture (0.43 m2) will focus the image of a single star onto a small (17 μm core) fiber, subtending 1.32 arcsec. Our telescope design calls for a spherical mirror, with a 4-lens assembly at prime focus that corrects not only for spherical aberration, but also for atmospheric dispersion down to 30° elevation, from 390 nm-1700 nm, and for rapid image motion caused by seeing or wind jitter. A method for rapid production of such mirrors has been tested, in which a disc of borosilicate float glass is slumped over a high-precision polished mandrel to an accuracy that greatly reduces subsequent optical finishing time. A method for active thermal control of mirror figure using Peltier devices will be incorporated. The projected cost of each unit telescope, when mass produced by the thousand, would then be approximately $8,000. The telescopes will be mounted in the open in groups of 20 located 12 m apart. The mirrors will be arrayed on either side of a central, pedestal-mounted alt-az drive using commercial worm gear bearings. Protection against rain and dust will be provided by automated covers above and below the mirrors, and by pointing the mirrors down (-20° elevation). The first LFAST array, some 150 m in diameter, will comprise 132 mounts carrying a total of 2,640 mirrors and having 1,200 m2 in collecting area. The light from all the fibers is combined at the central spectrographs, with little increase in etendue, by a 5 x 528 array of adjacent hexagonal lenses. A telecentric lens is used to reimage the lens array at the entrance slits of two echelle spectrographs. Together, these two cover simultaneously the full 390 nm-1700 nm spectral range of the star being observed. The targeted cost for the installed LFAST telescope and fiber array is $60M.