The Large Fiber Array Spectroscopic Telescope: status update of the prototype telescope

Abstract

The Large Fiber Array Spectroscopic Telescope, a part of the Eric and Wendy Schmidt Observatory System (LFAST), is designing and building a new optical telescope composed of an array of small “unit telescopes”, which can be replicated economically and at scale to produce an arbitrarily large incoherent aperture. Our first prototype telescope, the LFAST- 20X, consists of twenty 0.76m, f/3.3 telescopes in a common structure, on a single altitude-azimuth mount. Each unit telescope illuminates a 1.4” diameter optical fiber at prime focus. This array provides the effective collecting area of a single aperture 3.4m telescope, but at a fraction of the cost of a traditional facility. Software control of the mount and individual telescope systems is challenging because of the large number of components that must be controlled, and so is being built from the ground up using frameworks designed for multiplexed industrial facilities. The LFAST optical system is designed to operate between 400nm and 1700nm, bounded by the grasp of our atmospheric dispersion corrector and the transmission of the prime focus corrector optics and fiber. The fiber feed for the array incorporates a micro-lens beam combiner to efficiently combine light from the individual telescopes into a single, large rectangular fiber, which forms the entrance object for a spectrograph. We split the light into two arms, a visible arm from 400 - 900nm, and a near-infrared arm from 950 - 1700 nm, and each arm feeds a single spectrograph. In this manuscript, we provide updates on the fabrication, assembly, and testing of the LFAST-20X, as well as describe our plans to operate this system as a working observatory on Kitt Peak after commissioning. LFAST-20X will be a queue operated facility. We are designing the operations to accommodate target of opportunity observations that require rapid follow-up, such as transients detected by the Vera Rubin Observatory and Argus, as well as more traditional programs seeking observations of solar system, stellar, and galactic targets. Larger arrays that are being considered for the future could carry-out the characterization of exoplanet atmospheres, rapid classification of bright transient sources including supernovae and gamma-ray-bursts, longer term surveys of very faint galaxy clusters to search for signatures of intermediate mass black holes, classical studies of stars, disks, and sub-stellar objects, and probe fundamental physics including the potential evolution of the fine structure constant. Other papers in these proceedings discuss the optical fabrication of the primary mirrors, mechanical design of the drive and structure, system throughput, telescope control software, instrumentation, integration and testing, and future instrumentation concepts for a larger array.