The Large Fiber Array Spectroscopic Telescope: An observatory designed for exoplanet spectroscopy

Detecting and characterizing the atmospheres of Earth-like exoplanets orbiting nearby stars requires new telescopes capable of providing high-contrast, high-resolution spectroscopy under severely photon limited conditions. Transit spectroscopy has demonstrated the capacity to measure the molecular components of planet atmospheres, and has been a successful tool for nearly two decades for understanding hot-Jupiter planets and other bright companions. Spectroscopic measurements obtained with resolution sufficient to cleanly separate individual molecular transitions are particularly valuable because they unambiguously identify the presence of an atmospheric constituent. Conversely, low-resolution spectroscopy or spectro-photometric observations frequently come with degeneracies that lead to uncertainty in the specific species detected. The penalty for requiring high-resolution spectroscopy is that large telescope collecting area is required to obtain even marginal signal-to-noise during relatively short transit duration. This problem is exacerbated for the case of terrestrial planets in the habitable-zone of their star, where the planet signal is small, and transits are infrequent due to orbital periods that are measured in months to years, rather than days for hot-Jupiters. Even the largest telescopes currently being constructed will be challenged to efficiently observe the atmospheres of true Earth analogs, since may potential transits will not be observable by a facility located at a single longitude. To truly expedite these detections requires an observatory with giant collecting area distributed around the globe, ideally at five to six independent locations, so that all transits of a potential Earth-like planet can be observed, and long duration transits can be followed throughout their entire duration. The Large Fiber Array Spectroscopic Telescope (LFAST) project is designing a new array telescope platform for operation at visible and near-infrared wavelengths. LFAST is one leg of the larger Eric and Wendy Schmidt Observatory System, and has a goal of providing significant spectroscopic follow-up capability to study exoplanet atmospheres and other astrophysics programs including energetic transients, as well as stellar, galactic, and extragalactic studies that are time-dependent or photon starved. LFAST is comprised of arrays of "unit telescopes", which can be replicated much less expensively than monolithic traditional telescopes, and so can obtain truly enormous collecting areas for reasonable costs. The LFAST design consists of twenty 0.76m, f/3.3 spherical mirrors mounted in a steel frame. Each primary has a corresponding top-end optical package that includes a spherical corrector, an atmospheric dispersion corrector optimized for 400 nm - 1700 nm, and a small guide camera covering an 8 arcmin field. An optical fiber is located at prime focus to carry light from the scientific target to a spectrograph located elsewhere. The structure has a centrally mounted altitude-azimuth drive system that provides the tracking and guiding for the overall system with a small wide-field guider, and each individual primary mirror has independent tip, tilt, and piston control to compensate for flexure or other misalignment. The combined collecting area of twenty LFAST mirrors is equivalent to a traditional 3.5m telescope, but at much lower cost and smaller footprint. Ten such systems has collecting area slightly larger than a 10-m class telescope. One hundred systems is equivalent to a 30-m class giant telescope. Fibers from individual telescopes are combined together to feed a microlens array and a set of re-imaging optics, which will blend the light from individual telescopes into a large rectangular core fiber, forming a single input object for spectroscopy. The LFAST team is currently assembling the first prototype LFAST 20-telescope system, and plans to begin operating it later this year as both an engineering testbed and a functioning scientific 3-m class observatory. This facility will include two spectrograph arms, covering both visible and near-infrared wavelengths using existing instrumentation, and may be suitable for some limited demonstration exoplanet spectroscopy on the brightest transiting targets. We plan to build off of the lessons learned in this small facility and rapidly expand to a larger array of at least 10-m class. This larger observatory will include state-of-the-art spectrographs covering the full LFAST bandpass, including high-resolution infrared capabilities designed to target exoplanet atmospheres. LFAST has a goal of constructing such telescope facilities at a cost of one tenth or less that of a traditional, equivalent aperture telescope. If successful, the 10-m class array will pave the way for a facility that is distributed longitudinally around the globe, with apertures equivalent to the giant telescopes currently under construction, but dedicated to spectroscopy. In this presentation, I will provide an overview of the LFAST program, and update the community on the status of the 20-telescope prototype currently under construction.