For the past six years, CCAT Observatory Inc., a nonprofit research organization formed by more than a dozen academic institutions led by Cornell University, has been developing a new telescope, the Fred Young Submillimeter Telescope (FYST, pronounced “feast”) and its supporting infrastructure to observe at submillimeter wavelengths. The infrastructure is currently under construction at an altitude of 5,600 m near the summit of Cerro Chajnantor in the Atacama Desert of northern Chile, one of the driest places on Earth. Since water vapor absorbs the wavelengths of interest, the site is arguably the best ground-based location for submillimeter observations due to its thin, ultradry atmosphere. Once complete, it will be the second highest observatory in the world.
The submillimeter (submm) regime, defined as wavelengths from 1.0 to 0.2 mm, or frequencies from 300 to 1,500 GHz, offers a unique window into astrophysics. The mapping speed of FYST, along with first generation focal plane instruments, will enable new large-scale surveys of the sky. CCAT will reveal the trail from interstellar clouds to star formation in the Milky Way; the process of galaxy assembly and evolution; the process of reionization of the universe from the first galaxies; the growth of large-scale structure in the universe and massive clusters of galaxies; and transient events from the growth of protostars to the most luminous explosive events in the universe. In addition, CCAT submm maps of the cosmic microwave background are critical to the search for primordial gravitational waves – the required signature of the inflationary epoch at the origin of the universe.
To achieve these goals, CCAT is building FYST, with a six-meter clear aperture primary mirror. The aperture is modest by today’s standards, but due to its low emissivity, low cross-polarization and large field of view that can illuminate several hundred thousand detectors, its mapping speed is enormous. Mechanically, FYST is unconventional, as the elevation axis can go from nadir (EL = -90°), to horizon (EL = 0°), zenith (EL = +90°), and then opposite horizon (EL = +180°). Also, the optics are fully contained within the elevation superstructure, which diminishes the wind loading, and combined with a retractable shutter, provides environmental protection for maintenance and stowing for inclement weather. The design of FYST is so compelling that the Simons Observatory chose to produce a copy with reduced tolerances for its large aperture telescope operating in the millimeter-wavelength regime, and it serves as the baseline design for the Chilean large aperture telescopes of the fourth generation, ground-based cosmic microwave background experiment (CMB-S4).
Any telescope, of course, is just a machine that collects and focuses light. Focal plane instruments are needed to enable the broad range of science topics outlined above, and there are two planned for FYST’s first light: Prime-Cam and the CCAT Heterodyne Array Instrument (CHAI). Both instruments use pulse tube cryocoolers to lower the instrument background and enable the superconducting detectors to collect the weak astrophysical signals. Liquid cryogens were ruled out early in the planning stage due to their expense and the remoteness of the site.
For CHAI, two cryostats are planned, one for low frequency (~475 GHz) and one for high frequency (~810 GHz). Each CHAI cryostat uses two cryocoolers running in parallel to achieve a 50 K thermal shield and a 4 K focal plane array. From an operations standpoint, a key feature of the CHAI cryostats is their ability to go into a standby mode where only one cryocooler is required to maintain system temperatures when the instrument is not observing. This is especially important for the power budget of the observatory and to help reduce its carbon footprint.
Prime-Cam will have multiple optics modules within a single large cryostat (roughly 1.8 m in diameter), also utilizing multiple cryocoolers for 80, 40 and 4 K thermal intercepts, as well as a dilution refrigerator providing a 100 mK bath for the microwave kinetic inductance detector arrays. The optics modules will encompass both continuum cameras and imaging spectrometers. The modular design of Prime-Cam allows for optics modules to be developed in parallel at member institutions, as well as provide a framework for staged deployment and upgrades.
Progress on the observatory is well underway. The telescope’s foundation is complete, as well as the installation of 8 km of power and fiber optic cables and hundreds of meters of grounding grid. The steel structure of the telescope is complete as well, and the FeNi36 alloy elevation superstructure is currently in fabrication. All bearing, drive, and servo components have passed their initial factory acceptance testing. The mirror backup structure, made of carbon-fiber-reinforced plastic, is nearly finished for the primary mirror and the secondary is in progress, and all the 146 lightweight aluminum mirror tiles have reached completion. CPI Vertex Antennentechnik GmbH, the prime contractor handling the detailed design and construction of FYST, is conducting a full trial assembly this year in Germany prior to shipping to Chile in 2024. We look forward to declaring first light in early 2025 and commencing upon our early science program.
Image: Rendering of the Fred Young Submillimeter Telescope. Credit: CPI Vertex Antennentechnik GmbH
Image 2: Carbon fiber-reinforced plastic truss assembly for the primary mirror backup structure of FYST. Credit: CCAT Observatory, Inc.
Image 3: Milling of the gearbox and main bearing interfaces on the central section of the telescope traverse welded assembly. Credit: CCAT Observatory, Inc.
