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The ground-based Stage-4 Cosmic Microwave Background Experiment (CMB-S4) is a forefront scientific endeavor aimed at mapping the cosmic microwave background (CMB) with unprecedented sensitivity. The cosmic microwave background is the afterglow of the Big Bang and provides crucial insights into the origin and evolution of the universe. CMB-S4 will enhance our understanding of the universe's history, from the highest energy density at the moment of the Big Bang to the formation and evolution of cosmic structures up to the present day.
CMB-S4 is a collaborative effort proposed to be jointly pursued by the U.S. Department of Energy, the National Science Foundation, and international partners. CMB-S4 will deploy the largest arrays of superconducting microwave detectors ever built. The receiver cryostats will be integrated into three different types of highly optimized survey telescopes.
The paper briefly describes the main elements of the proposed CMB-S4 construction project and the key technologies required to build the survey telescopes. The CMB-S4 project management organization is designed as a unified single project integrating the complex organization and support from the two funding agencies. A possible project schedule is introduced, which maps out mass-producing large quantities of superconducting detector wafers, superconducting readout electronics, and testing of final focus module assemblies.
The integration and testing program for the Simons Observatory Large Aperture Telescope optics tubes
Comparing complex impedance and bias step measurements of Simons Observatory transition edge sensors
We explore observing strategies for both small (0.42 m) aperture telescopes (SAT) and a large (6 m) aperture telescope (LAT). We study strategies focused on small sky areas to search for inflationary gravitational waves as well as strategies spanning roughly half the low-foreground sky to constrain the effective number of relativistic species and measure the sum of neutrino masses via the gravitational lensing signal due to large scale structure. We present these strategies specifically considering the telescope hardware and science goals of the SO, located at 23° South latitude, 67.8° West longitude.
Observations close to the Sun and the Moon can introduce additional systematics by applying additional power to the instrument through telescope sidelobes. Significant side lobe contamination in the data can occur even at tens of degrees or more from bright sources. Therefore, we present several strategies that implement Sun and Moon avoidance constraints into the telescope scheduling.
Scan strategies can also be a powerful tool to diagnose and mitigate instrumental systematics either by using multiple scans to average down systematics or by providing null tests to diagnose problems. We discuss methods for quantifying the ability of an observation strategy to achieve this.
Strategies for resolving conflicts between simultaneously visible fields are discussed. We focus on maximizing telescope time spent on science observations. It will also be necessary to schedule calibration measurements, however that is beyond the scope of this work. The outputs of this study are algorithms that can generate specific schedule commands for the Simons Observatory instruments.
The large aperture telescope receiver (LATR) is coupled to the SO six-meter crossed Dragone telescope and will be 2.4 m in diameter, weigh over 3 metric tons, and have five cryogenic stages (80 K, 40 K, 4 K, 1 K and 100 mK). The LATR is coupled to the telescope via 13 independent optics tubes containing cryogenic optical elements and detectors. The cryostat will be cooled by two Cryomech PT90 (80 K) and three Cryomech PT420 (40 K and 4 K) pulse tube cryocoolers, with cooling of the 1 K and 100 mK stages by a commercial dilution refrigerator system. The secondo component, the small aperture telescope (SAT), is a single optics tube refractive cameras of 42 cm diameter. Cooling of the SAT stages will be provided by two Cryomech PT420, one of which is dedicated to the dilution refrigeration system which will cool the focal plane to 100 mK. SO will deploy a total of three SATs.
In order to estimate the cool down time of the camera systems given their size and complexity, a finite difference code based on an implicit solver has been written to simulate the transient thermal behavior of both cryostats. The result from the simulations presented here predict a 35 day cool down for the LATR. The simulations suggest additional heat switches between stages would be effective in distribution cool down power and reducing the time it takes for the LATR to reach its base temperatures. The SAT is predicted to cool down in one week, which meets the SO design goals.
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