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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.
We describe the DESI corrector optics, a series of six fused silica and borosilicate lenses. The lens diameters range from 0.8 to 1.1 meters, and their weights 84 to 237 kg. Most lens surfaces are spherical, and two are challenging 10th-order polynomial aspheres. The lenses have been successfully polished and treated with an antireflection coating at multiple subcontractors, and are now being integrated into the DESI corrector barrel assembly at University College London.
We describe the final performance of the lenses in terms of their various parameters, including surface figure, homogeneity, and others, and compare their final performance against the demanding DESI corrector requirements. Also we describe the reoptimization of the lens spacing in their corrector barrel after their final measurements are known. Finally we assess the performance of the corrector as a whole, compared to early budgeted estimates.
Management of light throughput and noise in all elements of the instrument is key to achieving the high-level DESI science requirements over the planned survey area and depth within the planned survey duration. The DESI high-level science requirements flow down to instrument performance requirements on system throughput and operational efficiency. Signal-to-noise requirements directly affect minimum required exposure time per field, which dictates the pace and duration of the entire survey. The need to maximize signal (light throughput) and to minimize noise contributions and time overhead due to reconfigurations between exposures drives the instrument subsystem requirements and technical implementation.
Throughput losses, noise contributors, and interexposure reconfiguration time are budgeted, tracked, and managed as DESI Systems Engineering resources. Current best estimates of throughput losses and noise contributions from each individual element of the instrument are tracked together in a master budget to calculate overall margin on completing the survey within the allotted time. That budget is a spreadsheet accessible to the entire DESI project.
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