Freeform Three-Mirror Anastigmatic Large-Aperture Telescope and Receiver Optics for CMB-S4

CMB-S4, the next-generation ground-based cosmic microwave background (CMB)observatory, will provide detailed maps of the CMB at millimeter wavelengths todramatically advance our understanding of the origin and evolution of theuniverse. CMB-S4 will deploy large and small aperture telescopes with hundredsof thousands of detectors to observe the CMB at arcminute and degreeresolutions at millimeter wavelengths. Inflationary science benefits from adeep delensing survey at arcminute resolutions capable of observing a largefield of view at millimeter wavelengths. This kind of survey acts as acomplement to a degree angular resolution survey. The delensing survey requiresa nearly uniform distribution of cameras per frequency band across the focalplane. We present a large-throughput, large-aperture (5-meter diameter)freeform three-mirror anastigmatic telescope and an array of 85 cameras for CMBobservations at arcminute resolutions, which meets the needs of the delensingsurvey of CMB-S4. A detailed prescription of this three-mirror telescope andcameras is provided, with a series of numerical calculations that indicateexpected optical performance and mechanical tolerance.