An off-axis three-mirror anastigmat is proposed for future cosmic microwave background observations. The telescope has a 5 m diameter primary, giving 1.5' angular resolution at lambda = 2 mm, which is sufficient for measurements of gravitational lensing and for galaxy cluster surveys. The design includes several key features, not previously combined in a large telescope, that are important for sensitive measurements, especially on large angular scales: (1) high throughput (8 degrees diameter diffraction-limited field of view at lambda = 1 mm, and 12 x 8 degrees at lambda = 3 mm, so a single telescope could support all the detectors for an optimistic, future experiment); (2) low scattering (all the mirrors are small enough to be monolithic, so there are no segment gaps); (3) full boresight rotation, over the full elevation range, for measuring polarization errors; and (4) a comoving shield or baffle around the entire telescope to control pickup. (c) 2018 Optical Society of America
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