Cold optical design for the large aperture Simons' Observatory telescope

The Simons Observatory will consist of a single large (6 m diameter) telescopeand a number of smaller (∼0.5 m diameter) refracting telescopesdesigned to measure the polarization of the Cosmic Microwave Backgroundto unprecedented accuracy. The large aperture telescopeis the same design as the CCAT-prime telescope, a modified Crossed Dragone design with a field-of-view of over 7.8 degrees diameterat 90 GHz. This paper presents an overview of the cold reimaging optics for this telescopeand what drove our choice of 350–400 mm diametersilicon lenses in a 2.4 m cryostatover other possibilities. We will also consider the future expandability of this design to CMB Stage-4 and beyond.