Optical Designs for Large Detector Arrays on Spherical-Primary Orbital/Sub-Orbital Telescopes

We discuss the guidelines to the optical design for relatively small-aperture sub-millimeter telescopes employing a spherical primary and large-format bolometer arrays. Although optical solutions for large ground optical telescopes using a spherical primary are discussed in the literature, these configurations are not acceptable for orbital or sub-orbital platforms. As a baseline telescope design we will use BLAST, the Balloon-borne Large-Aperture Sub-millimeter Telescope, which will be employing bolometer arrays at 250 m, 350m and 500 m, and will carry a 2.0 m spherical primary mirror during the first flight. We examine two different sets of optical solutions that might be used on future spherical-primary small aperture telescopes: first, we discuss configurations employing on-axis optical elements to correct the spherical aberration introduced by the primary. Then, we present several optical solutions using off-axis, cooled mirrors. These configurations are discussed in relation to optical performance and various operational design constraints. A preliminary optical design for BLAST is also discussed.