  SPT-3G is a detector system for the 10m diameter South Pole Telescope,
comprising 16,000 millimeter-wave bolometers. It is used for a deep Cosmic
Microwave Background survey of the Southern sky. This paper describes the
geometry of the secondary mirror, which is a section of a prolate spheroid, in
several useful coordinate systems. There is application to off-axis mirrors in
general. A geometric theorem is proven, relating to the Dragone condition: the
intersection of a prolate spheroid and any plane is an ellipse; the lines
connecting points on that ellipse to either focus compose a right circular
cone; the central axes of the two cones from the two foci intersect outside the
interior of the spheroid.
