A single Kepler science module with two CCDs and a single field flattening lens mounted onto an Invar carrier. Each of the 21 CCD science modules are covered with lenses of sapphire. The lenses flatten the field of view to a flat plane for best focus. Sapphire is highly refractive and is strong, so a thin, light lens can be used and still be durable. Sapphire is also very resilient to extremes of hot and cold and is radiation resistant- that is, its optical properties don’t degrade significantly with radiation impact.
