catadioptric

adj.
Of or relating to an optical system that uses both reflective and refractive optical devices.

Catadioptric (or ‘mirror’) lenses are a combination of reflective and transmissive elements. They are a form of telephoto objective in which the prime focusing elements are spherical mirrors, the refractive elements being employed chiefly to correct aberrations (Fig. 1). Catadioptric objectives are small and light compared with traditional long-focus lenses, but cannot be stopped down, and instead use neutral-density filters.

Fig. 1

— Graham Saxby