this super telephoto lens introduced in mid-1970's is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/8)
virtual elimination of chromatic aberration as uses mirror rather than lenses
also means focus position for infrared photography is the same as for light photography
central mirror:
allows much more compact lens length as the optic path is reflected internally twice
by necessity, blocks some of the light and thus causes:
decrease in image contrast
characteristic “doughnut” shaped bokeh imagery in out of focus areas
designed by OPCON Associates, a company founded by Ellis Betensky, who used to be a senior optical designer at Perkin-Elmer.
their ruggedness is partially due to their solid, one-piece construction, unlike most production mirror lenses
specs
600mm focal length and field of view (4.2°) at infinity focus