this rare super telephoto lens was 1st introduced in 1970 and is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/11)
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
Nikon stopped making mirror lenses in 2005
specs
2000mm focal length and field of view (1.2°) at infinity focus
fixed f/11 aperture (a 10“ f/10 telescope in astronomic terms)