this super telephoto lens introduced in ? is a modified Maksutov catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/10)
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
the Rubinar 1000mm f/10 is a later version of this lens
specs
1000mm focal length and field of view (2.5°) at infinity focus
fixed f/10 aperture
lens elements, ? groups
closest focus 7.3m (24')
M116 rear filter thread
M42 lens mount thread
122mm (4.8“) diameter, 250mm (9.8”) long and 1.9kg (4.2lb)