In 1992, I gave in to curiosity and lens lust and purchased the 600mm mirror lens from Sigma. The front element has a mirror (in reverse) at its center, forcing the light to travel three times through the lens and thereby allowing for a compact and relatively inexpensive design. This has some side effects, for instance the notorious donut shaped bokeh.
Then it’s manual, fixed aperture at f8, and not that sharp. I have maybe used this lens on four film rolls, which is very little.
When looking at the old negatives, the keepers from back then also show benefits.
The unruly bokeh can be used to separate foreground and background. Instead of the silky smooth blur caused by rounded aperture blades that has become the gold standard of every fast lens, here we can simultaneously blur and shake up the background.
This dramatic difference between foreground and background is hard to achieve (and maybe not desired) with modern glass. I used to experiment more.
The Inner Frame 