The 4th iteration of something I've been working on for a while(the first version was literally made of cardboard). An adjustable aperture for a cheap 500mm f/6.3 mirror(catadioptric) lens. These lenses are available for $120-150 online, which seems like a pretty sweet deal. However, the DOF is razor thin, and even if you can manage to get what you want in focus, the contrast and sharpness is pretty abysmal. Which, honestly, is to be expected from a fixed f/6.3 aperture. So I made an adjustable aperture that screws on like a lens filter(fits anything with a 95mm filter size, which most of these lenses have), and gives you a useable DOF, and improves the contrast/sharpness a bit, though at the cost of the unique “donut” bokeh of these lenses.

Assembly Instructions: insert the post into the base so that the threaded portion faces forward. fit the cover over the post, and tighten the nut down to secure it in place.