When selecting lenses for space applications, it is generally a good idea to make sure they are of fixed focal length and fixed aperture, in order to prevent the adjustments from shaking during vibe. This means you can’t focus the lenses like you normally would do to a physical camera. Instead, you must use a special procedure called shimming.
To focus a fixed focal length lens, you manually vary the distance between the image plane (i.e. the image sensor) and the lens. This is done by screwing/unscrewing the lens within the lens mount. To hold the lens in a particular distance within the lens mount, you use something called shims.
Shims (also called spacers) are nothing more than small donut-shaped spacers that you can place between the top of the lens holder and the bottom of the lens. This way, you artificially increase the height of the lens holder so that the lens when “fully threaded in” is at its optimum focus.
Shims are precision machined to be very specific “heights”. These heights are generally measured in “thou” (thousandths of an inch). They range from 0.1 thou to 50 thou.
To determine the correct “shim height”, you would follow the procedure below:
Once you’ve placed down the right amount of shims, you need to glue the shims in place with a vacuum-safe glue. One commonly used epoxy in the industry is EC 2216. You want to place the glue such that it creates bonding between the lens, the shims, and the lens holder. You don’t need to glue the entire circumference of the setup (epoxy is expensive!) - just create “lines of glue” from the lens down to the holder.