Hackaday project 326 has built a monocular that can image in the infrared spectrum, with parts cannibalised from his previous thermal microscope project.
Initially using a Newtonian reflector design, Project 326 was unable to get the scope to work well with infrared light, suffering from a diffraction limit.
Switching to a Keplerian design with a custom engineered first surface mirror gave much better results.
Two sets of lenses provided 5x and 10x magnification, with the lenses reducing sensitivity but nonetheless yielding successful results.
The posts describes some of the complexities and challenges of amateur optics design for the IR spectrum, and provides links to previous thermal imaging projects.
