I'm pretty sure you're correct, although I believe that the part that's capturing photons also needs to be heavily protected from the environment, and you also need something to prevent to many photons from getting to it and burning it out (e.g., almost all gen 3 NODs are autogated so that someone shining a flashlight at you won't wreck your image intensifier tubes.)
It's one of those things that can get pretty overwhelming to try and research as a consumer, because it gets really technical really fast.