An external optical relay assembly to allow an infrared camera with a fixed aperture to be used with a variety of fore optics, including refractive compound lenses, reflective telescopes, and reflective/refractive lenses, by providing an external, cooled aperture, that can be adjusted to provide effective f-number matching to the fore optic, allowing any f-number fore optic to be used with the infrared camera. This allows users of large families of similar telescopes, for example, to use their inventory of infrared Ritchie-Chrétien telescopes with a single infrared camera, regardless of f-numbers.