Mirrors, for a variety of reasons. But basically it comes down to mirrors being as close to an exact re-creation as physically possible, while all the lenses and imaging materials (either film or detector) distort the image. Even at their very best, lenses will be always be worse because we’re jamming all that light through a (relatively) small hole. Mirrors don’t really have that problem. Mirrors are also typically flat, which minimizes other sources of distortion.
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