The need to realign the optical elements of a telescope for best performance after the telescope has been disassembled, frequently moved, or given rough treatment. Usually required relatively frequently with reflector telescopes (particularly with large fast focal ratio systems), very rarely with catadioptrics, and almost never with a refractor. Recollimation is very easy with a catadioptric; still easy, but a little more time consuming, with a reflector; and difficult and best left to the manufacturer with a refractor.

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