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.