Focal reducer
Also called: reducer, f/6.3 reducer.
A lens that shortens a telescope's focal length — an f/6.3 reducer turns an f/10 SCT into a wider-field instrument.
Reducers are the opposite of Barlows: they lower the magnification of every eyepiece and widen the field. They're mainly used on long-focus SCTs, whose narrow fields can't otherwise frame big objects.