A device that effectively lowers the focal length of a telescope. Usually used to "speed up" the f/ ratio for photography. Also referred to as a telecompressor. Commonly used on refractors and catadioptrics. Can be thought of as the opposite of the barlow lens.
a camera which changes the focal ratio, or the scale, of the telescope
a lens system that mounts on the rear cell of a scope in front of a flip-mirror, camera, or diagonal and eyepiece