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

a lens that sits in front of your camera

A focal reducer is used to decrease the focal ratio (and focal length) of a telescope. A smaller focal ratio yields a faster optical system and thus a shorter exposure time. The trade off is reduced image scale, but this is usually acceptable in return for a wider field of view and shorter exposure. Focal reducers typically attach to the back of the telescope, just ahead of the camera in the optical path. They are made in a variety of focal reduction factors, from 0.18x to 0.8x. Popular focal reducers for SCTs are 0.33x and 0.63x reducers, often called f/3.3 and f/6.3 reducers since most SCTs have an inherent focal ratio of f/10 and thus the reducers yield these new focal ratios. A focal reducer is a highly recommended accessory for imaging. back to the top back to the top back to the top back to the top back to the top back to the top