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