focal reducer
A focal reducer is a positive lens (or combination of lenses), which is usually placed in front of the focal plane in the converging beam from the telescope.
A focal reducer is an auxiliary lens, or combination of lenses, used mainly for photographic or CCD-imaging purposes. It reduces a telescope's effective focal length, allowing images to be taken much faster, and also gives a wider field of view.
Some focal reducers may be used visually to obtain a wider field of view, but others are intended for photographic/CCD use only.
The opposite of a focal reducer – the focal extender or Barlow lens – has negative optical power and serves to increase the focal length of the telescope, and hence narrow the field of view.
