( also known as “Focal Reducers” )  

                       Telecompressors are positive optical elements, useful mainly for photographic work, 
                       but also for visual observation.

They short the focal length and increase the speed of the optical system,
and, within certain limits, also the field.

Factors that allows T to furtherly reduce F:  

Without changing the distance between O and P,
the same reduction of F is possible:

By equal reduction of F :


R= F/ (F+D);     D=F/R – F  

Telecompressors, also known as “focal reducers”, are 3- or 4-lenses systems; 
they are often devised also as "field flatteners"  and are usually quite expensive.

You can nevertheless try with inexpensive achromats (Binoculars lenses of 50 mm diameter).
The results can be sometime for the beginner quite interesting.
See the image of the Dumbbell Nebula, obtained with a 10 inch Schmidt-Cassegrain telescope, using a home made focal reducer.