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 TELE-COMPRESSORS  PROPERTIES
  ( 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.

• T=Telecompressor

• O=Objetiv

• F=Focal length

• P=Focal plane

• R=Reduction

• D=Distance between T and P  

Factors that allows T to furtherly reduce F:  

•  increase of  T-dioptric power

•  mayor approach of T to O

•  mayor distance between T and P
  

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

• using T of higher dioptric power and pushing it at time towards P;

• using T of lower dioptric power and pushing it towards O.

By equal reduction of F :

• increasing dioptries a/o approaching T to P increases vignetting;

• reducing dioptries a/o pushing T towards O, reduces vignetting.

Formula:

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.

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