( also known as “Focal Reducers” )
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.
D=Distance between T and P
Factors that allows T to
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.
equal reduction of F :
increasing dioptries a/o approaching T to P increases vignetting;
reducing dioptries a/o pushing T towards O, reduces vignetting.
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.