Focal Reducer Calculator

Last update: 2006-01-04

Focal reducer is a positive lens between telescope and CCD camera which shortens the focal length of the optical system. This has advantages for CCD imaging:

short focal length means easier tracking/guiding
faster focal ratio shortens exposure times
larger true field of view

The disadvantages are vignetting and problems for reaching focus on some systems. This calculator can be used to calculate the effective focal length and focal ratio. The focus shift can be used to estimate the ability to reach focus with the telescope. The calculator also estimates the diameter of the imaging circle.

Telescope

D = Diameter of the objective =

F = Focal length of the objective =

Reducer

d = Diameter of the reducer =

f = Focal length of the reducer lens =

X1 = Reducer to CCD distance =

S = Focus shift with reducer =

X2 = Reducer to objective (primary) distance =

System focal length with reducer =

Focal ratio with reducer =

Reduction factor =
Width of the unvignetted field (imaging circle) =

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Telescope
D = Diameter of the objective =
F = Focal length of the objective =

Reducer
d = Diameter of the reducer =
f = Focal length of the reducer lens =
X1 = Reducer to CCD distance =

S = Focus shift with reducer =
X2 = Reducer to objective (primary) distance =
System focal length with reducer =
Focal ratio with reducer =
Reduction factor =
Width of the unvignetted field (imaging circle) =