30
ASTRONOMY FORMULAS FOR YOUR
ZHUMELL DOBSONIAN TELESCOPE
MAGNIFICATION
To determine the magnification of a telescope and eyepiece
combina¬tion, divide the telescope focal length be the eye-
piece focal length.
Magnification (x) = Telescope Focal Length (mm)/Eyepiece
Focal Length (mm)
Ex: 3mm Eyepiece with a 203x1200mm telescope.
Magnification = 1200mm/32mm
Magnification = 37.5x
FOCAL RATIO
To determine the focal ratio of a telescope, divide the focal
length of the telescope by the aperture.
Focal Ratio (f/x)= Telescope Focal Length (mm)/Aperture (mm)
Ex: Focal Ratio of a 203x1200mm telescope.
Focal Ratio (fx)= 1200mm/203mm
Focal Ratio (fx)= f/5.9
