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Magnification:
The magnification corresponds to the difference between observation with the naked eye and
observation through a magnifying device like a telescope. If a telescope configuration has a magnification
of 30x, then an object viewed through the telescope will appear 30 times larger than it would with the
naked eye. To calculate the magnification of your telescope setup, divide the focal length of the telescope
tube by the focal length of the eyepiece. For example, a 20mm eyepiece in a telescope with a 1000mm
focal length will result in 50x power, which will make the object appear 50 times larger. If you change the
eyepiece, the power goes up or down accordingly.
Focal ratio
The focal ratio of a telescope is determined by dividing the telescope’s focal length by its aperture
(usually expressed in millimeters). It plays a key role in determining a telescope’s field of view and
significantly impacts imaging time in astrophotography. For example, a telescope with a focal length of
1000mm and a 100mm clear aperture has a focal ratio of f/10.
Exit Pupil
The exit pupil is the diameter of the beam of light coming out of the eyepiece. To calculate exit pupil,
divide the focal length of your eyepiece by your telescope’s focal ratio. For example, if you use a 20mm
eyepiece with an f/5 telescope, the exit pupil would be 4mm.
Eye Relief
Eye relief is all about a comfortable viewing experience because it is the distance at which you need to
position your eye from the eyepiece’s outermost surface to enjoy the full field of view. This characteristic
is of special concern to observers who wear glasses to correct an astigmatism, because a long enough
eye relief is necessary to allow room
for glasses.
Exit Pupil =
Eyepiece Focal Length
Telescope Focal Ratio
Eyepiece Lens
Long Eye Relief Distance
Eyepiece Lens
Short Eye Relief Distance
Eyepiece Lens
Exit Pupil (mm)
Eyepiece Lens
Long Eye Relief Distance
Eyepiece Lens
Short Eye Relief Distance
Eyepiece Lens
Exit Pupil (mm)
Eyepiece Lens
Long Eye Relief Distance
Eyepiece Lens
Short Eye Relief Distance
Eyepiece Lens
Exit Pupil (mm)
Telescope Focal Length
1000mm
Eyepiece
Focal Length
20mm
Aperture
102mm
Magnification =
Telescope Focal Length
Eyepiece Focal Length
Focal Ratio =
Telescope Focal Length
Telescope Aperture
