National Geographic STAR APP50 Instruction Manual page 14

Aperture
102mm
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.
Focal length (Telescope):
The focal length is the distance in millimeters between the objective lens or primary mirror and the
point at which entering light rays converge — otherwise known as the focal point. The focal lengths of
the telescope tube and the eyepiece are used to determine magnification.
Focal length (Eyepiece):
The focal length is the distance in millimeters between the center of the first lens element in an
eyepiece and the focal point. The focal lengths of the telescope tube and the eyepiece are used
to determine magnification. Short eyepiece focal lengths produce higher magnifications than long
eyepiece focal lengths.
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.
Magnification =
Focal Ratio =
Exit Pupil (mm)
Eyepiece Lens
Telescope Focal Length
1000mm
Telescope Focal Length
Eyepiece Focal Length
Telescope Focal Length
Telescope Aperture
Exit Pupil =
14 14
Eyepiece
Focal Length
20mm
Eyepiece Focal Length
Telescope Focal Ratio