National Geographic STAR APP50 Instruction Manual page 13

Telescope Terms to Know:
Objective Lens
Aperture
(mm)
Aperture
(mm)
Aperture:
This figure, which is usually expressed in millimeters, is the diameter of a telescope's light-gathering
surface (objective lens in a refractor or primary mirror in a reflector). Aperture is the key factor in
determining the brightness and sharpness of the image.
Objective Lens:
The objective lens is the main light-gathering component of a refractor telescope. It is actually
composed of several lens elements.
Diagonal:
This accessory houses a mirror that deflects the ray of light 90 degrees. With a horizontal telescope
tube, this device deflects the light upwards so that you can comfortably observe by looking downwards
into the eyepiece. The image in a standard diagonal mirror appears upright, but rotated around its
vertical axis (mirror image). To get an image without this rotation, you will need to use a special
diagonal with an erect image prism.
Eyepiece:
An eyepiece is an optical accessory comprised of several lens elements. It determines the magnification
of a particular observing setup.
Primary Mirror:
The primary mirror is the principle light-gathering surface of a reflector telescope.
Secondary Mirror:
A secondary mirror is a small mirror that sits at a 45° angle in relation to the primary mirror of a
reflecting telescope. Light from the primary mirror is reflected back up the tube to the secondary
mirror. The light is directed from this mirror up into the eyepiece.
Focal Length Telescope
(mm)
Eyepiece
Focuser
Focal Point
Secondary
Mirror
Focal Length Telescope
(mm)
Focal Length
Eyepiece
(mm)
13 13
Eyepiece
Focuser
Diagonal Focal Point
Primary Mirror
(Objective)
Focal Length
Eyepiece
(mm)