6
Barlow Lens
2 Lens Elements
Huygenian Eyepiece
2 Lens Elements
Light
Kellner Eyepiece
3 Lens Elements
Light
Plössl Eyepiece
4 Lens Elements
Light
Light
Eye Lens
Field Lens
Eye Lens
Field Lens
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.
Barlow Lens:
A Barlow lens effectively increases the focal length of a telescope. It is inserted between the eyepiece
and the focuser/diagonal (depending on the optical setup) and multiplies the magnification power of the
eyepiece.
For example, a 2x Barlow will double the magnification of a particular eyepiece.
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.
Objective Lens
Aperture
(mm)
Eyepiece
Diagonal Focal Point
Focal Length Telescope
(mm)
Focal Length
Eyepiece
(mm)
Focuser
Telescope Terms to Know:
