exit pupil

exit pupil - The smallest diameter of the image of an eyepiece’s field of view when the eyepiece is used with a particular telescope.

eye lens - The lens in an eyepiece which is nearest the eye.

eye relief - The maximum distance between your eye and the eyepiece where the entire field of view of the eyepiece is visible.

eyepiece - The part of the telescope from which the observer obtains a view. Also called an ocular, it is this part that determines the magnification of the telescope.

f-ratio - A value equal to the focal length of a lens or mirror divided by the aperture.

field lens - The lens in an eyepiece which magnifies the apparent field of view to prevent “tunnel vision”.

finder scope - Either a small telescope or non-magnifying sighting instrument mounted on the side of the main telescope. It is used to locate objects to be viewed through the main telescope.

focal length - Distance from a lens or mirror where light from an infinitely distant object is focused.

focal point - The point where light from an infinitely distant point source is focused by a lens or mirror.

fork equatorial mount - A good mount for astrophotography since its polar mount is parallel to the Earth’s rotational axis. It is easier for novices to use than the German Equatorial Mount because the telescope may be freely rotated up or down without any polar axis rotation.

Galilean refractor - Simplest of all telescopes. Has a single element convex lens as its objective and is highly susceptible to chromatic aberration.

German equatorial mount - A telescope mount with its polar axis aligned with the Earth’s rotational axis which also has its declination axis attached to the top of its polar axis. This mount is found mainly on refractors and Newtonian reflectors and works well for astrophotography purposes.

Huygens eyepiece - An eyepiece containing an eye lens and a field lens both of which are simple lenses. Each lens has one convex side. Unlike the Ramsden eyepiece, the convex side of both lenses faces away from the observer’s eye. Has better color correction than a single lens eyepiece, but still causes a fair amount of false color. Though the Ramsden exhibits slightly more chromatic aberration, it yields far less spherical aberration than the Huygens.