achromatic lens
A telescope lens composed of two lenses ground from different kinds of glass and designed to bring two selected colors to the same focus and correct for chromatic aberration.
reflecting telescope
A telescope that uses a concave mirror to focus light into an image.
light-gathering power
The ability of a telescope to collect light; proportional to the area of the telescope objective lens or mirror.
resolving power
The ability of a telescope to reveal fine detail; depends on the diameter of the telescope objective.
diffraction fringe
Blurred fringe surrounding any image caused by the wave properties of light. Because of this, no image detail smaller than the fringe can be seen.
magnifying power
The ability of a telescope to make an image larger.
light pollution
The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects.
prime focus
The point at which the objective mirror forms an image in a reflecting telescope.
secondary mirror
In a reflecting telescope, the mirror that reflects the light to a point of easy observation.
Cassegrain focus
The optical design of a reflecting telescope in which the secondary mirror reflects light back down the tube through a hole in the center of the objective mirror.
Newtonian focus
The focal arrangement of a reflecting telescope in which a diagonal mirror reflects the light out the side of the telescope tube for easier access.
Schmidt-Cassegrain focus
The optical design of a reflecting telescope in which a thin correcting lens is placed at the top of a Cassegrain telescope.
sidereal drive
The motor and gears on a telescope that turn it westward to keep it pointed at a star.
equatorial mounting
A telescope mounting that allows motion parallel to and perpendicular to the celestial equator.
alt-azimuth mounting
A telescope mounting capable of motion parallel to and perpendicular to the horizon.