chromatic aberration

chromatic aberration - False color caused by the prism effect in a simple lens.

collimation - The alignment of a reflector telescope’s objective.

coma - A distortion of an image caused by an extremely steep reflecting surface. See Understanding Optical Defects.

common alt-azimuth mount - Most common form of the alt-azimuth mount. This term is used to distinguish it from the more sophisticated Dobsonian mount which is also an alt-azimuth mount.

concave - An inwardly curving surface.

convex - An outwardly curving surface.

corrector plate - The transparent optical element in the front of a Schmidt-Cassegrainian telescope. This element cancels out defects in the objective mirror.

Dawes’ Limit - The theoretical maximum resolution that a particular telescope can obtain.

deep sky object - Any object beyond our solar system, such as: a galaxy, star cluster, or interstellar nebula.

diagonal eyepiece holder - An accessory for telescopes with rear mounted eyepieces. This device makes it more comfortable to view when the telescope is pointed toward the vertical.

diffraction - Scattering of light due to wave interference. In reflector telescopes, diffraction causes a loss of sharpness which is proportionate to the size of the secondary mirror.

Dobsonian mount - A special form of alt-azimuth mount invented by John Dobson of the San Francisco Sidewalk Astronomers.

element - Any lens or mirror.

Erfle - A type of eyepiece derived from the Kellner eyepiece. It gives a much wider apparent field of view than a Kellner by introducing a third achromatic element between the eye lens and field lens.