optical aberrations

optical aberrations - Defects in a telescope’s optics. (See chromatic aberration, coma, and astigmatism).

orthoscopic eyepiece - A relatively advanced eyepiece offering excellent correction from chromatic aberration, coma, and astigmatism. The only standard eyepiece which surpasses it is the Plössl.

Plössl eyepiece - The most advanced of the standard eyepiece types. It offers both a wider field of view and better overall correction than the orthoscopic eyepiece.

polar axis - On fork equatorial and German equatorial mounts, the axis which is parallel to the Earth’s rotational axis.

Ramsden 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 Huygens eyepiece, the convex side of the field-lens faces toward the observer’s eye, while the convex side of the eye lens faces away from the observer’s eye. Has better color correction than a single lens eyepiece but has slightly worse color correction than the Huygens. However, the Ramsden has much less spherical aberration than the Huygens.

reflector - A telescope whose objective is a concave mirror.

refractor - A telescope whose objective is a lens.

resolution - The smallest possible detail visible through a telescope under existing conditions.

Schmidt - An astrographic camera whose objective is a spherical mirror and whose optical defects are corrected by a corrector plate.

Schmidt-Cassegrainian - A hybrid telescope. It is essentially a Cassegrainian reflector whose steeply curved objective mirror is corrected by a Schmidt style corrector plate.

secondary - The smaller mirror in a reflector telescope which reflects the light from the objective mirror to the eyepiece.

Telrad - Modern electronic version of a finder scope with a “heads-up display” showing a bull’s eye for lining up a target object.

true field of view - The amount of sky seen when viewing through a telescope.