Eyepiece Prism

(1) A telescope accessory that is used to obtain spectra of stars. It can be used instead of an objective prism and has most of the advantages of the latter. The eyepiece prism is mounted near the focal plane of the eyepiece, where the cross section of the light beam is considerably smaller than near the objective; therefore, the illuminated diameter of the eyepiece prism is comparatively small. However, to obtain a spectrum of sufficient linear size in this case, considerable angular dispersion of light is needed. Therefore, direct-vision prisms of complex design are used as eyepiece prisms.

(2) A reflecting prism mounted behind the eyepiece of small telescopes, such as universal and satellite telescopes, or straight-tube theodolites. Such prisms facilitate the sighting of objects that are visible at small zenith distances if the eyepiece of the tube is located too close to the body of the instrument.