Objective Prism

objective prism [äb′jek·tiv ′priz·əm]

(optics)

A large prism, usually having a small angle, which is placed in front of the objective of a photographic telescope to make spectroscopic observations.

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Objective Prism 

one of the spectroscopic prisms used in astronomy; it is mounted in front of the objective lens or mirror of a telescope to produce images of the spectra of celestial bodies in the focal plane. The combination of an objective prism with a camera is called a prismatic camera.

Objective prisms have small angles of refraction and correspondingly low angular dispersion, but because of the great focal length of the objective the linear dimensions of the spectrum are sufficient to determine the spectral class of a star. Spectroscopic devices that use an objective prism do not require a slit, which places them among the highest-transmission devices used in astronomy. The spectra of all the bodies in the telescope’s field of view can be photographed simultaneously; such photographs are convenient for large-scale studies of stellar spectra, searches for novas, and so on. The disadvantages of objective prisms are their large size and weight; therefore, they are seldom used in instruments where the diameter of the objective exceeds 50 cm.

REFERENCE

Kurs astrofiziki i zvezdnoi astronomii, vol. 1. Moscow-Leningrad, 1951.