objective prism

objective prism

A narrow-angle prism placed in front of the primary mirror or objective lens of a telescope. The prism disperses the incident light very slightly so that each star image is spread out into a small spectrum (see dispersion). A transmission diffraction grating (called an objective grating) is now often used for the same purpose. The spectra of a field of stars can thus be recorded in a single photographic or electronic image and the chief spectral features of the stars can be quickly assessed (see spectral types).
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.


Kurs astrofiziki i zvezdnoi astronomii, vol. 1. Moscow-Leningrad, 1951.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

objective prism

[äb′jek·tiv ′priz·əm]
A large prism, usually having a small angle, which is placed in front of the objective of a photographic telescope to make spectroscopic observations.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Because Stock used an objective prism, these plates yielded the spectra and magnitudes needed to pick clusters out from the teeming masses of field stars.
Key to the success of this time- and labor-intensive project was the development of an objective prism, which fits over a telescope's main lens and creates a spectrum of every star in the field of view.
The results are very similar to those obtained by using a costly objective prism in front of the telescope's aperture.
For example, in the Fall 1995 issue of CCD Astronomy, England's Maurice Gavin showed a series of impressive spectra obtained with an objective prism placed at the entrance of his telescope.
In his CCD Astronomy article, Gavin showed a methane absorption feature in the planet's spectrum at 7250 angstroms, but the resolution was low due to Jupiter's large size and the small dispersion of the objective prism at red wavelengths.
The paper reported that Barnard had outfitted a motor-slewed telescope with an objective prism and three slits tuned to the bright hydrocarbon bands commonly found in the spectra of comets.
Full-aperture objective prisms on Schmidt cameras have proven a powerful survey tool, as in searches for quasars and the automated spectral typing of stars.