objective prismA 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).
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.