Slitless Spectrograph

an astronomical spectrograph that is mounted on large telescopes and used to obtain the spectra of faint celestial bodies—stars, comets, planetary nebulas, and the like. The light source in a slitless spectrograph is not a narrow slit as in an ordinary spectrograph. The image of the celestial object formed in the focal plane of the telescope serves directly as the light source. The absence of light loss on the slit is the primary advantage of the slitless spectrograph. The instrument makes it possible to photograph the spectra of several objects simultaneously. However, owing to the continuous motion of nonhomogeneities in the earth’s atmosphere, through which the light of the celestial object passes, its image in the telescope is not completely point-like; it constantly oscillates in the focal plane. This limits the resolving power of the slitless spectrograph; because of this, it is usually used only for investigating the continuous spectra of celestial objects. The other shortcoming of the slitless spectrograph is the telescope’s relatively low penetrating power, which is determined by the strong influence of the night sky’s glow.

Usually the slitless spectrograph has a field of 10–20 minutes of arc and a dispersion of 150–500 angstroms (Å) per mm. The slitless spectrograph of the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR, which is mounted on a 2.6-m reflector, the largest in the USSR, has a resolving power of 18 Å/mm with a dispersion of 180 Å/mm and a 1:4 aperture ratio and an angular diameter of the star’s image of 2 seconds of arc. Its penetrating power is 12th—13th stellar magnitude.

an astronomical instrument for photographing stellar spectra using an objective prism, a glass or quartz prism positioned in front of the instrument’s objective. The assembly is attached to an equatorial mounting.

The most convenient type of slitless spectrograph is equipped with a direct-vision prism; a less convenient type uses a simple trihedral prism with an angle of refraction of 1°-60°. However, absorption of light is always greater in the direct-vision prism, which makes it difficult to photograph faint stars. With a slitless spectrograph, the spectra of several stars can be simultaneously photographed in the focal plane of the objective. This is one of the instrument’s advantages in comparison to a slit spectrograph, which gives a spectral image of only one object. The slitless spectrograph, with its wide field of view, has the further advantage of utilizing all the light from a star, while in the slit spectrograph, stellar image tremor causes some of the light to be trapped by the edges of the slit. Thus the slitless spectrograph produces a brighter image but has low resolution, and weak lines in stellar spectra are usually washed out.

Slitless spectrographs are used in the spectral classification of stars, in spectrophotometry of continuous spectra, and in mass determination of the radial velocities of faint stars.