astronomical spectrograph[‚as·trə′näm·ə·kəl ′spek·trə‚graf]
a spectral device for photographing the spectra of celestial bodies. It is mounted in the telescope’s focus so that the actual image of the star, planet, nebula, or similar object is formed in its slit. Obtaining the spectra of faint astronomical sources requires extremely long exposure times (dozens of minutes or hours), during which the spectrographed object changes its position relative to the horizon. At the same time, the position of the telescope directed at the source is also changed. In order to avoid the image’s shifting in the astronomical spectrograph’s slit, the spectrograph-telescope system must be completely rigid. Moreover, the astronomical spectrograph itself is thermostatically controlled, since changes of even 0.1°C can cause shifting of the spectral lines, introducing errors into radial velocities of up to 5 km/sec.
Dispersion in stellar astronomical spectrographs usually ranges from 100 to 10 A/mm and is limited by the design properties of the astronomical spectrograph mounted on the telescopes. High dispersion is obtained in the stationary coudé focus—up to 1 A/mm during observations of bright stars using 3- to 5-m reflectors. Dispersions from 500 to 2,000 Å/mm are used for faint objects, and in special cases dispersions of up to 10,000 Å/mm are used. High-aperture cameras with very short focal lengths are used for such objects, most commonly the Schmidt telescopes. For spectrography of extremely faint objects, astronomical spectrographs are mounted in the primary focus of the telescope and the slits, on whose edges loss of light occurs, are even dispensed with.
Slitless and nebular spectrographs and prism cameras are varieties of astronomical spectrographs. In slitless spectrographs, spectrally resolved images are obtained not only from the object that is located in the optical axis but from other objects. The prism camera works in a similar fashion. The camera is aimed at the area of the sky being studied; a prism without a collimating lens is placed before the camera lens. In a nebular spectrograph, the collimating lens is absent: the slit, which is placed far from the prism (diffraction grating), makes it possible to isolate light from relatively small areas of the sky. In the case of spectral observations of the sun, which gives a huge flux of light, stationary long-focusing spectrographs with dispersions from 0.1 A/mm are used. Successful use is made of echelle gratings permitting photography of large regions of the spectrum with high dispersion using very high-order spectra and mirror optics.
The wavelengths of spectral lines in astronomical spectrographs are determined by devices introducing light into the spectrograph from laboratory sources, the spectral reso lution of which gives a comparison spectrum.
REFERENCEMartynov, D. Ia. Kurs prakticheskoi astrofiziki, 2nd ed. Moscow, 1967. Chapter 1, sec. 8.
D. IA. MARTYNOV