an instrument for observing the spectra of faint, extended objects of finite angular diameter that merge with the background of the sky, in particular, gaseous nebulae within our galaxy (mainly those with bright-line emission spectra). In the classical version, a nebular spectrograph consists of a slit prism spectrograph with a collimator having a focal length of up to several tens of meters (the small divergence of the rays in this case obviates the need for an objective lens), with a one- or two-prism or diffraction-grating dispersion system, and a camera with a fast, short-focus mirror or lens. Because of the great demagnification of the image, the entrance slit may be very wide—in practice, up to 250 mm wide with a length (height) up to 1,000 mm. A nebular spectrograph is mounted and used in such a way that the whole portion of the sky being photographed can be observed from the entrance pupil of the camera lens through the prism system and slit.
In a nebular spectrograph, the interfering background of the sky is resolved into a bright-line emission spectrum consisting of a few lines (bands) on a faint, continuous background. The contrast between the spectrum of a gaseous nebula and those parts of the background spectrum that are free of lines is a hundred or more times greater than with direct photography. The long collimators of nebular spectrographs have necessitated the development of special designs, which are convenient for mountainous locations (see Figure 1). At the north end of the coelostat C–C, a special mount supports on one side a camera K with two prisms; on the other side, the mount supports a plane mirror M2 with a screen containing the entrance slit. Light from the object is reflected by the polar mirror M1 (placed perpendicular to the direction of the celestial pole PN) through the prisms into the camera. There are two guide telescopes T1 and T2 (T1 is shown) and an independent, smaller mirror (not shown) for obtaining the spectrum of a comparison star (with different coordinates). In the foreign literature, all fast spectrographs are called nebular spectrographs.
REFERENCESMartynov, D. Ia. Kurs prakticheskoi astrofiziki, 2nd ed. Moscow, 1967.
Dimitroff, G., and J. Baker. Teleskopy i prinadlezhnosti k nim. Moscow-Leningrad, 1947. (Translated from English.)
O. A. MEL’NIKOV