A high resolution spectrograph for the vacuum ultraviolet

Abstract

The construction of a 21-foot Eagle mounting vacuum spectrograph employing a 30,000 line per inch concave grating is described. In this instrument, the slit is horizontal, the plateholder vertical, and both are displaced from the rowland plane. This arrangement, originally designed by Harrison, is quite convenient and simplifies evacuation problems. The bilateral slit can be adjusted without breaking the vacuum, and a movable diaphragm is provided for various purposes. The grating, newly ruled and blazed for the 5000 to 6000A region, is normally employed in that order of the vacuum ultraviolet which falls on the blaze. In the fourth order the reciprocal plate dispersion is 0.3A/mm and an observed resolving power of about 300,000 has been obtained. The plate chamber is cylindrical and possesses an aluminum door through the center of which are mounted various controls for operating the cassette. The method of optical adjustment is described briefly and the techniques peculiar to this field such as light sources, order separation, low temperature work, and wavelength standards are discussed. The high resolving power is demonstrated by a number of photographs of room and low temperature absorption spectra of nitric oxide, and emission spectra of atomic oxygen. The optical theory of the mounting, which permits the slit and plateholder to lie off the Rowland plane, is discussed with respect to the theory given by Beutler. This treatment has been examined in detail and a number of corrections and extensions have been made. It is shown that the off-plane mounting causes no new aberrations not already present in the usual in-plane mounting provided that the off-plane angle is kept below 1°.