Improvements in the 21-Foot Normal Incidence Vacuum Spectrograph

Abstract

The twenty-one foot normal incidence vacuum spectrograph previously described by the writer has been improved by the provision of 30,000 line per inch glass and speculum gratings in addition to the 15,000 line per inch grating previously used, and by the installation of a simpler and much more rapid pumping system. The previous range of 2000 to 1000A has been extended so that the instrument can be used from 6000 to 300A, and orders up to and including the sixth have been photographed, giving dispersions ranging between 2.72A/mm and 0.22A/mm with the advantages of an almost normal spectrum. When a constant value for the dispersion is used over a 16 inch plate the correction factor seldom exceeds 0.3A. The errors introduced when Rowland's method of coincidences is used with these Wood gratings appear to be much less than 0.01A. Wave-lengths of numerous lines of the hot-spark spectra of copper and nickel, and of the high frequency discharge spectra of argon and neon, have been measured with the instrument, and the accuracy of many of the measurements appears to be in the neighborhood of ±0.005A. The resolving power thus far attained, while still below the theoretical limit of the gratings, is far above that previously claimed for this region. Instead of the four four-stage mercury condensation pumps previously used, a single steel oil condensation pump of eight inch internal diameter is used, together with a trap in which either solid CO2 or circulating tap water can be used as refrigerant. All high vacuum connections are of 8 inch steel tubing, and a satisfactory stopcock of this bore has been constructed. A new type of vacuum gate for installation between the main spectrograph body and the camera box is described. A vacuum of about 2×10−4 mm is found sufficient to photograph the complete spectrum covered by the instrument without appreciable absorption in the optical path of 42 feet; a higher vacuum can be obtained in the tube quite readily.