Profile of the Hβ Line in the Spectrum of Zodiacal Light

Abstract

As a result of the observations of Blackwell and Ingham¹ it is now believed that the zodiacal light is almost entirely due to the scattering of sunlight by dust particles concentrated into the plane of the solar system. Because of the orbital velocities of these dust particles, the Fraunhofer absorption lines in the spectrum of zodiacal light should show Doppler shifts. During June–August of 1962, observations were made at Chacaltaya (long. 68° 68′ W.. lat. 16° 21′ S., alt. 5.5 km) to measure this Doppler shift for the Hβ Fraunhofer line, using a specially constructed, high-luminosity, photoelectric spectrometer. The spectrometer was of the Ebert type, with the resolving power or luminosity increased by using it in series with a large Fabry–Perot etalon². The addition of the etalon improved the performance of the system by a factor of 10 over the best that could be realized from a conventional spectrometer design. The etalon plates were of 90 mm useful diameter, polished by Hilger and Watts, Ltd., so that the gap was uniform to better than λ/50 at 5461 Å, and coated by Grubb Parsons with seven-layer dielectric reflecting coatings. The etalon was enclosed in a pressure vessel which could be filled to a controlled pressure with propane gas. This made it possible to vary the refractive index in the gap between the etalon plates and so change the order of interference, and hence the wave-length, passed by the interferometer. The Fabry–Perot fringes were focused by a plano-convex lens of focal length 47.5 cm on the entry slit of the Ebert spectrometer, which utilized mirrors of focal length 68 cm and a 128 mm by 208 mm plane grating (Bausch and Lomb No. 33–53–27–57) worked in fifth order. The grating of the spectrometer was imaged through the exit slit on to a photomultiplier (E.M.I. 6256 S), especially selected for low dark noise, which fed a pulse counting system with digital output.