An astronomical photoelectric spectrophotometer

An instrument is described for the direct recording of astronomical spectral profiles. It consists of a prism monochromator whose output is recorded photoelectrically as the wave-length transmitted is varied. Compensation for atmospheric seeing effects is essential when narrow slits are used, and is provided by recording, as the compensated profile, the ratio of the monochromator output to the undispersed light collected after the entrance slit. The performance of this compensation, and the limiting effects of shot noise on the accuracy of the instrument, are discussed. The instrument has been used with a fairly high resolution (about A) to record stellar absorption line profiles, and with a low resolution, as a slitless spectrograph, to measure the relative intensities of emission lines of planetary nebulae.