Electronographic Image Tube Development at the Royal Greenwich Observatory

Abstract

This chapter discusses electronographic image tube development at the Royal Greenwich Observatory. The Image Tube Department at the Royal Greenwich Observatory was set up for the development of electronographic image tubes, both for stellar field photometry and for the recording of spectra. Of the existing tubes, those because of Lallemand and Kron have reasonably large image areas, but they are not easy to operate because the electronographic plate is introduced into the same vacuum environment as the photocathode. An observatory using these tubes should have a high-vacuum laboratory for the preparation of the tubes (including photocathode processing). The Spectracon, on the other hand, is simple to operate because of its mica window, which enables the electronographic film to be exposed externally to the tube. The image size is limited to about 30 × 15 mm2 because the mica window, which is only 4 μm thick, has to withstand atmospheric pressure. The solution that was adopted in this research was to incorporate a mica window but to protect it from atmospheric pressure by a vacuum lock. This overcomes the limitation on window size set by strength considerations and minimizes the possibility of mica breakage.