Introduction

Even before the launches of the Einstein and EXOSAT observatories, it was clear that the next major step forward in EUV astronomy should be a survey of the entire sky, along the lines of the X-ray sky surveys of the 1970s. Such a survey was necessary to map out the positions of all sources of EUV radiation and determine the best directions in which to observe. Indeed, the groups at University of California, Berkeley had been selected by NASA to fly such an experiment on the Orbiting Solar Observatory (OSO) J satellite but, unfortunately, the OSO series was cancelled after the flight of OSO-I. Following the success of the Apollo–Soyuz mission in 1975, scientific interest was revived in the survey concept and the Extreme Ultraviolet Explorer (EUVE) mission was subsequently approved in 1976. Interest in the EUV waveband was also growing in Europe. Having successfully flown a series of imaging X-ray astronomy experiments between 1976 and 1978, the Massachussetts Institute of Technology (MIT)/University of Leicester collaboration sought a new direction of research, with the imminent launch of Einstein, and began development of a new imaging telescope operating in the EUV. This was seen as a direct extension of the mirror technology already refined in the soft X-ray and was combined with the MCP detector expertise acquired from work on the Einstein HRI.