The first ESA mission entirely devoted to X-ray astronomy, launched in 1983 May; the name is short for European X-ray Observatory Satellite. Exosat carried three instruments: an imaging telescope which operated at 0.05–2 keV (0.6–25 nm); a proportional counter array; and a gas scintillation proportional counter array; together they spanned 1–50 keV (0.025–1.24 nm). Exosat was placed into a highly elliptical orbit with a period of 96 hours, allowing extended observations of variable X-ray sources. It operated until 1986.
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=32 ESA mission website.