TD-1A satellite developed by the European Space Research Organization and launched in March 1972. It made a systematic ultraviolet survey of the whole sky, with one experiment on board recording spectrophotometric data at 3.5 nanometer resolution on thousands of stars in the wavelength range 135–274 nm. All hot O and B stars were recorded down to 9th magnitude in addition to observations of many of the brighter cooler stars of later spectral types. A catalog of more than 30 000 ultraviolet sources has been compiled from the data. A second UV instrument detected UV spectra, at about 0.2 nm resolution, of brighter preselected stars and a catalog of these data has also been prepared.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006