Flight spare of the Orbiting Solar Observatory (OSO 4) x-ray spectrometer

Flight spare of the Orbiting Solar Observatory (OSO 4) x-ray spectrometer. This launched in October 1967 and while a tape recorder failed in May 1968, it did continue to function in a limited capacity until December 1971. The spectrometer on OSO 4 gathered data on solar coronal X-ray active regions (ARs) and impulsive events.

The Orbiting Solar Observatory Programme was a series of NASA space telescopes primarily designed to study the 11-year sun spot cycle in UV and X-ray spectra. While these satellites were NASA-led missions, the University of College London's Mullard Space Science Laboratory (MSSL) were pioneers in early UV and X-ray astronomy due to their work on Skylark rocket payloads. The programme launched eight satellites between March 1962 and June 1975.