The Orbiting Solar Observatory was a series of American space telescopes primarily intended to do what?

The Orbiting Solar Observatory (abbreviated OSO) Program was the name of a series of American space telescopes primarily intended to study the Sun, though they also included important non-solar experiments.

Eight were launched successfully into Low Earth orbit by NASA between 1962 and 1975 using Delta rockets. Their primary mission was to observe an 11-year sun spot cycle in UV and X-ray spectra.

The initial seven (OSO 1–7) were built by Ball Aerospace, then known as Ball Brothers Research Corporation (BBRC), in Boulder Colorado. OSO 8 was built by Hughes Space and Communications Company, in Culver City, California.