The first fully imaging x-ray telescope in space, launched by NASA in 1978. Originally named "HEAO-2" (High Energy Astrophysics Observatory 2), it was renamed for Albert Einstein upon launch.
The second of NASA's three High Energy Astrophysical Observatories, HEAO-2, renamed Einstein after launch, was the first fully imaging X-ray telescope put into space. The few arcsecond angular resolution, the field-of-view of tens of arcminutes, and a sensitivity several 100 times greater than any mission before it provided, for the first time, the capability to image extended objects, diffuse emission, and to detect faint sources. It was also the first X-ray NASA mission to have a Guest Observer program.