The Roentgen Satellite, ROSAT, a Germany/US/UK collaboration, was launched on June 1, 1990 into a low Earth orbit and operated for almost 9 years.
It was sensitive in the 0.1-2.4 keV band. The two X-ray instruments were the Position Sensitive Proportional Counter (PSPC) which had a FOV 2 ° diameter and an energy resolution of 0.43. It had a 20’’ angular resolution.
The High Resolution Imager (HRI) had a FOV of 38′ square with a ~ 5 arcsec spatial resolution (FWHM).
The sensitivity limit was ~1 x 10⁻¹³ erg cm⁻² s⁻¹. The X-ray all-sky survey catalog contains more than 135000 objects.
See also https://www.mpe.mpg.de/ROSAT