Past observatories include SMART-1, which contained an X-ray telescope for mapping lunar X-ray fluorescence, ROSAT, the Einstein Observatory (the first fully imaging X-ray telescope), the ASCA observatory, EXOSAT, and BeppoSAX.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) featured curved mirrors whose multilayer coatings reflect and focus low-energy X-rays or extreme ultraviolet light the way optical telescopes focus visible light.