Heliostats, movable mirrors, were used to concentrate the sun'srays on a central tower of initially water, and later, salt.Solar One and Solar Two were test sites, and neither operatesanymore. The array of mirrors has been converted to be part of agamma ray telescope (the mirrors now focus the brief and very weakflashes of light generated by cosmic gamma rays as they enter theearth's atmosphere onto a detector).Another plant, Solar Tres, (later Gemasolar) was built in Spain,using the technology of Solar One and Solar Two, and three timesbigger. Its molten salt storage means it can run during 15 hours ofdarkness. During the summer of 2013 the plant ran 24/7 for 36consecutive days.