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Pioneering thermal solar power plant Solar One at Daggett, California. 1818 heliostats (mirror assemblies) concentrate the sun's energy onto a receiver that produces steam to run a turbine and generator. 10-MW-Solar One was completed 1981, It morphed later into Solar Two, adding more heliostats and changing the heat transfer fluid from oil to a molten salt compound. Operational until 1999. Today the field of heliostats is used in astronomical research. (KEYSTONE/Georg Gerster)