the sun in the sky. However, some older types of heliostat incorporate solar trackers, together with additional components to bisect the sun-mirror-target angle. A siderostat is a similar device which is designed to follow a fainter star, rather than the sun. Large-scale projects In a solar-thermal power plant, like those of The Solar Project or the PS10 plant in Spain, a wide field of heliostats focuses the sun's power onto a single collector to heat a medium such as water or molten salt. The medium travels through a heat exchanger to heat water, produce steam, and then generate electricity through a steam turbine. A somewhat different arrangement of heliostats in a field is used at experimental solar furnaces, such as the one at Odeillo, in France. All the heliostat mirrors send accurately parallel beams of light into a large paraboloidal reflector which brings them to a precise focus. The mirrors have to be located close enough to the axis of the paraboloid to reflect sunlight into it along lines parallel to the axis, so the field of heliostats has to be narrow. A closed loop control system is used. Sensors determine if any of the heliostats is slightly misaligned. If so, they send signals to correct it. It has been proposed that the high temperatures generated could be used to split water producing hydrogen sustainably. Small-scale projects Smaller heliostats are used for daylighting and heating. Instead of many large heliostats focusing on a single target to concentrate solar power (as in a solar power tower plant), a single heliostat usually about 1 or 2 square meters in size reflects non-concentrated sunlight through a window or skylight. A small heliostat, installed outside on the ground or on a building structure like a roof, moves on two axes (up/down and left/right) in order to compensate for the constant movement of the sun. In this way, the reflected sunlight stays fixed on the target (e.g. window). Genzyme Center, corporate headquarters of Genzyme Corp. in Cambridge, Massachusetts, uses heliostats on the roof to direct sunlight into its12-story atrium. In a 2009 article, Bruce Rohr suggested that small heliostats could be used like a solar power tower system. Instead of occupying hundreds of acres, the system would fit in a much smaller area, like the flat rooftop of a commercial building, he said. The proposed system would use the power in sunlight to heat and