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This section describes the two largest solar heliostat arrays located in the US. The STACEE collaboration has conducted tests and obsersevations at both of these sites: Details of the STACEE instrument concept are described below.
The National Solar Thermal Test Facility
The National Solar Thermal Test Facility (NSTTF) is a facility for solar energy and concentrated sunlight research. The NSTTF is managed by Sandia National Laboratories under contract from the US Department of Energy. The STACEE experiment makes use of the Central Receiver Test Facility (CRTF) at NSTTF consisting of 220 sterable heliostat mirrors (each with nearly 40 square meters of light collecting area) and a 200-foot tall central tower with a series of test bays for locating instruments.
Daylight solar energy research operations at the National Solar Thermal Test Facility at Sandia National Laboratories, near Albuquerque NM.
Aerial view of the central tower and heliostats at the NSTTF at Sandia National Laboratories.
Solar Two
Solar Two is an experimental solar power plant located near Barstow, California with approximately 1800 heliostats. Southern California Edison (SCE), a private utility company, operates the facility in collaboration with the U.S. Department of Energy and several other utility and technology groups.
Daylight power operations at Solar Two.
What is the STACEE concept?
The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new experiment, currently under construction, dedicated to the study of high energy light (gamma rays) from astrophysical sources. The STACEE collaboration is a group of university scientists from several institutions in the United States and Canada who are carrying out the construction of the experiment. STACEE will use as its primary collection mirror the large field of solar heliostat mirrors at the National Solar Thermal Test Facility of Sandia National Laboratories. Although the heliostats have been designed for the collection and concentration of sunlight for solar energy research, the STACEE collaboration has demonstrated that it is also possible to use these same heliostats at night to collect the quick flashes of blue Cherenkov light that result from gamma ray air showers. These gamma rays originate from astrophysical sources at galactic and extragalactic distances. When they enter the earth's atmosphere, the gamma rays interact with air molecules, creating an extensive shower of high-energy particles. These particles go faster than the speed of light in air, and thus emit light, or Cherenkov radiation. This radiation (at optical and ultraviolet wavelengths) is beamed to the ground, where it is readily collected by the heliostat mirrors. The very large mirror area allows an observatory to be built that has a much lower energy threshold and much greater sensitivity than present day observatories.
The STACEE concept is to use the heliostats at Sandia at night for gamma-ray astronomy. STACEE will use the heliostats to reflect Cherenkov light produced in air showers onto a secondary mirror. The secondary will focus the Cherenkov light onto a camera of photomultiplier tubes, which will turn the light into a measurable electric signal.
  The reason solar heliostat arrays are so ideal for a gamma ray obseravtory is that they contain an enormous mirror area. For example, the NSTTF at Sandia offers 220 large heliostats, each equivalent in size to a 7 meter reflector. These large areas can be exploited to detect energies at a threshold ten times lower than that achieved by current instruments, thus providing a means of opening the important window in the gamma ray spectrum.
For a complete summary of the current status of the STACEE experimental program, including instrument details, publications, and science motivation, see the main STACEE Collaboration Home Page at UCLA.
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