For the most part, astronomers prefer to use their telescopes during the dark of the moon and for some projects the moon adds so much background light that telescopes go unused every month for the eight to ten nights surrounding full moon. For example, there are arrays of telescopes used to measure the intensity and the direction of electromagnetic radiation (Cherenkov radiation) caused when cosmic rays enter the earth's atmosphere. Because these signals are very weak and the resulting light very faint, the telescopes typically sit idle nearly a third of the nights. However, research projects that investigate brighter objects like nearby stars can still be conducted while the moon is shining. In order to make use of such available telescopes during "bright time," Dr. Stephan LeBohec of the University of Utah is planning to use them during bright time as an intensity interferometer to measure the diameters of stars. This kind of measurement has been done before using intensity interferometry, but the technique has fallen out of favor and been superseded by so-called phase interferometers. Phase interferometry works well for cooler, red stars where the red and infrared light has longer wavelengths. However, for stars like our yellow sun as well as for hotter and bluer stars intensity interferometry performs much better. NSF's Division of Astronomical Sciences Advanced Technologies and Instrumentation Program is providing funding for Dr. LeBohec to acquire and develop the instrumentation necessary for the intensity interferometry measurements to be made on the Cherenkov telescopes.
