Astronomers build larger telescopes not only to collect more energy but also to resolve finer detail in the objects to be studied. While groundbased optical telescopes with "filled" apertures of up to 10 m have been built, and 30 m and larger-aperture designs are being studied, another method of observations permits equivalent apertures spanning tens to hundreds of meters to be used. Radio astronomers have used these instruments, called interferometers, for decades, with equivalent apertures spanning thousands of kilometers. Optical astronomers have now designed interferometer systems that rely on two (or more) widely-separated telescopes to collect light, and combine the energy to resolve detail that is equivalent to a filled aperture with diameter equal to the separation of the telescopes.