A garnet crystal doped with the rare earth neodymium emits infra-red light of the required wavelength. When Galileo trained his hand-made telescope on the night sky in 1604, he was the first to see lunar craters, Jupiter's moons and the phases of Venus, all with equipment more primitive than modern binoculars. But Galileo, and countless astronomers who followed him, laboured under restrictions imposed by more than just primitive equipment. Their telescopes improved with time but, until quite recently, they could only observe visible light.
Visible light forms a tiny slice of the electromagnetic spectrum.
In the past 50 years, scientists have added immensely to their understanding of the universe by exploring it through these forms of radiation, but limitations still remain. Much of the universe is dark matter, which emits virtually no electromagnetic radiation. Gravitational waves seem to offer astrophysicists the best hope of studying objects composed of dark matter, but the problems are considerable. Static objects, even those that are large, will not emit gravitational waves. These, Einstein predicted, are produced when objects are accelerated, or when strong gravitational fields interact dynamically.