telescope Instrument for enlarging a distant object or studying electromagnetic radiation from a distant source. Optical telescopes can use lenses (refracting telescopes) or mirrors (reflecting telescopes); catadioptric telescopes use both in combination. The lens or mirror is the telescope's main light-gathering part (objective), and its diameter, known as the aperture of the telescope, determines its magnifying power. The point at which the objective concentrates the light from the source is its focus, and the distance from the focus to the objective is its focal length. Refracting telescopes were invented by Dutch optician Hans Lippershey (1608) and Italian astronomer Galileo (1609). The main disadvantage with refracting telescopes was chromatic aberration. This problem was solved in reflecting telescopes by combining lenses so their aberrations cancelled each other out. In 1668, English scientist Sir Isaac Newton built an early astronomical reflector. Modern observatories are built on mountain peaks in order to improve ‘seeing’ and to observe infrared waves from celestial bodies. Earth-bound telescopes have limitations because the incoming radiation has to pass through the Earth's atmosphere. This ceases to be a problem with telescopes in Earth orbit, such as the Hubble Space Telescope (HST). Orbiting telescopes can also detect other types of electromagnetic radiation more easily, such as ultraviolet radiation, X-rays and gamma radiation. Radio telescopes are complex electronic systems that detect and analyse radio waves from beyond the Earth. In 1937, the first radio telescope was built by US radio engineer Grote Reber. Radio interferometers are arrays of smaller dishes that permit the investigation of even more distant radio sources.