Optical telescopes use the same part of the electromagnetic spectrum in which the human eye operates. The larger the aperture, the more light is collected and the more objects you can see. Astronomers often want to see objects which are very faint so their telescopes are often very large.

Optical astronomical observatories are often located on the tops of hills. One reason for this is to get above the clouds which block out the visible light from stars and prevent observations. Sometimes they are sent into orbit around the Earth to get above the fluctuations in the atmosphere which can also make observations more difficult.

Famous optical observatories include the Hubble Space Telescope, Paranal in Chile, the Anglo-Australian Observatory, the CFHT on Hawaii and the ING on La Palma.