• Telescope with a lens rather than a mirror as the main light-collecting component. Refractors have been surpassed by reflectors among the biggest telescopes but are sometimes used for work in areas like astrometry where immense light-gathering capacity is not critical. A good refractor’s main lens is a compound of several lenses designed to preserve natural colour faithfully (achromatic), and in which each piece of glass must be optically flawless. This accounts for the high cost of good telescope lenses and the fact that no refractor larger than 1m has ever been built.