Electromagnetic radiation and optical telescopes

An optical telescope gathers light from a distant object, forms an image and either magnifies it through an eyepiece or sends it to a camera. In a refractor the objective is a lens (Galileo began observing with one in 1609; the largest refractor is Yerkes in the USA, 102 cm), in a reflector it is a concave mirror (Newton, 1668). Today’s large telescopes are almost all reflectors, because a big mirror can be supported from behind while a lens can be held only at its edge and sags under its own weight: Keck (two, 10 m each), the VLT (four, 8.2 m), GTC (10.4 m), SALT (≈ 11 m). The European Southern Observatory’s 39 m ELT is under construction. To reduce atmospheric turbulence adaptive optics and high, dry, cloud-free sites are used. In Uzbekistan the Maidanak Observatory in Kashkadarya region (≈ 2700 m, Ulugh Beg Astronomical Institute of the Academy of Sciences) stands at such a site. The Sun may be observed only through a proper solar filter; pointing an unfiltered telescope at the Sun blinds the observer and ruins the instrument.