ABSTRACT
The subject of astronomical telescopes has been treated by many authors. This chapter explains astronomical telescopes. It reviews the light-gathering power is another important characteristic in a telescope. The atmosphere has large inhomogeneities in the index of refraction owing to variations in the pressure, to air currents, and to variations in the temperature. Scintillation is observed only with small apertures, mainly with the naked eye, producing what is commonly known as twinkling. The larger the telescope, the smaller the effect of the scintillation. The diameter of the seeing enlarged image depends on the astronomical site. In a medium-sized city, it may be as large as 2 or 3 arcsec or even more. In the best astronomical sites in a high mountain on the best nights, it may be of the order of one-tenth of an arcsec. The magnitude of a star is an indication of its brightness. The greater the magnitude, the fainter the star.
