The telescope

Abstract

A telescope is an optical system for collecting and focusing radiation. The optical system as a rule modifies the apparent area of the light source (i.e., changes the magnification), but it should be remembered that radiance of a light beam cannot be increased by optical means (see Section 1.2.1). As a matter of fact, radiance is effectively reduced due to transmission losses in and at the surfaces of the optical components, which modify the function s_t(λ) in Eq. (1.14). This is especially true for spectral radiance at wavelengths where transmission or reflection losses in the optics are important.