Telescopes I. Important properties A. Light gathering ability. Amount of light gathered is determined by the area of the objective. The objective in some small telescopes is a lens. A lens works by refraction of light, i.e. bending of light when it crosses a surface. The two lens surfaces are designed so that all of the light rays from any point are brought together at a single point in the focal plane of the lens.
Big lenses are hard to make and are subject to chromatic aberration .
So big telescopes always use large curved mirrors to collect the light from a source into the focal plane. The largest reflector objectives have diameters of 10-12'. The images formed by reflecting telescopes in their focal planes can be recorded at the side of the telescope in Newtonian configuration, at the bottom in the Cassegrain configuration, or at the prime focus . Two point sources can be resolved if the two sources form distinguishable images. Small density fluctuations in the atmosphere change the refraction of the light slightly so that images of stars seem to dance around or twinkle.