In the field of optical telescopy, two different methods are used to magnify distant objects; each through the principles of refraction or reflection. Consequently, the two varieties of optical telescopes available are known as the refracting telescope or the reflecting telescope. This article will describe the processes involved in both types of optical telescope.
The refracting telescope is constructed of two convex lenses placed a specific distance apart. The lens closest to the object being viewed is called the objective lens, whilst the other is the eyepiece lens. The objective lens has a relatively long focal length. Light rays from a distant object are considered to be parallel and are brought to a focus, forming a real image, in the focal plane of the objective lens. The eyepiece lens in positioned so that its focal point is concurrent with that of the objective lens. Rays from the real image then pass through the eyepiece lens and emerge parallel. Note however that the image formed is inverted, but is fine since the user is observing the stars.
The eye must still carry out its normal function of focusing parallel rays from a distant object; however, the visual angle has been increased, resulting in magnification. The parallel rays originally traveling from the object would form a very small visual angle at the eye. This would result in a small image projected onto the retina of the eye, and will be detected as a relatively small object compared to the magnified object of the telescope.
To collect light from very distant and faint objects, large diameter telescopes are needed. Larger telescopes form brighter images, but their size causes practical problems. The largest optical telescopes in the world are all reflecting telescopes: they use a mirror rather than a lens as the main light-collecting element. These telescopes still employ eyepiece lenses. The mirrors used have their reflecting surface at the front, unlike the back-silvered mirrors used commonly in bathrooms due to the associated aberration. The advantages of using mirrors rather than lenses are numerous. Unlike lenses, large mirrors can be supported from behind and can be more easily shaped into parabolic surfaces to reduce spherical aberration. Large flawless mirrors are much cheaper to produces than large flawless lenses. Mirrors are not subject to chromatic aberration.
There are two types of optical reflecting telescopes in common use, the Newtonian reflector and the Cassegrain reflector. The principle behind their operation is the same for the refracting telescope, to produce an image at the focal plane of the eyepiece lens. The light-collecting mirror and eyepiece lens need to be arranges so that their focal planes collide. To do this, a second smaller mirror is used to redirect the rays, otherwise the observer’s head would need to be inside the telescope itself. The Newtonian reflector simply uses a flat mirror to redirect rays to the side of the telescope. This is simply for more convenient viewing when astronomers are manipulating the position of these enormous telescopes. In the Cassegrain reflector a small hyperbolic mirror reflects the rays back through an opening in the main mirror.
– Hienemann Physics 11, 2nd edition
– Wikipedia (Telescope)