There are two main designs of telescopes
Astronomy can be defined as the study of all electromagnetic radiation from outer space. This definition encompasses the nature of the field, that the objects actually under study – the planets, stars, galaxies, et. al – are quite unreachable and such, information and knowledge we have of them stems not from direct experimentation but from observation of their emitted radiation.
For most of astronomy’s history, that radiation was in the form of light and the primary tool for gathering and observing that light was the human eye. Without any aids, early astronomers such as Tycho Brahe, the Mayans and the Egyptians were able to develop a very detailed understanding of the stars and their motions.
Ever since Galileo pointed his telescope towards the heavens, astronomy has been revolutionized. Far from being a study of celestial motions, astronomy evolved to the study of the actual celestial objects. The telescope allowed for direct observation of these specks of light traveling in the velvet sky.
Moreover, the telescope allowed us to see what is impossible to perceive with the naked eye. Even the most basic telescope available today can reveal craters on the moon, or the moons of Jupiter and the rings of Saturn.
There are two main designs of telescopes – reflecting and refracting. A telescope’s main function is to do one thing – gather light and focus it to one point. Refractor telescopes do this using lenses – curved pieces of glass which light passes through. The type of lens used by refractor telescopes are convex lenses which make parallel rays of light (emanating from the astronomical object) converge to a single point.
The path of the light gets bent as it passes through the lens resulting in all the light rays being focused to a single point. In the case of reflecting telescopes, light is focused through the use of curved mirrors. Light being reflected by the mirror is directed to a single point. The shape of mirrors used in reflector telescopes can include parabolas, hyperbolic and elliptical curves depending on the specific design of the reflector telescope.
Today, the largest telescopes are all reflector telescopes. This is because to some inherent advantages in reflector telescopes. First, we need to realize that a telescope is a very precise optical device. With a reflector telescope, we only need to have one very precisely made side – the mirrored side of the glass reflector. Compare this to a lens where light has to pass through.
A lens would have two surfaces which need to be perfectly polished, resulting in twice the amount of effort. Couple this with the fact that most contemporary refractor designs rely on multiple lenses to correct for optical aberrations. Secondly, reflector telescopes are more structurally sound at large sizes. Since light does not have to pass through the mirror, a support structure can be placed directly underneath the mirror.
The support structure would be much harder to create for a refractor telescope. Firstly, the shape of the convex lenses means that it is heaviest at the center. Secondly, even if it is heaviest at the center, the support structure can only be made at the edges due to the need for light to pass through the lens.
As we said earlier, the primary purpose of a telescope is to gather light. A common misconception is that telescopes are used to simply make far away things look nearer, that is a telescope is supposed to magnify the view of objects. The magnification of a telescope is not a function of a telescope itself but a function of the telescope’s focal length and the focal length of the eyepiece used in viewing.
The magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece. This means that a single telescope can have many different magnifications depending on the eyepiece used. One might be tempted to think that telescopes can have infinite magnification simply by using eyepieces with shorter focal length.
In reality, the highest magnification for a telescope is around 50 x the aperture (the diameter of the telescope’s objective optical element) in inches. This limitation is due to several factors. First, high magnifications will result in a dimmer image. Secondly, aberrations in the atmosphere and optical train become more obvious with high magnifications.
Secondly, there is a limit to magnification as optical limitations would place a threshold on the available resolution produced by the telescope. Any magnification beyond this threshold only produces empty magnification as not additional detail can be resolved.
The ability of the telescope to resolve details is inversely proportional to its aperture. This is set by the Rayleigh, Dawes and Sparrow’s limit which all say that the limiting resolution of an optical system is some factor divided by the diameter of the optic. Simply put, the larger the telescope, the higher its resolving power.