No discussion of how telescopes work would be complete without exploring the many various forms of telescopes, because not all telescopes work alike. Yet all telescopes share on thing in common - they collect radiation. Often this radiation is in the form of light. Telescopes have the additional tasks of enhancing the resolution of an object by increasing its apparent angular diameter, and determining the location of the observational target in relation to its surroundings.
'Optical' telescopes collect light along the visible spectrum and relay that information to either the viewing human eye, or to a camera or computer for future study. There are several types of optical telescopes, depending on whether mirrors, lenses, or a combination of both to collect and relay data.
'Radio' telescopes collect light along the electromagnetic spectrum far beyond the small light range that we can see. Other telescopes may monitor X-Rays, microwaves, gamma rays, or other forms of radiation. Various instruments such as polarimeters, photographic plates, and photometers are used along with telescopes to enhance, amplify, correct, or record the imagery.
Optical telescopes
The most commonly used telescopes in visual astronomy are 'reflector' telescopes. Commonly called 'mirror telescopes', these use two mirrors to capture light and then reflect it in a focused beam into an eyepiece or onto a photographic plate. The first mirror is usually parabolic - i.e. it is shaped into a concave parabola. Sometimes the mirror is spherical, but the parabolic mirrors are more common.
Light enters the telescope and hits this first mirror which is positioned at the very back of the telescopic tube. The nature of the parabolic mirror is such that all light hitting it strait on is reflected back to a single point. This point is the 'focus' of the parabola. By changing the shape of the parabola, the focus can be changed, allowing reflector telescopes to be many lengths and sizes. The focus point is situated behind the secondary mirror, so all light bouncing off of the first mirror hits the second.
The second mirror, unlike the first, is either a convex hyperbola or it is flat. Reflector telescopes that use a secondary flat mirror are called Newtonian Reflectors after their inventor - Sir Isaac Newton. The flat mirror is at an angle, and reflects the light out the telescope's eyepiece. Newtonian telescopes are usually smaller telescopes compared to the ones using a hyperbolic secondary mirror, which are known as Cassegrain telescopes. In a Cassegrain telescope, the primary mirror has the addition of having a small hole bored through the middle. Light bouncing off the secondary mirror is then reflected through this hole to the secondary focus, which is just outside the telescope's eyepiece.
There are other, often more complicated configurations of mirrors that allow for more accurate light focus or larger telescopes, but most telescopes used in amateur astronomy are either Newtonian or Cassegrain reflector telescopes. In more high end Cassegrain telescopes, a corrective lens is placed at the entrance to enhance the quality of the image even further, making them excellent for deep space observation or stellar photography.
'Refractor' telescopes are the other main form that Optical telescopes take. In a refractor telescope, two lenses are positioned at opposite ends of the cylindrical body of the telescope. The largest lens is called the 'objective', and the light first enters through this. The light is bent by the curvature of the lens inward, and the path of the light actually crosses itself before reaching the second mirror. The second mirror is much smaller and is called the 'eyepiece'; in essence the 'eyepiece' acts as a magnifying glass. The light is turned upside down by the first mirror, then turned upside down again by the second - so the image reaches the human eye or photographic plate correctly. More lenses can be added to correct and amplify the image still further, but refractor telescopes are rarely larger than one meter in aperture. ('Aperture' refers to the diameter of the largest lens, mirror, or collection plate in a telescope.)
In many large observational telescopes, the primary mirror or lens is not in one piece, but is made of many smaller units usually spherical or hexagonal in shape. The aperture in these telescopes (referring to their combined diameter) is in some cases as wide as ten meters, or over thirty feet. As technology advances, more of these 'mosaic' telescopes are being made.
Radio telescopes
Radio telescopes measure electromagnetic radiation far beyond the visible spectrum. Frequencies lower than one hundred megahertz and up to hundreds of gigahertz can be observed. Through the aperture of the telescope or through an antenna a radio telescope receives the radiation. A receiver called a radiometer then boosts this energy signal and records it onto a computer, magnetic tape, strip chart, or other recording apparatus.
Most radio telescopes are set up similar to Reflector telescopes, using mirrors to reflect and focus the radio signals. Those that do not use parabolic reflectors use dipole antennas instead to collect the photons, and connect many dipoles together in an array to increase accuracy, resolution and range.
While to collect high frequency emissions the collection surface has to be perfectly smoothed and continuous due to the short photon wavelength, telescopes designed for the observation of lower frequency photons are often metal meshes.
Radio telescopes, unlike optical telescopes, have to be massive to obtain good resolution. The world's largest radio telescope is a three-hundred and five meter mesh dish. Still, to approach the resolution of an optical telescope, the antenna array would have to be several kilometers wide. By placing several telescopes near to each other in an array however, such as the twenty-seven telescope 'Very Large Array' in New Mexico, smaller telescopes can be combined and effectively act as one super telescope. The Very Large array has an effective aperture of thirty five kilometers, far surpassing the resolution of any optical telescope currently in use.
X-Ray telescopes
Far more complex than optical and radio telescopes, X-Ray telescopes use several specialized mirrors placed inside each other to direct the radiation to a collection point.
In the majority of X-Ray telescopes, a cone is set inside the main tube of the telescope. The light enters the base of the cone, where it grazes a sequence of mirrors. Because x-rays striking a surface perpendicularily are absorbed, not reflected, everything is set up in an X-Ray telescope to refract the light by bouncing it off the sides - hence the conical shape. Laid into the inner surface of the first half of the cone is a mirror with a concave parabolic surface.
Eventually, as the cone narrows, the parabolic surface changes into a hyperbolic surface by the addition of a second mirrored surface. The X-Rays missing the parabolic mirror will be reflected by the hyperbolic mirror, and vice versa. In both cases, the X-Rays then reflect and meet at a mutual focal point where a detector is installed. Detectors usually take the form of a gas filled box with one or more anode wires running through the middle. A cathode is formed by the sides of the box, and a current is created by the difference in potential between cathode and anode as the gas is ionized by incoming X-Ray radiation.
Other telescopes
There are many other forms of telescopes measuring different forms of radiation, all making valuable contributions to the study of the universe. Infrared telescopes are similar to optical, and ultraviolet similar in construction to X-Ray telescopes. Telescopes very in length, aperture, mounting and style, and they work in many different ways. Still, one thing stays the same.
In all telescopes, energy is gathered and then modified so that it can be observed, recorded, and studied in the most accurate manner possible far beyond what would be possible by eye alone. All manner of energy, even gravity can be studied through various telescopic apparatuses. From the simplest backyard reflector telescope to the deepest underground neutrino detector, telescopes allow human kind a glimpse into to secrets of our universe.
