Have you ever looked at the stars and wondered what is out there? Telescopes can magnify and clarify many of the heavenly bodies in our galaxy. I already knew telescopes could range in complexity from two lenses to complex systems of lenses and mirrors to fancy radio telescopes. I want to find out about the advantages and disadvantages of different types of telescopes, the principles of telescopes and their mechanisms, as well as the applications of each kind of telescope.

When eight of the nine planets aligned in our solar system, I became interested in obtaining a telescope to view the stars. This aligning of the planets is extremely rare, and to view all eight planets one needs a telescope. The wide variety of telescopes made me curious about their differences and applications. I hope to become more familiar with all aspects of the techniques of astronomical viewing. Also, I would like to learn why radio telescope arrays provide images as clear as a single radio telescope with a diameter the distance apart of the farthest placed radio telescope dishes of the array.

In the 13th century, European scientists understood the optical properties of lenses and mirrors. A Dutch optician known as Hans Lippershey (1570 - 1619) is credited with the invention of the optical telescope in 1608 (Ridpath 12). A telescope is a device used to enhance images of a certain wavelength of the electromagnetic spectrum. There are many types of telescopes including the refractor, reflector, radio, x-ray, ultraviolet, gamma ray, neutrino, and infrared (McHenry 505). The most important characteristic to any type of telescope is its aperture, or diameter of the objective focal device. The objective is the primary mirror or lens that initiates the focusing and viewing of electromagnetic waves. The aperture determines its electromagnetic radiation gathering ability (Pealer). Resolving power is another major characteristic of a telescope. The resolving power is a function of the ability of the instrument to discern between two objects that have very little angular separation (McHenry 506). Telescopes must be finely adjusted and built to minimize any defects or aberrations in the image that a fault could cause (Fredrick 414).

The refracting telescope was the first type of telescope invented (Ridpath 12). The simplest refractor consists of two lenses and a long tube. In a refracting telescope, light enters through and is refracted by the objective lens. Then light travels the length of the tube and is focussed in the eye piece lens. This kind of telescope is used only to view the visible light portion of the electromagnetic spectrum (McHenry 505). In certain applications this design can be highly advantageous. The first main advantage is that there is no central blockage or any other diffracting causing agent in the path of incoming light, thereby conserving the visual image. The second main advantage is that there is minimal light lost during refraction and transmission of light through the telescope. The third advantage is that refracting telescopes are very sturdy and keep their alignment over time (Fredrick 415). However, refractors are limited in their size and use. Their objective lens must be small enough so as to not cause warpage or bending of the lens under its own weight (Nash). Also, no matter how well built the refractor is, lenses cause different wavelengths of light to focus at different points. This property causes defects in the image viewed called chromatic aberrations (Fredrick 414).

Reflecting telescopes have many different configurations. The simplest of which consists of one concave mirror, one flat mirror, one lens, and a tube. In a reflector, light enters the telescope, travels the length of the tube to the concave mirror which causes the image to reflect back thereby partially focusing. Then a flat mirror inside the tube reflects the light again to a lens which finishes the focussing process. Reflecting telescopes are used to view ultraviolet and infrared light as well as visible light (McHenry 507). Beside a lack of chromatic aberration, the reflector has many other advantages over the refractor. Reflecting telescopes can be much smaller in size than refractors of the same diameter because the focus is folded back onto itself before being viewed in the eye piece. Reflecting telescope can also be made with a larger aperture since the primary mirror can be supported from behind without causing interference in the image. The primary disadvantage of reflectors when compared to refractors is that the secondary mirror is inside the light gather tube. This blockage causes distortions in the incoming image. Also, reflectors must be focussed much more precisely than refractors to achieve clear, quality images (Fredrick 415). However, recent advances in telescope technology has let scientists link optical reflecting telescopes together to synthesize the precision of a larger telescope ("Coasting To a Sharper Image." 155). Radio telescopes are a quickly evolving technology. They are usually used to study radio emissions of 30 MHz to 300 GHz. Most often scientists use them in their study of stars, galaxies, quasars, and other astronomical objects in this wave range (McHenry 510). Radio telescopes have two basic components, a large radio antenna and a radio receiver. The most common type of radio telescope is the one with a parabolic dish and a feed horn to collect data (McHenry 511). A technique used to synthesize a large radio telescope is called an array or an interferometer. An array consists of many smaller radio telescope dishes, which are all linked together by computers. Computers integrate the images received to synthesize one image with the quality of one huge radio telescope dish (Fredrick 417). The largest single dish radio telescope has a 305 meter diameter (the Arecibo) in Arecibo, Puerto Rico. However, the largest array is the Very Large Array near Socorro, New Mexico which consists of 27 parabolic dishes each of which measures 25 meters in diameter (McHenry 516). The major application of the radio telescope is to study objects that emit waves by one of the following: thermal radiation from a solid body, thermal radiation from a hot gas, synchrotron radiation from electrons in weak magnetic fields, line radiation from atomic or molecular transitions that occur in space, and pulsed radiation from the rapid rotation of neutron stars (McHenry 513).

There are a myriad of other types of telescopes. The Schmidt-Cassegrain telescope is a combination of the best features of reflecting and refracting telescopes. A refracting correction plate enables the telescope to produce a clear optical picture with very little distortion that is usually present in reflectors and refractors (McHenry 508). The infrared telescope is similar to the design of the Schmidt-Cassegrain but has a thin monolithic primary mirror (McHenry 516). Ultraviolet telescopes are similar to the infrared but sometimes the mirror is plated with gold and uses filters to collect the ultraviolet radiation. X-ray telescopes are built radically different than other telescopes since X-ray photons have a much higher level of energy. X-ray telescopes work on a grazing-incidence principle in which mirrors are angled slightly off parallel to incoming X-rays and then another set of mirrors continues this slight reflection to a focus point. The mirrors employed in X-ray telescopes are gold coated to increase accuracy (McHenry 517). Gamma ray telescopes have the same design as X-ray telescopes. The difference between the X-ray and gamma ray telescopes is that the data collection devices are finely tuned to examine different parts of the electromagnetic spectrum (McHenry 518). Space telescopes appear similar to land-based reflecting telescopes, however, they have significant advantages. The source of these advantages is simple, they are outside the earth's distorting atmosphere. However, they have significant restrictions as well. Space telescopes are much more expensive than land based telescopes and are not readily repaired. They have to be transported to space on a rocket and remotely maneuvered and controlled just like any other unmanned spacecraft. Neutrino telescopes are the latest in telescope technology. They are used to study neutrino emissions from stars. For many of the aforementioned telescopes, there are no comparisons except among refractors, reflectors, and Schmidt-Cassegrain telescopes since astronomy of other than visible light and radio waves is rare and present technology is in its infancy (Nash).

Telescopes have many applications in society. Without telescopes, we would know very little of the universe. Space exploration started on land with the use of the naked eye later enhanced by telescopes once they were invented. Maps of the sky and discoveries of new types of heavenly bodies, like black holes and quasars, can all be attributed to the use of telescopes. As humans expand their borders into outer space, telescopes and probes are used to map and view new regions of the universe long before manned spacecrafts venture outward. Everybody can access or purchase telescopes for stargazing or collecting data. The type of individual or university use or study determines the type of telescope to be procured. The appropriate type of telescope varies depending on which wavelength of the electromagnetic spectrum is to be scrutinized and how the data will be collected. Telescopes cost a significant amount of money since the processes by which telescope mirrors and lenses are made is highly precise to prevent defects. Astronomy is a precise science and a wide variety of telescopes are available for many specific applications (Nash).

Telescopes have given humans the capacity to view distant objects in our vast universe. The natural wonder and mystery of the universe lured the eyes of men to the night sky and encouraged dreams of outer space. Telescopes have provided answers to many questions about the cosmos, though they have created even more that are yet to be answered. The limits of traditional telescopes will surely be conquered by invention as it has been in the past. Therefore, the future is bright and well focussed (Nash).