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Introduction to Telescopes

Dr. Edwin Hubble (1889-1953) peering through the Hooker telescope (near Pasadena, California), one of the oldest and largest observatory telescopes still in operation today. (Courtesy: NASA)
What are the techniques used by astronomers to study the stars? The image one might first think of is the lonely Renaissance scientist staring through a refracting telescope or perhaps the modern astronomer peering into the night sky in a large observatory. However, as we shall learn in this section, much more can be received from the light of stars than simply awe-inspiring sights.

The first manner in which people learned about the stars and the planets was, of course, with their naked eyes, and it was with the eyes alone that the modern constellations were conceived and the first five of our planets (Mercury, Venus, Mars, Jupiter, and Saturn) were revealed. Optical telescopes (which come in two types: reflectors and refractors) are designed mainly to gather light and reveal more detail than can be seen with the naked eye. Other types of telescopes include the much larger radio telescopes as well as space telescopes – including infrared (IR), ultraviolet (UV), x-ray and gamma-ray telescopes.

Modern lenses, the cornerstone of optical telescopes, were only first made available in the Western world in the thirteenth century through the glass-makers of Florence and Venice, who had perfected the art of grinding and polishing lenses. Although lenses had long been applied toward the production of eyeglasses, the first well-documented evidence of them being used to produce a telescope comes only from the seventeenth century. The basic principle of upon which lenses work is their ability to refract light, a fact outlined by a mathematical relationship many of us learned in high school:

n1 sinq1 = n2 sinq2

where n is a dimensionless constant called the index of refraction (and the 1 and 2 subscripts refer to the medium through which the ray of light is traveling);q1 is the angle of incidence; and q2 is the angle of reflection (see illustration below).

Refraction of light through a medium like glass or acrylic.
The refractive index, n, of a vacuum is exactly 1, while that of air is approximately 1.0003, although this value changes with temperature. The refractive indices for glass vary from 1.5 to 1.8. Someone who is spear-fishing can witness an example of refraction: in order to catch a fish, one must aim at a spot slightly below where the fish is seen because the light rays coming off of the fish are refracted when they travel from the water to the air. Another example would be the way a pen placed on a clear glass of water gives the appearance of being bent due to the refraction of the light.

This principle of refraction when applied to lenses results in the formation of images. The geometric shape of lenses either results in light traveling parallel to the lens to either converge (i.e., a double convex lens) or diverge (i.e., a double concave lens). Each type of lens has a focal point, a value that depends on the amount of curvature the lens has. This focal length can be closely estimated by assuming a “thin lens” – that is, a lens where the thickness is small compared to the object distance, the image distance, or the radii of curvature of the lens.

A. A. Converging Lens: Notice how incident rays parallel to the principal axis converge through the focal point. Also, rays that enter through the focal point exit the lens parallel to the principal axis.

B. Diverging Lens: Notice how the incident rays traveling parallel to the principal axis refract through the lens in such a way as to diverge in a path in line with the focal point. Also, rays entering toward the focal point refract such that they then travel parallel to the principal axis.
Unfortunately, the spherical lenses produced in the “real world” are not the same as the ideal “thin” lenses of physics, resulting in a flaw called spherical aberration. Spherical aberration results in less than perfectly sharp images because the light rays that are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. Another related problem is chromatic aberration, and it is the result of light of different wavelengths (that is, colors) refracting differently and thus also focusing at different focal distances. These problems can and do have an impact on the quality of images that refracting telescopes produce.

The other principle of optics used in telescopes is reflection, specifically the kind of reflection that results from curved mirrors. As with lenses, the rays will either converge or diverge depending on whether the mirror surface is convex or concave and both types of mirrors have focal lengths that depend on the amount of curvature of the lens.

The principle of reflection
It was in 1608 when a glass-maker named Hans Lipperhey announced the invention of the telescope when he applied for a patent in the Netherlands for “a certain device by means of which all things at a very great distance can be seen as if they were nearby, by looking through glasses.” Although Lipperhey was denied a patent on the grounds that the telescope was too easy to copy, he was commissioned and well paid to make some binoculars. Soon afterward the news about this new technology spread throughout Europe.

Cartoon of the principle behind a refracting telescope (not to scale).
Optical telescopes require at minimum two lenses, the objective and the eyepiece. The simplest kind of telescope is called a refractor. In a refracting telescope, the light is collected through the objective lens and then enlarged with the eyepiece, which basically acts as magnifying glass that enlarges the image produced by the objective lens (see figure below). Unless corrected for with another lens or a prism, this image will appear inverted. The amount of magnification is dependent on the ratio of the focal lengths of each lens – that is, the ratio of the distance from each lens to the focused image. Written as an equation:

m = -fobj / feye

where m is the magnification, fobj is the distance from the objective lens to the focal point and feye is the distance from the eyepiece to its focal length.

A simple magnifier: Notice how the virtual image that is viewed through the lens results in magnification. This is the same principle on which the eyepiece of a telescope works.
Although one might consider magnification as the most important factor in a telescope’s performance, several other numbers are equally vital. Light gathering power determines how bright the objects appear and is very important for proper viewing of galaxies and other faint objects. Making the objective larger in diameter can increase the amount of light a lens can gather. Another factor in a refracting telescope is the resolving power, which concerns how well it can discern two distant objects whose angular separation is small, like a pair of twin stars. In addition, the field of view must be considered, since if an astronomer wants to view meteors she would want a wider field of view than if she were attempting to find galaxies.

The Subaru-Japan National Large Telescope located on Mauna Kea in Hawaii. (Courtesy: Subaru Telescope )
Despite the use of refracting telescopes throughout the centuries, all modern optical telescopes used by professional astronomers are reflectors, and for good reason: when using a reflecting telescope to view the night sky there is no chromatic aberration; only one mirror needs to be precise (instead of two or more lenses); and the mirrors (unlike lenses) do not sag because they can be supported not only on the sides but on the back as well. The largest reflecting telescope is the Subaru-Japan National Large Telescope located at the dormant volcano Mauna Kea in Hawaii.

Reflecting telescopes take advantage of the fact that concave mirrors cause light to converge and convex mirrors cause light to diverge. The kinds of reflectors that the amateur astronomer can buy come in two main types: Newtonian and Cassegrain. Newtonian reflectors (named after their inventor, Sir Issac Newton) reflect the rays of light back up the tube to another smaller flat mirror and into an eyepiece on the side of the tube. This is somewhat more convenient for viewing than the Cassegrain reflector, which has the eyepiece at the end of the tube. In both cases the size of the image depends on the focal length of the mirror.

A Newtonian reflecting telescope. The flat secondary mirror lies at a 45° angle to the telescope axis, thus allowing the eyepiece to be located on the side of the telescope for easier viewing.

A Cassegrain reflecting telescope. Note the hole in the center of the primary mirror that permits viewing
Other, non-optical telescopes are also used to reveal the mysteries of our universe: among these are radio telescopes. Radio telescopes are much, much larger than the ones described above because of the longer wavelength of radio. Since a reflecting surface cannot have irregularities greater than about 1/5th the wavelength of the radiation being collected, radio telescopes are easier to configure since radio waves are about 100,000 times longer than light. For example the Arecibo Radio Observatory in Puerto Rico has a 305 meter reflecting surface.

The Arecibo Radio Observatory in Puerto Rico. (Courtesy: NASA)

The Hubble Space Telescope being serviced by NASA’s Space Shuttle (Courtesy: NASA)
Another type of telescope is a space telescope. Why bother putting a telescope in space? There are several reasons for spending millions of dollars putting one in orbit. First, from space one can view the part of the electromagnetic spectrum obstructed by the Earth’s atmosphere (infrared, ultraviolet, x-rays, and gamma-rays). Second, there is no light pollution in space. Third, the lack of an atmosphere results in a lack of light distortion due to atmospheric turbulence. Besides the familiar Hubble Space Telescope, other space telescopes include the CHANDRA X-ray Observatory, launched in July 1999 to get high resolution X-ray images from high energy regions of the universe, such as the remnants of exploded stars (see figure below), and the Compton Gamma Ray Observatory, which was removed from orbit by NASA in June 2000 bringing to an end to a successful 9-year mission. In 2002 the SIRTF (Space Infrared Telescope Facility) will be launched, allowing unprecedented infrared images of our galaxy and the far reaches of the Universe.

Crab Nebula: the left image is an optical image from the Palomar Observatory in San Diego, while the image on the right is the same image but from the CHANDRA X-ray Observatory (Courtesy: NASA). Note how different the details are for the x-ray image than for the optical image.

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