Telescope
The telescope is an instrument that collects and analyzes the radiation emitted by distant sources. The most common type is the optical telescope, a collection of lenses and/or mirrors that is used to allow the viewer to see distant objects more clearly by magnifying them or to increase the effective brightness of a faint object. In a broader sense, telescopes can operate at most frequencies of the electromagnetic spectrum, from radio waves to gamma rays. The one characteristic all telescopes have in common is the ability to make distant objects appear to be closer (from the Greek tele meaning far, and skopein meaning to view).
The first optical telescope was probably constructed by the Dutch lens-grinder, Hans Lippershey, in 1608. The following year, Galileo Galilei built the first astronomical telescope from a tube containing two lenses of different focal lengths aligned on a single axis (the elements of this telescope are still on display in Florence, Italy). With this telescope and several following versions, Galileo made the first telescopic observations of the sky and discovered lunar mountains, four of Jupiter's moons, sunspots, and the starry nature of the Milky Way. Since then, telescopes have increased in size and improved in image quality. Computers are now used to aid in the design of large, complex telescope systems.
The primary function of a telescope is that of light gathering. As will be seen below, resolution limits on telescopes would not call for an aperture much larger than about 30 in (76 cm). However, there are many telescopes around the world with diameters several times this. The reason is that larger telescopes can see further because they can collect more light. For example, the 200 in (508 cm) diameter reflecting telescope at Mt. Palomar, California can gather 25 times more light than the 40 in (102 cm) Yerkes telescope at Williams Bay, Wisconsin, the largest refracting telescope in the world. The more light a telescope can gather, the more distant the objects it can detect, and therefore larger telescopes increase the size of the observable universe.
Unfortunately, scientists are not able to increase the resolution of a telescope simply by increasing the size of the light-gathering aperture to as large a size as needed. Disturbances and nonuniformities in the atmosphere limit the resolution of telescopes to somewhere in the range of 0.5–2 arc seconds, depending on the location of the telescope. Telescope sights on top of mountains are popular because the light reaching the instrument has to travel through less air, and consequently the image has a higher resolution. However, a limit of 0.5 arc seconds corresponds to an aperture of only 12 in (30 cm) for visible light: larger telescopes do not provide increased resolution but only gather more light.
Magnification is not the most important characteristic of telescopes as is commonly thought. The magnifying power of a telescope is dependent on the type and quality of eyepiece being used. The magnification is given simply by the ratio of the focal lengths of the objective and eyepiece. Thus, a 0.8 in (2 cm) focal length eyepiece used in conjunction with a 39 in (100 cm) focal length objective will give a magnification of 50. If the field of view of the eyepiece is 20°, the true field of view will be 0.4°.
Most large telescopes built before the twentieth century were refracting telescopes because techniques were readily available to polish lenses. Not until the latter part of the nineteenth century were techniques developed to coat large mirrors, which allowed the construction of large reflecting telescopes.
Refracting telescopes, i.e. telescopes that use lenses, can suffer from problems of chromatic and other aberrations, which reduce the quality of the image. In order to correct for these, multiple lenses are required, much like the multiple lens systems in a camera lens unit. The advantages of the refracting telescope include having no central "stop" or other diffracting element in the path of light as it enters the telescope, and the alignment and transmission characteristics are stable over long periods of time. However, the refracting telescope can have low overall transmission due to reflection at the surface of all the optical elements, and the largest refractor ever built has a diameter of only 40 in. (102 cm): lenses of a larger diameter will tend to distort under their own weight and give a poor image. Additionally, each lens needs to have both sides polished perfectly and be made from material which is of highly uniform optical quality throughout its entire volume.
All large telescopes, both existing and planned, are of the reflecting variety. Reflecting telescopes have several advantages over refracting designs. First, the reflecting material (usually aluminum), deposited on a polished surface, has no chromatic aberration. Second, the whole system can be kept relatively short by folding the light path, as shown in the Newtonian and Cassegrain designs below. Third, the objectives can be made very large since there is only one optical surface to be polished to high tolerance, the optical quality of the mirror substrate is unimportant, and the mirror can be supported from the back to prevent bending. The disadvantages of reflecting systems are: 1) alignment is more critical than in refracting systems, resulting in the use of complex adjustments for aligning the mirrors and the use of temperature insensitive mirror substrates, and 2) the secondary or other auxiliary mirrors are mounted on a support structure which occludes part of the primary mirror and causes diffraction.
Catadioptric telescopes use a combination of lenses and mirrors in order to obtain some of the advantages of both. The best-known type of catadioptric is the Schmidt telescope or camera, which is usually used to image a wide field of view for large area searches. The lens in this system is very weak and is commonly referred to as a corrector-plate.
The limits to the resolution of a telescope are, as described above, a result of the passage of the light from the distant body through the atmosphere, which is optically nonuniform. Stars appear to twinkle because of constantly fluctuating optical paths through the atmosphere, which results in a variation in both brightness and apparent position. Consequently, much information is lost to astronomers simply because they do not have sufficient resolution from their measurements. There are three ways of overcoming this limitation: setting the telescope out in space in order to avoid the atmosphere altogether, compensating for the distortion on a ground-based telescope, and/or stellar interferometry. The first two methods are innovations of the 1990s and are expected to lead to a new era in observational astronomy.
The best-known and largest orbiting optical telescope is the Hubble Space Telescope (HST), which has an 8 ft (2.4 m) primary mirror and five major instruments for examining various characteristics of distant bodies. After a much-publicized problem with the focusing of the telescope and the installation of a package of corrective optics in 1993, the HST has proved to be the finest of all telescopes ever produced to date. The data collected from HST is of such a high quality that researchers can solve problems that have been in question for years, often with a single photograph. The resolution of the HST is 0.02 arc seconds, a factor of around twenty times better than was previously possible, and also close to the theoretical limit since there is no atmospheric distortion. An example of the significant improvement in imaging that space-based systems have given is the Doradus 30 nebula, which prior to the HST was thought to have consisted of a small number of very bright stars. In a photograph taken by the HST it now appears that the central region has over 3,000 stars.
Another advantage of using a telescope in orbit is that the telescope can detect wavelengths such as the ultraviolet and various portions of the infrared, which are absorbed by the atmosphere and not detectable by ground-based telescopes.
In 1991, the United States government declassified adaptive optics systems (systems that remove atmospheric effects), which had been developed under the Strategic Defense Initiative for ensuring that a laser beam could penetrate the atmosphere without significant distortion.
A laser beam is transmitted from the telescope into a layer of mesospheric sodium at 56–62 mi. (90–100 km) altitude. The laser beam is resonantly backscattered from the volume of excited sodium atoms and acts as a guide-star whose position and shape are well-defined except for the atmospheric distortion. The light from the guide-star is collected by the telescope and a wavefront sensor determines the distortion caused by the atmosphere. This information is then fed back to a deformable mirror, or an array of many small mirrors, which compensates for the distortion. As a result, stars located close to the guide-star come into a focus, which is many times better than can be achieved without compensation. Telescopes have operated at the theoretical resolution limit for infrared wavelengths and have shown an improvement in the visible region of more than 10 times. Atmospheric distortions are constantly changing, so the deformable mirror has to be updated every five milliseconds, which is easily achieved with modern computer technology.
Telescopes collect light largely for two types of analysis: imaging and spectrometry, with the better known being imaging. The goal of imaging is simply to produce an accurate picture of the objects that are being examined. In past years, the only means of recording an image was to take a photograph. For long exposure times, the telescope had to track the sky by rotating at the same speed as Earth, but in the opposite direction. This is still the case today, but the modern telescope no longer uses photographic film but a charged coupled device (CCD) array. The CCD is a semiconductor light detector, which is 50 times more sensitive than photographic film and is able to detect single photons. Being fabricated using semi-conductor techniques, the CCD can be made very small, and an array typically has a spacing of 15 microns between CCD pixels. A typical array for imaging in telescopes will have a few million pixels. There are many advantages of using the CCD over photographic film or plates, including the lack of a developing stage and that the output from the CCD can be read directly into a computer and the data analyzed and manipulated with relative ease.
The second type of analysis is spectrometry, which means that the researcher wants to know what wavelengths of light are being emitted by a particular object. The reason behind this is that different atoms and molecules emit different wavelengths of light; measuring the spectrum of light emitted by an object can yield information as to its constituents. When performing spectrometry, the output of the telescope is directed to a spectrometer, which is usually an instrument containing a diffraction grating for separating the wavelengths of light. The diffracted light at the output is commonly detected by a CCD array and the data read into a computer.
For almost 40 years, the Hale telescope at Mt. Palomar was the world's largest with a primary mirror diameter of 200 in (5.1 m). During that time, improvements were made primarily in detection techniques, which reached fundamental limits of sensitivity in the late 1980s. In order to observe fainter objects, it became imperative to build larger telescopes, and so a new generation of telescopes is being developed. These telescopes use revolutionary designs in order to increase the collecting area; 2,260 ft2 (210 m2) is planned for the European Southern Observatory. This new generation of telescopes will not use the solid, heavy primary mirror of previous designs, whose thickness was between 1/6 and 1/8 of the mirror diameter, but will use a variety of approaches to reduce the mirror weight and improve its thermal and mechanical stability. These new telescopes, combined with quantum-limited detectors, distortion reduction techniques, and coherent array operation, will allow astronomers to see objects more distant than have been observed before.
One of this new generation, the Keck telescope located on Mauna Loa in Hawaii, is currently the largest operating telescope, using a 32 ft (10 m) effective diameter hyperbolic primary mirror constructed from 36 6 ft (1.8 m) hexagonal mirrors. The mirrors are held to relative positions of less than 50 nanometers using active sensors and actuators in order to maintain a clear image at the detector.
Because of its location at over 14,000 ft (4,270 m), the Keck is useful for collecting light over the range of 300–1100 nm. In the late 1990s, this telescope was joined by an identical twin, Keck II, which resulted in an effective mirror diameter of 279 ft (85 m) through the use of interferometry.
Most of the discussion so far has been concerned with optical telescopes operating in the range of 300–1100 nm. However, valuable information is contained in the radiation reaching us at different wavelengths, and telescopes have been built to cover wide ranges of operation, including radio and millimeter waves, infrared, ultraviolet, x rays, and gamma rays.
Infrared telescopes (operating from 1–1000 æm) are particularly useful for examining the emissions from gas clouds. Because water vapor in the atmosphere can absorb some of this radiation, it is especially important to locate infrared telescopes in high altitudes or in space. In 1983, NASA launched the highly successful Infrared Astronomical Satellite, which performed an all-sky survey, revealing a wide variety of sources and opening up new avenues of astrophysical discovery. With the improvement in infrared detection technology in the 1980s, the 1990s will see several new infrared telescopes, including the Infrared Optimized Telescope, a 26.2 ft (8 m) diameter facility, on Mauna Kea, Hawaii.
Several methods are used to reduce the large thermal background which makes viewing infrared difficult, including the use of cooled detectors and dithering the secondary mirror. This latter technique involves pointing the secondary mirror alternatively at the object in question and then at a patch of empty sky. Subtracting the second signal from the first results in the removal of most of the background thermal (infrared) noise received from the sky and the telescope itself, thus allowing the construction of a clear signal.
Radio astronomy was born on the heels of World War II, using the recently developed radio technology to look at radio emissions from the sky. The first radio telescopes were very simple, using an array of wires as the antenna. In the 1950s, the now familiar collecting dish was introduced and has been widely used ever since.
Radio waves are not susceptible to atmospheric disturbances like optical waves are, and so the development of radio telescopes over the past 40 years has seen a continued improvement in both the detection of faint sources as well as in resolution. Despite the fact that radio waves can have wavelengths which are meters long, the resolution achieved has been to the sub-arc second level through the use of many radio telescopes working together in an interferometer array, the largest of which stretches from Hawaii to the United States Virgin Islands (known as the Very Long Baseline Array).