Telescope
Telescope
Overcoming resolution limitations
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. The word telescope is derived from the Greek tele meaning far, and skopein meaning to view.
The first optical telescope was probably constructed by German-born Dutch lensmaker Hans Lippershey (1570–1619), in 1608. The following year, Italian astronomer and physicist Galileo Galilei (1564– 1642) 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 galaxy. 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.
Operation of a telescope
Light gathering
The primary function of a telescope is that of radiation gathering, in many cases 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 value. The reason for this occurrence is that larger telescopes can see further because they can collect more light. The 200 in (508 cm) diameter reflecting telescope at Mt. Palomar, California, for instance 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 light gathering power grows as the area of the objective increases, or the square of its diameter if it is circular. 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.
Resolution
the resolution, or resolving power, of a telescope is defined as being the minimum angular separation between two different objects that can be detected. The angular resolution limit, q, of a telescope operating under ideal conditions is given by the simple formula:
where λ is the wavelength of radiation being detected and D is the limiting aperture of the telescope, usually the diameter of the objective, or primary optic. Unfortunately, astronomers 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 is need. Disturbances and non-uniformities in the atmosphere limit the resolution of telescopes positioned on the surface of Earth to somewhere in the range 0.5 to 2 arc seconds, depending on the location of the telescope. Telescope sights on top of mountains are popular since 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
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°.
Types of telescope
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
A simple, uncorrected refracting telescope is shown in Figure 1.
The parallel light from a distant object enters the objective, of focal length f1, from the left. The light then comes to a focus at a distance f1 from the objective. The eyepiece, with focal length f2, is situated a distance f1+ f2 from the objective such that the light exiting the eyepiece is parallel. Light coming from a second object (dashed lines) exits the eyepiece at an angle equal to f1/f2 times the angle of the light entering.
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 stability of the alignment and transmission characteristics 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. In addition, 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 that is of highly uniform optical quality throughout its entire volume.
Reflecting telescopes
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 that occludes part of the primary mirror and causes diffraction.
Figure 2 shows four different focusing systems for reflecting telescopes.
These are a) the prime focus, where the detector is simply placed at the prime focus of the mirror; b) the Newtonian, where a small, flat mirror reflects the light out to the side of the telescope; c) the Cassegrain, where the focus is located behind the plane of the primary mirror through a hole in its center and d) the Coudè, where the two flat mirrors provide a long focal length path as shown.
Catadioptric telescopes
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.
Overcoming resolution limitations
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 non-uniform. 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, namely 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 have lead to a new era in observational astronomy.
Space telescopes
The best known and biggest 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. 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, close to the theoretical limit since there is no atmospheric distortion, and a factor of around twenty times better than was previously possible. 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 about Earth 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.
Adaptive optics
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. The principle behind adaptive optical telescope systems is illustrated in Figure 3.
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 telescope collects the light from the guide-star 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 that are 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 ten 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.
Recording telescope data
Telescopes collect light largely for two types of analysis, imaging and spectrometry. The better known is imaging, the goal of which 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 the Earth, but in the opposite direction. This is still the case today, but the modern telescope no longer uses photographic film but a charge-coupled device (CCD) array. The CCD is a semiconductor light detector, which is fifty times more sensitive than photographic film, and is able to detect single photons. Being fabricated using semiconductor techniques, the CCD can be made to be 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 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.
Modern optical telescopes
For almost 40 years the Hale telescope at Mt. Palomar (San Diego, California) 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 for the 2000s and beyond. These telescopes use revolutionary designs in order to increase the collecting area; 2,260 ft2 (210 m2) is being used for the European Southern Observatory (ESO), which operates observatories in Chile; the organization is headquartered near Munich, Germany.
This new generation of telescopes does not use the solid, heavy primary mirror of previous designs, whose thickness was between one-sixth and one-eighth of the mirror diameter. Instead, it uses a variety of approaches to reduce the mirror weight and improve its thermal and mechanical stability, including using many hexagonal mirror elements forming a coherent array; a single large meniscus mirror (with a thickness one-fortieth of the diameter), with many active support points which bend the mirror into the correct shape; and, a single large mirror formed from a honeycomb sandwich (Table 1). In 2005, one of the first pictures taken by ESO was of 2M1207b, an exo-solar planet (a planet orbiting a star other than the sun) orbiting a brown dwarf star about 260 light-years away (where one light-year is the distance that light travels in vacuum in one year). These new telescopes, combined with quantum-limited detectors, distortion reduction techniques, and coherent array operation allow astronomers to see objects more distant than have been observed before.
One of this new generation, the Keck telescope located on Mauna Kea in Hawaii, is currently the largest operating optical/infrared 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 nm 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 nm to 30 æm. In the late 1990s, Keck I was joined by an identical twin, Keck II. Then, in 2001, the two telescopes were linked together through the use of interferometry for an effective mirror diameter of 279 ft (85 m).
Alternative wavelengths
Most of the discussion so far has been concerned with optical telescopes operating in the range from 300 to 1,100 nanometers (nm). However, valuable information is contained in the radiation reaching Earth 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
Infrared telescopes are particularly useful for examining the emissions from gas clouds. Since 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 saw several new infrared telescopes, including the Infrared Optimized Telescope, a 26 ft (8 m) diameter facility, on Mauna Kea, Hawaii. In August 2003, NASA launched the Spitzer Space Telescope (formerly the Space Infrared Telescope Facility; named after Lyman Spitzer, Jr., who first suggested placing telescopes in orbit in the 1940s). It is in orbit about the sun (a heliocentric orbit), in which it follows behind Earth’s orbit about the sun, slowly receding away from Earth each year. Its primary mirror is about 2.8 ft (85 cm) in diameter, with a focal length that is twelve times the diameter of the primary mirror.
Several methods are used to reduce the large thermal background that 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 telescopes
Radio astronomy was developed following 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 forty 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). The largest working radio telescope is the Giant Meterwave Radio Telescope in India. It contains 14 telescopes arranged around a central square and another 16 positioned within three arms of a Y-shaped array. Its total interferometric baseline is about 15.5 mi (25 km). Construction is being made on the Low Frequency Array (LOFAR), which is a series of radio telescopes located across the Netherlands and Germany. As of September 2006, LOFAR has been constructed and is in the testing stage. When operational, it will have a total collecting area of around 0.4
Table 1. Major Ground-Based Optical Telescopes. (Thomson Gale.) | ||
---|---|---|
Major ground-based optical telescopes | ||
Name | Collector area | Design type |
Multi-mirror Telescope | 33 m2 | 6.5 m honeycomb |
Conversion Kitt Peak, Arizona | glass | |
Magellan Las Campanas, | 50 m2 | 8 m honeycomb glass |
Chile | ||
Keck Telescope Mauna Kea, Hawaii | 76 m2 | 36 × 1.8 m hexagonal array, |
Keck I and II Mauna | 152 m2 | two 36 × 1.8 m |
Key, Hawaii mirror | hexagona arrays, spaced by ~ 75 m | |
Columbus, Arizona | 110 m2 | 2 × 8.4 m honeycomb glass |
Very Large Telescope Cerro Paranal, Chile | 210 m2 | 4 × 8.2 m diameter meniscus |
KEY TERMS
Chromatic aberration —The reduction in image quality arising from the fact that the refractive index in varies across the spectrum.
Objective —The large light collecting lens used in a refracting telescope.
Reflecting telescope —A telescope that uses only reflecting elements, i.e., mirrors.
Refracting telescope —A telescope that uses only refracting elements; i.e., lenses.
Spectrometry —The measurement of the relative strengths of different wavelength components that make up a light signal.
square miles (one square kilometer). The Square Kilometer Array (SKA), which is scheduled to be completed in 2010, partially operational in 2015, and fully operational in 2020, will become the most sensitive radio telescope ever built. Its final location has yet to be decided, although Western Australia and South Africa are prime candidates.
See also Spectroscopy.
Resources
BOOKS
Comins, Neil F. Discovering the Universe. New York: W.H. Freeman and Co., 2005.
Halpern, Paul. Brave New Universe: Illuminating the Darkest Secrets of the Cosmos. Washington, DC: Joseph Henry Press, 2006.
Kerrod, Robin. Hubble: The Mirror on the Universe. Toronto, Canada, and Buffalo, NY: Firefly Books, 2003.
Lang, Kenneth R. Parting the Cosmic Veil. New York: Springer, 2006.
Levy, David H. Cosmology 101. New York: Ibooks, 2003.
Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.
Moore, Patrick. Eyes on the Universe: The Story of the Telescope. London, UK, and New York: Springer, 1997.
Iain A. McIntyre
Telescope
Telescope
The telescope is an instrument which 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 a 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.
Operation of a telescope
Light gathering
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 for this is that larger telescopes can see further because they can collect more light. The 200 in (508 cm) diameter reflecting telescope at Mt. Palomar, California, for instance 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 light gathering power grows as the area of the objective increases, or the square of its diameter if it is circular. 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.
Resolution
The resolution, or resolving power, of a telescope is defined as being the minimum angular separation between two different objects which can be detected. The angular resolution limit, q, of a telescope operating under ideal conditions is given by the simple formula:
where λ is the wavelength of radiation being detected and D is the limiting aperture of the telescope, usually the diameter of the objective, or primary optic. Unfortunately, we 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 we need. Disturbances and nonuniformities in the atmosphere limit the resolution of telescopes to somewhere in the range 0.5-2 arc seconds, depending on the location of the telescope. Telescope sights on top of mountains are popular since 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
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°.
Types of telescope
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
A simple, uncorrected refracting telescope is shown in Figure 1.
The parallel light from a distant object enters the objective, of focal length f1, from the left. The light then comes to a focus at a distance f1 from the objective. The eyepiece, with focal length f2, is situated a distance f1+f2 from the objective such that the light exiting the eyepiece is parallel. Light coming from a second object (dashed lines) exits the eyepiece at an angle equal to f1/f2 times the angle of the light entering.
Refracting telescopes, i.e. telescopes which 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 .
Reflecting telescopes
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 .
Figure 2 shows four different focusing systems for reflecting telescopes.
These are a) the prime focus, where the detector is simply placed at the prime focus of the mirror; b) the Newtonian, where a small, flat mirror reflects the light out to the side of the telescope; c) the Cassegrain, where the focus is located behind the plane of the primary mirror through a hole in its center and d) the Coudé, where the two flat mirrors provide a long focal length path as shown.
Catadioptric telescopes
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.
Overcoming resolution limitations
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, namely 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 .
Space telescopes
The best known and biggest 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. 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, close to the theoretical limit since there is no atmospheric distortion, and a factor of around twenty times better than was previously possible. 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.
Adaptive optics
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. The principle behind adaptive optical telescope systems is illustrated in Figure 3..
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 that are 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 ten 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.
Recording telescope data
Telescopes collect light largely for two types of analysis, imaging and spectrometry. The better known is imaging, the goal of which is simply to produce an accurate picture of the objects which 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 fifty times more sensitive than photographic film, and is able to detect single photons. Being fabricated using semiconductor techniques, the CCD can be made to be 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 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.
Modern optical telescopes
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 for the 1990s and beyond. 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, including using 1) many hexagonal mirror elements forming a coherent array, 2) a single large meniscus mirror (with a thickness 1/40 of the diameter), with many active support points which bend the mirror into the correct shape and 3) a single large mirror formed from a honeycomb sandwich.
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
Name | Collector Area | Design Type |
Multi-mirror Telescope Conversion Kitt Peak, Arizona | 33 m2 | 6.5 m honeycomb glass |
Magellan Las Campanas, Chile | 50 m2 | 8 m honeycomb glass |
Keck Telescope Mauna Kea, Hawaii | 76 m2 | 36 x 1.8 m hexagonal array |
Keck I and II Mauna Key, Hawaii mirror | 152 m2 | two 36 x 1.8 m hexagonal arrays, spaced by ~ 75 m |
Columbus, Arizona | 110 m2 | 2 x 8.4 m honeycomb glass |
Very Large Telescope Cerro Paranal, Chile | 210 m2 | 4 x 8.2 m diameter meniscus |
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 nm 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 300 nm-30 æm. 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.
Alternative wavelengths
Most of the discussion so far has been concerned with optical telescopes operating in the range 300 nm-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
Infrared telescopes (operating from 1-1000 æm) are particularly useful for examining the emissions from gas clouds . Since 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, an 26 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 telescopes
Radio astronomy was developed following 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 forty 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).
See also Spectroscopy.
Resources
books
Consolmagno, Guy, and Dun M. Davis. Turn Left at Orion. Cambridge, UK: Cambridge University Press, 1989.
Field, George, and Donald Goldsmith. The Space Telescope. Chicago: Contemporary Books, 1989.
Malin, David. A View of the Universe. Cambridge: Sky Publishing, 1993.
Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.
Parker, Barry. Stairway to the Stars. New York: Plenum, 1994.
Tucker, Wallace, and Tucker, Karen. The Cosmic Inquirers. Cambridge: Harvard University Press, 1986.
periodicals
Martin, Buddy, Hill, John M., and Angel, Robert. "The New
Ground-Based Optical Telescopes." Physics Today (March 1991).
Iain A. McIntyre
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Chromatic aberration
—The reduction in image quality arising from the fact that the refractive index in varies across the spectrum.
- Objective
—The large light collecting lens used in a refracting telescope.
- Reflecting telescope
—A telescope which uses only reflecting elements, i.e., mirrors.
- Refracting telescope
—A telescope which uses only refracting elements, i.e., lenses.
- Spectrometry
—The measurement of the relative strengths of different wavelength components which make up a light signal.
Telescope
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).
See also Atmospheric composition and structure; SETI; Space and planetary geology
Telescope
Telescope
There is much confusion and debate concerning the origin of the telescope. Many notable individuals appear to have simultaneously and independently discovered how to make a telescope during the last months of 1608 and the early part of 1609. Regardless of its origins, the invention of the telescope has led to great progress in the field of astronomy.
The Origin of the Telescope
Contrary to popular belief, Galileo Galilei (1564–1642) did not invent the telescope, and he was probably not even the first person to use this instrument in astronomy. Instead, the latter honor may be attributed to Thomas Harriot (1560–1621). Harriot developed a map of the Moon several months before Galileo began observations. Nevertheless, Galileo distinguished himself in the field through his patience, dedication, insight, and skill.
The actual inventor of the telescope may never be known with certainty. Its invention may have been by a fortuitous occurrence when some spectacle maker happened to look through two lenses at the same time. Several accounts report that Hans Lipperhey of Middelburg in the Netherlands had two lenses set up in his spectacle shop to allow visitors to look through them and see the steeple of a distant church. However, this story cannot be verified.
It is known that the first telescopes were shown in the Netherlands. Records show that in October 1608, the national government of the Netherlands examined the patent application of Lipperhey and a separate application by Jacob Metius of Alkmaar. Their devices consisted of a convex and concave lens mounted in a tube. The combination of the two lenses magnified objects by 3 or 4 times. However, the government of the Netherlands considered the devices too easy to copy to justify awarding a patent. The government did vote a small award to Metius and employed Lipperhey to devise binocular versions of his telescope. Another citizen of Middelburg, Zacharias Janssen, had also made a telescope at about the same time but was out of town when Lipperhey and Matius made their applications.
News of the invention of the telescope spread rapidly throughout Europe. Within a few months, simple telescopes, called "spyglasses," could be purchased at spectacle-maker's shops in Paris. By early 1609, four or five telescopes had made it to Italy. By August of 1609, Thomas Harriot had observed and mapped the Moon with a six-power telescope.
What Galileo Discovered
Despite Harriot's honor as the first telescopic astronomer, it was Galileo who made the telescope famous. At the time, Galileo was Professor of Mathematics at the University of Padua in Italy. Somehow, he learned of the new instrument that had been invented in Holland, although there is no evidence that he actually saw one of the telescopes known to be in Italy. Over the next several months in 1609 and 1610, Galileo made several progressively more powerful and optically superior telescopes using lenses he ground himself. Galileo used these instruments for a systematic study of the night sky. He saw mountains and craters on the Moon, discovered four satellites of Jupiter, viewed sunspots, observed and recorded the phases of Venus, and found that the Milky Way galaxy consisted of clouds of individual stars.
Galileo summarized his discoveries in the book Sidereus Nuncius (The Starry Messenger ) published in March of 1610. Others working at around the same time claimed to have made similar discoveries—others certainly observed sunspots—but Galileo gathered all of his observations together and wrote about them first. Consequently, he is generally credited with their discovery.
The observation of Venus's phases was especially important to Galileo. According to Ptolemaic theory , Venus would show crescent and "new" phases, but it would not go through a complete cycle of phases. The Ptolemaic model never placed Venus on the opposite side of the Sun as seen from Earth, so Venus would never appear "full." Yet Galileo clearly observed a nearly full Venus. He also observed that the four satellites of Jupiter orbited the planet, conclusively demonstrating that there was at least one instance of an orbital center other than Earth, a clear contradiction to the Ptolemaic model.
Adapting the Telescope
Galileo apparently had no real knowledge of how the telescope worked but he immediately recognized its military value, as well as its entertainment value. He set about building a version that is commonly known as a "Galilean telescope." It had a convex lens as a primary objective (the lens in front) and a concave lens as the eyepiece. The focal point of the objective lens was behind the eyepiece, and the eyepiece served primarily to form the upright image desired for terrestrial observation.
Johannes Kepler was arguably the first person to give a concise theory of how light passed through the telescope and formed an image. Kepler also discussed the various ways in which the lenses could be combined in different optical systems, improving on Galileo's design. Kepler's design used convex lenses for both the primary objective and the eyepiece. However, in spite of his theoretical understanding, there is no evidence that Kepler ever actually tried to put together a telescope.
Telescopes built following Kepler's design were not practical for military applications or everyday use because they inverted and reversed the images and showed people upside-down. However, their greater magnification, brighter image, and wider angle of view made them best for astronomical observations where the inverted image made no difference. The telescope rapidly came into common astronomical use during the 20 years after it was invented.
Unfortunately, it soon became evident that the refracting telescope had a great disadvantage. The main problem with early telescopes was the low quality of their glass and the poor manner in which the lenses were ground. However, even the best lenses had two inherent defects. One defect resulted because the objective lens did not bend all wavelengths equally, and this resulted in the red part of the light-beam being brought to a focus at a greater distance from the objective. An image of a star viewed through an astronomical telescope from this period seemed to be surrounded by colored fringes. This defect is known as "chromatic aberration."
The other problem resulted when the surface of the lens was ground to a spherical shape. Rays passing through the edge of the lens were brought to a focus at a different distance than rays passing near the center of the lens. This defect is called "spherical aberration." A lens can be ground to a different shape (all modern optical instruments use "aspheric" lenses) but the lens grinders of Galileo's time did not possess the technology to do this.
One remedy for both chromatic and spherical aberrations was to make telescopes with extremely long focal length lenses (so that the lenses did not have much curvature), but this required telescopes several meters long. These telescopes were cumbersome and difficult to use. Another solution for chromatic aberration, unknown at the time, was to use an objective lens made from two different kinds of glass glued together. This method greatly reduces chromatic aberration.
Development of the Reflecting Telescope
During the 1680s, Cambridge University was often closed for fear of the Plague. When this occurred, physicist Isaac Newton would retreat to his country home in Lincolnshire. During one of these intervals, Newton began trying to unravel the problem of chromatic aberration.
Newton allowed a beam of sunlight to pass through a glass prism and observed that the beam was split into a rainbow of colors. On the basis of this and other experiments, he decided (incorrectly, it turns out) that the refractor could never be cured of chromatic aberration. Newton consequently developed a new type of telescope, "the reflector," in which there is no objective lens. The light from the object under observation is collected by a curved mirror, which reflects all wavelengths equally.
Newton likely did not originate the idea of a reflecting telescope. Earlier, in 1663, Scottish mathematician James Gregory had suggested the possibility of a reflector. However, Newton was apparently the first person to actually build a working reflector.
Newton created a reflecting telescope with a 2.5-cm (centimeter) metal mirror and presented it to the Royal Society in 1671. But using a reflecting mirror instead of a reflecting lens created another problem. The light beam is reflected back up the tube but it cannot be observed without blocking the light entering the tube. Gregory had suggested inserting a curved secondary mirror that would reflect the light back through a hole in the center of the primary mirror. However, the technology needed to grind the complex mirror surfaces required for Gregory's design did not exist at the time. Newton solved this problem by introducing a second flat mirror in the middle of the tube mounted at a 45-degree angle so that the light beam is reflected out the side of the tube. The eyepiece was then mounted to the side of the tube. This design is still known as a "Newtonian reflector." Since Newton's time, several other reflecting telescope designs have been developed.
Newtonian reflectors were not free of problems. Metal mirrors were hard to grind. The mirror surface tarnished quickly and had to be polished every few months. These problems kept the Newtonian reflector from being widely accepted until after 1774, when new designs, polishing techniques, the use of silvered glass, and other innovations were developed by William Herschel. Herschel discovered the planet Uranus in 1781 using a telescope he had made. He continued to build reflecting telescopes over the next several years, culminating in an enormous 122-cm instrument completed in 1789.
Herschel's 122-cm telescope remained the largest in the world until 1845, when the Irish astronomer, William Parsons, the third Earl of Rosse, completed an instrument known as the Leviathan, which had a mirror diameter of 180 cm. Lord Rosse used this instrument to observe "spiral nebulae," which are now known to be other galaxies. Throughout the eighteenth and nineteenth centuries, telescopes of ever-increasing size were built.
Modern Telescopes
The twentieth century saw continued improvement in telescope size and design. Larger telescopes are preferred for two reasons. Larger instruments gather more light. Astronomical distances are so great that most objects are not visible to the unaided eye. The Andromeda galaxy (M31) is generally considered the most distant object that can be seen with the naked eye, and it is the closest galaxy to Earth outside of the Milky Way. To see very far out into space requires large telescope objectives. This is another reason for the general preference of astronomers for reflecting telescopes. It is easier to build large mirrors than it is to build large lenses.
A second reason for the general trend toward large instruments is resolving power. The ability of a telescope to separate two closely spaced stars (or see fine detail in general) is known as resolving power. If R is the resolving power (in arc seconds), is the wavelength (in micrometers) and d is the diameter of the objective (in meters) then: R = 0.25 . As d gets larger, R gets smaller (smaller is better).
During most of the twentieth century, astronomical images were recorded on photographic film. Later in the century, most observatories and research astronomers switched to solid state devices called CCDs (chargecoupled devices). These devices are much more sensitive to low light levels, and they also have the advantage of creating an electronic image that can be fed directly into a computer where it can be immediately processed.
Earth's atmosphere continues to challenge the progress of astronomy. Infrared and ultraviolet wavelengths do not pass through the atmosphere, so astronomy in those parts of the spectrum is generally done by balloon-based telescopes or by satellites. A bigger problem with the atmosphere is its inherent instability. Even on the clearest of nights, images jiggle and quiver due to small variations in the atmosphere. One way to get around this problem is to get above the atmosphere.
The Hubble Space Telescope (named after Edwin Hubble, who discovered the galactic redshift ) is a satellite based optical and infrared observatory. The spectacular images from "Hubble" have pushed back the frontiers of astronomical knowledge, while raising many new questions.
Ground-based large telescope design also continues to evolve. Very large mirrors suffer from many problems. To make them stiff, they must be very thick, which makes them very heavy. Modern telescopes use thin mirrors with many different supports that can be independently controlled by a computer. As the mirror is moved, the supports continually adjust to compensate for gravity, keeping the shape of the mirror within precise tolerances.
As of 2001, the largest telescope in the world is the twin mirror Keck Telescope. The Keck consists of two matched mirrors housed in separate buildings. Each telescope housing stands eight stories tall and each mirror weighs 300 tons. The mirrors are not made of one solid piece of glass. Instead, each mirror combines thirty-six 1.8-m (meter) hexagonal cells combined to form a collecting area equivalent to one 10-m mirror. Since the twin mirrors are separated by a wide distance, the Keck telescope has a much greater resolving power than either mirror alone would have.
The other advantage of the Keck telescope is its position on the top of an extinct volcano, Mauna Kea, in Hawaii. The atmosphere is very stable and the mountaintop, at 4 km (kilometers), is so high that the telescopes are above most of Earth's atmosphere. The European Southern Observatory in Chile is constructing a similar telescope that will combine light from four8.2-m mirrors working as a single instrument. These and similar instruments around the world promise to reveal even more about our universe.
see also Astronomer.
Elliot Richmond
Bibliography
Chaisson, Eric, and Steve McMillan. Astronomy Today, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1993.
Giancoli, Douglas C. Physics, 3rd ed., Englewood Cliffs, NJ: Prentice Hall, 1991
Pannekoek, Anton. A History of Astronomy. New York: Dover Publications, 1961.
Sagan, Carl. Cosmos. New York: Random House, 1980.
Telescope
Telescope
Background
A telescope is a device used to form images of distant objects. The most familiar kind of telescope is an optical telescope, which uses a series of lenses or a curved mirror to focus visible light. An optical telescope which uses lenses is known as a refracting telescope or a refractor; one which uses a mirror is known as a reflecting telescope or a reflector. Besides optical telescopes, astronomers also use telescopes that focus radio waves, X-rays, and other forms of electromagnetic radiation. Telescopes vary in size and sophistication from homemade spyglasses built from cardboard tubes to arrays of house-sized radio telescopes stretching over many miles.
The earliest known telescope was a refractor built by the Dutch eyeglass maker Hans Lippershey in 1608 after he accidentally viewed objects through two different eyeglass lenses held a distance apart. He called his invention a kijker, "looker" in Dutch, and intended it for military use. In 1609, the Italian scientist Galileo Galilei built his own telescopes and was the first person to make astronomical observations using them. These early telescopes consisted of two glass lenses set within a hollow lead tube and were rather small; Galileo's largest instrument was about 47 inches (120 cm) long and 2 inches (5 cm) in diameter. Astronomers such as Johannes Kepler in Germany and Christian Huygens in Holland built larger, more powerful telescopes throughout the 1600s. Soon these telescopes got too large to be easily controlled by hand and required permanent mounts. Some were more than 197 feet (60 m) long.
The ability to construct enormous telescopes outpaced the ability of glassmakers to manufacture appropriate lenses for them. In particular, the problems caused by chromatic aberration (the tendency for a lens to focus each color of light at a different point, leading to a blurred image) became acute for very large telescopes. Scientists of the time knew of no way to avoid this problem with lenses, so they designed telescopes using curved mirrors instead.
In 1663, the Scottish mathematician James Gregory designed the first reflecting telescope. Alternate designs for reflectors were invented by the English scientist Isaac Newton in 1668 and the French scientist N. Cassegrain in 1672. All three designs are still in use today. In the 1600s, there was no good way to coat glass with a thin reflective film, as is done today to make mirrors, so these early reflectors used mirrors made out of polished metal. Newton used a mixture of copper, tin, and arsenic to produce a mirror which could only reflect 16% of the light it received; today's mirrors reflect nearly 100% of the light that hits them.
It had been known as early as 1730 that chromatic aberration could be minimized by replacing the main lens of the telescope with two properly shaped lenses made from two different kinds of glass, but it was not until the early 1800s that the science of glassmaking was advanced enough to make this technique practical. By the end of the 19th century, refracting telescopes with lenses up to a meter in diameter were constructed, and these are still the largest refracting telescopes in operation.
Reflectors once again dominated refractors in the 20th century, when techniques for constructing very large, very accurate mirrors were developed. The world's largest optical telescopes are all reflectors, with mirrors up to 19 feet (6 m) in diameter.
Raw Materials
A telescope consists of an optical system (the lenses and/or mirrors) and hardware components to hold the optical system in place and allow it to be maneuvered and focused. Lenses must be made from optical glass, a special kind of glass which is much purer and more uniform than ordinary glass. The most important raw material used to make optical glass is silicon dioxide, which must not contain more than one-tenth of one percent (0.1%) of impurities.
Optical glasses are generally divided into crown glasses and flint glasses. Crown glasses contain varying amounts of boron oxide, sodium oxide, potassium oxide, barium oxide, and zinc oxide. Flint glasses contain lead oxide. The antireflective coating on telescope lenses is usually composed of magnesium fluoride.
A telescope mirror can be made from glass that is somewhat less pure than that used to make a lens, since light does not pass through it. Often a strong, temperature-resistant glass such as Pyrex is used. Pyrex is a brand name for glass composed of silicon dioxide, boron oxide, and aluminum oxide. The reflective coating for telescope mirrors is usually made from aluminum, and the protective coating on top of the reflective coating is usually composed of silicon dioxide.
Hardware components that are directly involved with the optical system are usually manufactured from steel or steel and zinc alloys. Less critical parts can be made from light, inexpensive materials such as aluminum or acrylonitrile-butadiene-styrene plastic, commonly called ABS.
The Manufacturing
Process
Making the hardware components
- 1 Metal hardware components are manufactured using standard metalworking machines such as lathes and drill presses.
- 2 Components made from ABS plastics (usually the external body of the telescope) are produced using a technique known as injection molding. In this process the plastic is melted and forced under pressure into a mold in the shape of the final product. The plastic is allowed to cool back into a solid, and the mold is opened to allow the component to be removed.
Making optical glass
- 3 The glass manufacturer mixes the proper raw materials with waste glass of the same type as the glass to be made. This waste glass, known as cullet, acts as a flux; that is, it causes the raw materials to react together at a lower temperature than they would without it.
- 4 This mixture is heated in a glass furnace until it has melted into a liquid. The temperature needed to form molten glass varies with the type of glass being made, but it is typically about 2550°F (1400°C).
- 5 The temperature of the molten glass is raised to about 2820°F (1550°C) to force air bubbles to come to the surface. It is then allowed to cool while being stirred constantly until it has reached about 1830°F (1000°C), at which point it is an extremely thick fluid. This viscous, molten glass is poured into molds with roughly the same shape as the lenses required.
- 6 After the glass has cooled to about 570°F (300°C), it must be reheated to about 1020°F (550°C) to remove internal stresses that form during the initial cooling period and which weaken the glass. It is then allowed to cool slowly to room temperature. This process is known as annealing. The final lens-shaped chunks of glass are known as blanks.
Making the lenses
The blanks are processed by the telescope manufacturer in three steps: cutting, grinding, and polishing. A mirror is formed in exactly the same way as a lens until the reflective coating is applied.
- 7 First a high-speed, rotating cylindrical cutter with a round diamond blade, known as a curve generator, shaves the surface of the lens until a close approximation of the desired curve is achieved. The cut lens is inspected with a spherometer to check the curvature and is recut if necessary. The time required for cutting varies greatly with the type of glass being cut and the kind of lens being shaped. A lens may require several cuttings, each of which may take anywhere from a few minutes to more than half an hour.
- 8 Several cut blanks are placed on a curved block in such a way that their surfaces line up as if they were all part of one large spherical curve. This is necessary so that the grinding machine can grind them all in the same way. A cast iron grinding surface known as a tool is pressed onto them. During grinding, the block of lenses rotates while the tool is free to move at random on top of it. Between the tool and the block flows a slurry containing water, an abrasive to do the grinding (usually silicon carbide), a coolant to prevent the lenses from being damaged by overheating, and a surfactant to keep the abrasive from settling out. The speed at which the block rotates, the force placed on the lenses, the exact contents of the slurry, and other variables are controlled by experienced opticians to produce the exact type of lens desired. Each lens is once again inspected with a spherometer and reground if necessary. The total grinding process may take anywhere from one hour to eight hours. The ground lenses are cleaned and moved to the polishing room.
- 9 The polishing machine is similar to the grinding machine, but the tool is made from pitch—a thick, soft, resinous substance derived from coal tar or wood tar. A pitch tool is made by placing tape around the circumference of a curved dish, pouring in hot, liquid pitch with other ingredients such as beeswax and jeweler's rouge, and letting it cool back into a solid. A pitch tool can polish about 50 lenses before it must be reshaped. Polishing proceeds in the same manner as grinding, but instead of an abrasive the slurry contains a polishing substance, usually cerium dioxide, in the form of a very fine pink powder. The polished lenses are optically inspected and repolished if necessary. The polishing procedure may take anywhere from half an hour to four or five hours. The lenses are cleaned and are ready for coating.
Applying coatings
- 10 To make a lens into a mirror, a very thin, very smooth coating of aluminum is applied. Aluminum is heated in a vacuum to form a vapor. A negative electro-static charge is applied to the surface of the lens so that the positively charged aluminum ions are attracted to it. Similar procedures are followed to apply a coating of silicon dioxide to protect the fragile surface of a mirror or to apply an antireflective coating of magnesium fluoride to the surface of a lens. The finished lens or mirror is inspected, labeled with a date of manufacture and a serial number, and stored until needed.
Assembling and shipping the telescope
- 11 The hardware components, lenses, and mirrors required to make a particular model of telescope are assembled by hand in an assembly line process. The completed telescope is packed with close-fitting expanded polystyrene foam to protect it from damage during shipping. The telescope is packed in a cardboard box and shipped to the retailer or consumer.
Quality Control
The most critical aspect of quality control for an optical telescope is the accuracy of the lenses and mirrors. During the cutting and grinding stages, the physical dimensions of the lens are measured very carefully. The thickness and the diameter of the lens are measured with a vernier caliper, an instrument which looks something like a monkey wrench. The outer, fixed jaw of the caliper is placed against one side of the lens and the inner, sliding jaw is gently moved until it meets the other side of the lens. In a classic vernier caliper, the dimensions of the lens are read very accurately using a scale which moves along with the inner jaw and which is compared with a stationary scale attached to the outer jaw. This type of caliper works much like a slide rule. There also exist electronic versions of this instrument, in which the measured dimension automatically appears on a digital display.
The curvature of a lens is measured with a spherometer, a device which resembles a pocket watch with three small pins protruding from its base. The outer two pins are fixed in place while the inner pin is free to move in and out. The spherometer is gently placed on the surface of the lens. Depending on the type of curve, the middle pin will either be higher than the other two pins or lower than the other two pins. The movement of the inner pin moves a needle on a calibrated dial on the face of the spherometer. This value is compared with the standard value that should be obtained for the desired curvature.
Tolerances vary with the type of lens being manufactured, but a typical acceptable variation might be plus or minus 0.0008 inches (20 micrometers). For a flat lens, generally one destined to become a flat mirror, the tolerance is much smaller, usually about plus or minus 0.00004 inches (1.0 micrometer).
During the polishing stage, these instruments are not accurate enough to ensure that the lens will work properly. Optical tests, which measure the way light is affected by the lens, must be used. One common test is known as an autocollimation test. The lens is placed in a dark room and is illuminated with a low intensity pinpoint light source. A diffraction grating (a surface containing thousands of microscopic parallel grooves per inch) is placed at the point where the lens should focus light. The grating causes an interference pattern of dark and light lines to form in front of and behind the focal point. The true focal point can thus be found precisely and compared with the theoretical focal point for the type of lens desired.
In order to test a flat lens, a lens that is known to be flat is placed face down on the lens that is to be tested, which rests on a piece of black felt. The microscopic gaps between the two lenses cause an interference pattern to appear when gentle pressure is applied. The light and dark lines are known as Newton's rings. If the lens being tested is flat, the lines should be straight and regular. If the lens is not flat, the lines will be curved.
The Future
The techniques used to produce excellent lenses and mirrors have been well under-stood for many years, and major innovations in this area are unlikely. One area of active research is in coating technology. New coating substances may be developed to provide better protection for mirrors and better prevention of loss of light through reflection for lenses.
A more dramatic area of progress is in the electronic accessories that accompany telescopes. Amateur astronomers will soon be able to obtain telescopes with built-in computer guidance systems that will enable them to automatically point the telescope at a selected celestial object and to track it night by night. They will also be able to attach video cameras to their telescopes and film such astronomical phenomena as lunar eclipses and the movements of planets and moons.
Where To Learn More
Books
Asimov, Isaac. Eyes on the Universe: A History of the Telescope. Houghton Mifflin, 1975.
Bell, Louis. The Telescope. Dover, 1981.
Manly, Peter L. Unusual Telescopes. Cambridge University Press, 1991.
Periodicals
Mullins, Mark. "A Truly Economical Telescope." Sky and Telescope, December 1993, pp. 91-92.
Nash, J. Madeleine. "Shoot for the Stars." Time, April 27, 1992, pp. 56-57.
Nelson, Ray. "Reinventing the Telescope." Popular Science, January 1995, pp. 57-59, 85.
—Rose Secrest
Telescope
Telescope
The telescope is an instrument that gathers light or some other form of electromagnetic radiation (from radio waves to gamma rays) emitted by distant sources. The most common type is the optical telescope, which uses a collection of lenses or mirrors to magnify the visible light emitted by a distant object. There are two basic types of optical telescopes—the refractor and the reflector. The one characteristic all telescopes have in common is the ability to make distant objects appear to be closer.
The first optical telescope was constructed in 1608 by Dutch spectacle-maker Hans Lippershey (1570–1619). He used his telescope to view distant objects on the ground, not distant objects in space. The following year, Italian physicist and astronomer Galileo Galilei (1564–1642) built the first astronomical telescope. 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 our Milky Way galaxy.
Words to Know
Black holes: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
Chromatic aberration: Blurred coloring of the edge of an image when white light passes through a lens, caused by the bending of the different wavelengths of the light at different angles.
Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.
Gamma ray: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.
Interferometry: In astronomy, the precise combining of light or radio waves collected by two or more instruments from one single celestial object.
Radiation: Energy transmitted in the form of subatomic particles or waves.
Radio wave: Longest form of electromagnetic radiation, measuring up to 6 miles (9.6 kilometers) from peak to peak.
Reflector telescope: Telescope that directs light from an opening at one end to a concave mirror at the far end, which reflects the light back to a smaller mirror that directs it to an eyepiece on the side of the tube.
Refractor telescope: Telescope that directs light through a glass lens, which bends the light waves and brings them to a focus at an eyepiece that acts as a magnifying glass.
Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.
X ray: Electromagnetic radiation of a wavelength shorter than ultraviolet radiation but longer than gamma rays that can penetrate solids and produce an electrical charge in gases.
Refractor telescopes
In a refractor telescope, light waves from a distant object enter the top of the telescope through a lens called an objective lens. This lens is convex—thicker at the middle than the edges. As light waves pass through it, they are bent (refracted) so that they converge (come together) at a single point, known as the focus, behind the objective lens. The distance between the objective lens and the focus is called the focal length. A second lens, the eyepiece, at the focus then magnifies the image for viewing. This is the type of telescope Galileo developed and used.
As refractor telescopes came into wider use, observers realized the instruments had a slight imperfection. Since, like a prism, a lens bends the different wavelengths (colors) that make up light through different angles, refractor telescopes produced a false color around any bright object. This defect is called chromatic aberration. Early astronomers tried to correct this problem by increasing the focal length, but the new instruments were very clumsy to use.
A solution to this problem came in 1729 when English scientist Chester Moore Hall (1703–1771) devised the achromatic lens: two lenses, made of different kinds of glass and shape, set close together. As light passes through the lenses, the false color brought about by the first lens is canceled out by the second lens. Hall went on to create the achromatic telescope in 1733. The lens itself was further developed by English optician John Dollard in 1758. His lens combined two or more lenses with varying chemical compositions to minimize the effects of aberration.
Reflector telescopes
In a reflector telescope, light waves from a distant object enter the open top end and travel down the tube until they hit a mirror at the bottom. This mirror is concave—thicker at the edges than in the middle. Because of this primary mirror's shape, the light waves are reflected back up the tube to a focus, where a small, flat secondary mirror reflects the image to an eyepiece on the side of the telescope. English physicist and mathematician Isaac Newton (1642–1727) developed the reflector telescope in 1668. English astronomer William Herschel (1738–1822) used an updated version when he discovered Uranus in 1781.
Even today, reflectors are perhaps the most prominent type of telescope. They are relatively inexpensive to build and maintain, produce little false color, and maintain a high resolution. The mirrors used in larger reflectors, however, often cause distortion due to the weight on the instrument. Newer reflectors incorporate mirrors of varying shapes (hexagonal glass segments, for example) and of lighter, more durable materials (such as Pyrex™).
Limits to ground-based telescopes
Earth's atmosphere provides an effective filter for many types of cosmic radiation. This fact is crucial for the survival of humans and other life-forms. However, since the atmosphere only allows visible light and radio waves to pass through it, celestial objects that emit other types of electromagnetic radiation cannot be viewed through telescopes on the ground. Many observatories have been constructed at high altitudes where the atmosphere is thinner—and where the glare of urban artificial light interferes less with viewing—but this improves the situation only slightly.
Space-based telescopes
One way astronomers have sought to overcome the distortion caused by the atmosphere and by city lights is by placing telescopes in space. The first of these instruments, placed in orbit around Earth during the 1970s, were small telescopes that could detect X rays, gamma rays, and
ultraviolet radiation. They discovered hundreds of previously unknown entities, including one likely black hole.
While many other space telescopes have been placed in orbit, the most well known is the Hubble Space Telescope (HST). Launched in April 1990 aboard the space shuttle Discovery, the HST has an 8 foot (2.4 meter) primary mirror and five major instruments for examining various characteristics of distant celestial bodies. Shortly after the HST began orbiting Earth, scientists learned that the curve in its primary mirror was off by just a fraction of a hair's width. This flaw caused light to reflect away from the center of the mirror. As a result, the HST produced blurry pictures.
In 1993, astronauts aboard the space shuttle Endeavor caught up with the HST and installed a group of three coin-sized mirrors around the primary mirror, which brought the light into proper focus. In 1997, another space shuttle crew conducted general repairs to the HST. Then in November 1999, the HST stopped working after its gyroscopes broke down. Without the gyroscopes, the telescope could not hold steady while focusing on stars, galaxies, and other cosmic targets. A month later, during an eight-day mission, astronauts aboard the space shuttle Discovery installed almost $70 million worth of new equipment on the HST, including a computer 20 times faster than the telescope's old one; new gyroscopes; batteries with voltage regulators to prevent overheating; a new guidance unit, data recorder, and radio transmitter; and steel sunshades to protect the telescope from solar damage.
Despite the need for repairs, the HST has proven to be the finest of all telescopes ever produced. The thousands of images it has captured—a comet hitting Jupiter, a nursery where stars are born, stars that belong to no galaxy, galaxies that house quasars, galaxies violently colliding—have amazed astronomers.
The future on the ground
Technological advances in the 1990s began to return astronomy to the ground. New observatories have sprung up on every continent, including Antarctica, housing telescopes that are able to capture celestial images almost as clearly as the HST. These new ground-based telescopes are far more advanced than previous ones. The cost of producing their light-gathering mirrors has been reduced, so the mirrors and the telescopes can be built even larger. Advances in photographic devices allow these telescopes to capture images in minutes instead of hours or entire nights.
The twin domes at the Keck Observatory complex on Mauna Kea in Hawaii house telescopes with mirrors roughly 32 feet (9.8 meters) in diameter. The Hobby-Eberly Telescope at the University of Texas McDonald Observatory near Fort Davis, Texas, was completed on December 12, 1996. Its mirror, made up of 91 hexagonal segments, measures 36 feet (11 meters) in diameter.
Perhaps the greatest advancement is the development of interferometry. Astronomical interferometry is the art of combining light or radio waves collected by two or more telescopes from a single celestial object. The information is fed into a computer, which precisely matches up the light-wave images gathered from the telescopes, peak for peak and
trough for trough. After distortion is removed through mathematical analysis, the resulting image is equal in sharpness to what a single telescope of enormous size can produce.
Sometime early in the twenty-first century, the world's largest telescope will be completed. On the summit of Cerro Paranal, an 8, 645-foot (2,635-meter) mountain in the Atacama Desert in northern Chile (an area considered to be the driest on Earth), stands the Paranal Observatory. The observatory houses the Very Large Telescope, which will consist of four telescopes, each containing a mirror almost 27 feet (8.2 meters) in diameter. In the language of the Mapuche, indigenous people who live in the area, the four unit telescopes are known as Antu ("Sun"), Kueyen ("Moon"), Melipal ("Southern Cross"), and Yepun ("Venus"). The four units were each operational as of the beginning of 2001, but had yet to be combined as an interferometer. Through interferometry (the precise combining of light or radio waves collected by two or more instruments from one single celestial object), the teamed instruments will have a light-gathering capacity greater than a single telescope with a mirror more than 52 feet (16 meters) in diameter.
[See also Gamma ray; Infrared astronomy; Interferometry; Radio astronomy; Ultraviolet astronomy; X-ray astronomy ]
telescope
telescope
tel·e·scope / ˈteləˌskōp/ • n. an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified. ∎ short for radio telescope.• v. [tr.] cause (an object made of concentric tubular parts) to slide into itself, so that it becomes smaller. ∎ [intr.] be capable of sliding together in this way: five steel sections that telescope into one another. ∎ crush (a vehicle) by the force of an impact. ∎ fig. condense or conflate so as to occupy less space or time: a way of telescoping many events into a relatively brief period.