Chapter 6

Radiation

- refers to anything that radiates away from a source. - Dangerous high-energy particles emitted from radioactive atoms are also called radiation

gamma-rays

- shortest - Electromagnetic wave with extremely short wavelength, high frequency, and large photon energy.

Light can be focused into an image in one of two ways

1. a lens refracts ("bends") light passing through it, or 2. a mirror reflects ("bounces") light from its surface.

modern optical telescopes

Reflecting telescopes with standard designs depicted on this page have capabilities limited by complexity, weight, and the turbulence in Earth's atmosphere. Modern design solutions are shown on the opposite page. The primary mirror makes light converge to a prime focus position high in the telescope tube, as shown at the right . Although the prime focus is a good place to image faint objects, it is inconvenient for large instruments. A secondary mirror can reflect the light through a hole in the primary mirror to a Cassegrain focus. This focal arrangement is the most common one for large telescopes. Icon Smaller telescopes are often built with a Newtonian focus, the arrangement that Isaac Newton used in his first reflecting telescope. The Newtonian focus is inconvenient for large telescopes, as shown at right. Icon Many small telescopes such as the one on the left use a Schmidt-Cassegrain focus. A thin correcting plate improves the image but is not curved enough to introduce serious chromatic aberration. Icon Shown below, observations using the Mayall Telescope at Kitt Peak National Observatory in Arizona can be made at either the prime focus or the Cassegrain focus. Note the human figure at lower right. Icon Telescope mountings must contain a sidereal drive to move the telescope smoothly westward, countering the eastward rotation of Earth. The earlier equatorial mount (far left) has a polar axis parallel to Earth's axis, but the mount used for the largest modern telescopes is alt-azimuth mount (altitude-azimuth; near left) moves like a cannon—up and down, left and right. Alt-azimuth mountings are simpler to build than equatorial mountings but require computer control to follow the stars. Icon Unlike traditional thick mirrors, thin mirrors, sometimes called "floppy" mirrors as shown at right, weigh less and require less massive support structures. Also, they cool rapidly at nightfall, and there is less distortion from uneven expansion and contraction. Icon Grinding a large mirror may remove tons of glass and take months, but new techniques speed the process. Some large mirrors are cast in a rotating oven that causes the molten glass to flow to form a concave upper surface. Grinding and polishing such a preformed mirror is much less time consuming. Icon Mirrors made of segments are economical because the segments can be made separately. The resulting mirror weighs less and cools rapidly. Icon Both floppy mirrors and segmented mirrors sag under their own weight. Their optical shapes must be controlled by computer-driven thrusters behind the mirrors, a technique called active optics. Icon The two Keck telescopes, each meters in diameter, are located atop the extinct volcano Mauna Kea in Hawai'i. Their two primary mirrors are composed of hexagonal mirror segments, as shown at right . - With large enough telescopes, astronomers can actually ride inside a prime-focus "cage," although observations are usually made using instruments connected to computers in a separate control room.

wavelength

The distance between successive peaks or troughs of a wave; usually represented by

powers of a telescope

-light gathering, resolving

refracting telescope design

- A refracting telescope's primary lens is much more difficult to manufacture than a mirror of the same size. The interior of the glass must be pure and flawless because the light passes through it. Also, if the lens is achromatic, it must be made of two different kinds of glass requiring four precisely ground surfaces. - use a big lens at the top of the tube to bend the light and bring it to a focus. That lens, the primary lens, has a long focal length and produces a tiny upside down image that is hard to view. A second, short focal length lens called the eyepiece is used to magnify the image and guide the light into the eye, making the image convenient to view.

achromatic lenses

- A telescope lens composed of two lenses ground from different kinds of glass and designed to bring two selected colors to the same focus and correct for chromatic aberration.

optical telescopes

- A telescope that gathers and focuses visible light - intended for the study of visible light - performs better on a high mountaintop where the air is thin and steady. But even in that situation, Earth's atmosphere spreads star images at visual wavelengths into blobs about .5 to 1 arc second in diameter.

reflecting telescopes

- A telescope that uses a concave mirror to focus light into an image. - uses a primary mirror to focus the light by reflection. In this particular reflector design, called a Cassegrain telescope, a small secondary mirror reflects the starlight back down through a hole in the middle of the primary mirror to the eyepiece lens. - the main mirror in a reflecting telescope - all large astronomical telescopes built since the start of the th century have been reflecting telescopes - the curved primary mirror at the bottom of the tube reflects the light to a focus. To make the light come out the bottom of the tube for convenient viewing, astronomers place a secondary mirror at the top of the tube to reflect the light back down the tube through a hole in the primary mirror to a focus. These optical surfaces are coated with a thin film of aluminum alloy to make them highly reflective.

angstrom (Å)

- A unit of distance = 1 A = 5*10^-10 m - often used to measure the wavelength of light. - 1/10th of a nanometer

spectrum

- An arrangement of electromagnetic radiation in order of wavelength or frequency. - visible light: extending from red to violet

seeing

- Atmospheric conditions on a given night. When the atmosphere is unsteady, producing blurred images, the seeing is said to be poor. - even under relatively good seeing conditions, the detail visible through a large telescope is limited not by its diffraction fringes but by the turbulence of the air through which the telescope must look.

radio waves

- Beyond the infrared part of the electromagnetic spectrum - Electromagnetic wave with extremely long wavelength, low frequency, and small photon energy. used for FM, television, military, government, and cell phone radio transmissions have wavelengths of a few centimeters to a few meters, whereas - AM and other types of radio transmissions have wavelengths of a few hundred meters to a few kilometers

diffraction fringes

- Blurred fringe surrounding any image caused by the wave properties of light. Because of this, no image detail smaller than the fringe can be seen. - Stars are so far away that their images are points, but the wavelike characteristic of light causes each star image to be surrounded with diffraction fringes, much magnified in this computer model. (b) Two stars close to each other have overlapping diffraction fringes and become impossible to detect separately. - he size of the diffraction fringes is inversely proportional to the diameter of the telescope. This means that the larger the telescope, the better its resolving power. However, the size of diffraction fringes is also proportional to the wavelength of light being focused. In other words, an infrared or radio telescope has less resolving power than an optical telescope of the same size.

telescopes

- Both refracting and reflecting telescopes form an image that is small, inverted, and difficult to observe directly - an eyepiece normally is used to magnify the image and make it convenient to view - Short focal-length lenses and mirrors must be made with more curvature than ones with long focal lengths. The surfaces of lenses and mirrors then must be polished to eliminate irregularities larger than the wavelengths of light

electromagnetic radiation

- Changing electric and magnetic fields that travel through space and transfer energy from one place to another—for example, light, radio waves, and the like. - light it is made up of both electric and magnetic fields - travels through space at a speed of 3*10^8 m/s (186,000 mi/s); this is commonly referred to as the speed of light, symbolized by the letter , but it is in fact the speed of all types of electromagnetic radiation.

- modern telescope mirrors

- Conventional primary mirrors are thick to prevent the optical surface from sagging and distorting the image as the telescope is moved around the sky. But large mirrors can weigh many tons, are difficult to support, and are expensive to make. Also, large mirrors take a long time to cool after nightfall. Changes in shape as the mirror cools down make the telescope difficult to focus and cause image distortions.

.X rays

- Electromagnetic radiation with short wavelengths, high frequencies, and high photon energies, between gamma-rays and ultraviolet radiation on the electromagnetic spectrum - shorter than UV rays

infrared (IR) radiation

- Electromagnetic radiation with wavelengths intermediate between visible light and radio waves. - Beyond the red end of the visible spectrum - wave lengths range from 700nm to 1mm (1 million nm) - skin senses it as heat - discovered in the year 1800, the first known example of "invisible light"

ultraviolet (UV)

- Electromagnetic radiation with wavelengths shorter than visible light but longer than X-rays. -electromagnetic waves shorter than violet

microwaves

- Electromagnetic wave with wavelength, frequency, and photon energy intermediate between infrared and radio waves. - Beyond the infrared part of the electromagnetic spectrum - used for cooking food in a microwave oven, as well as for radar and some long-distance telephone communications, have wavelengths from a few millimeters to a few centimeters

short wavelengths

- Extremely short-wavelength, high-frequency photons, such as X-rays and gamma-rays, have high energies and can be dangerous. Even ultraviolet photons have enough energy to harm you

lower-energy infrared photons

- Individually they have too little energy to affect skin pigment, a fact that explains why you can't get a tan from a heat lamp. Only by concentrating many low-energy photons in a small area, as in a microwave oven, can you transfer significant amounts of energy

reflecting telescope usage

- Light does not pass through any glass, so reflecting telescopes do not suffer from chromatic aberration. That color distortion has made reflecting telescopes the preferred form among modern astronomers. Also notice that the focused light in a reflecting telescope goes up the tube and back down to the focus. That makes the tube shorter than that of a similar refracting telescope, and among large telescopes, a short tube weighs significantly less and is easier to support without vibration. A short tube also means the observatory building and dome can be smaller. Another important advantage is that astronomers can make mirrors much bigger than lenses. That makes giant telescopes possible.

three important points about telescope design and ten new terms that describe optical telescopes and their operation:

1) Conventional-design reflecting telescopes use large, solid, heavy mirrors to focus starlight to a prime focus, or by using a secondary mirror, to a Cassegrain focus (pronounced KASS-uh-grain). Other telescopes have a Newtonian focus or a Schmidt-Cassegrain focus. Icon 2) Telescopes must have a sidereal drive to follow the stars. An equatorial mount with motion around a polar axis is the conventional way to provide that motion. Today, astronomers can build simpler, lighter-weight telescopes on alt-azimuth mounts that depend on computers to move the telescope so that it follows the apparent motion of stars as Earth rotates without having an equatorial mount and polar axis. Icon 3) Active optics, computer control of the shape of a telescope's main mirrors, allows the use of thin, lightweight mirrors—either "floppy" mirrors or segmented mirrors. Reducing the weight of the mirror reduces the weight of the rest of the telescope, making it stronger and less expensive. Also, thin mirrors cool and reach a stable shape faster at nightfall, producing better images during most of the night.

radio astronomer disadvantages

A radio astronomer works under two disadvantages relative to optical astronomers: poor resolution and low signal intensity. Recall that the resolving power of a telescope depends on the diameter of the primary lens or mirror but also on the wavelength of the radiation. At very long wavelengths like those of radio waves, the diffraction fringes are quite large. This means that images or maps from individual radio telescopes generally don't show such fine details as are seen in optical images. The second handicap radio astronomers face is the low intensity of the radio signals. You learned previously that the energy of a photon depends on its wavelength. Photons of radio energy have such long wavelengths that their individual energies are quite low. The cosmic radio signals arriving on Earth are astonishingly weak—as little as one-billionth the strength of the signal from a commercial radio station. To get detectable signals focused on the antenna, radio astronomers must build large collecting areas either as single large dishes or by combining arrays of smaller dishes. Even then, because the radio energy from celestial objects is so weak, it must be strongly amplified before it can be measured and recorded.

airborne telescopes

In addition to the atmospheric windows at visual and radio wavelengths you have already learned about, there are also a few narrow windows at short infrared wavelengths accessible from the ground, especially from high mountains such as Mauna Kea (Figure 6-13). However, most infrared wavelengths are blocked, especially by water vapor absorption. Also, Earth's atmosphere itself produces a strong infrared "glow." Observations at very long infrared wavelengths can only be made using telescopes carried to high altitudes by aircraft or balloons or launched entirely out of the atmosphere onboard spacecraft. (Notice that the reasons to put an infrared telescope above the atmosphere are not the same as the reasons to send an optical telescope into space.) Starting in the 1960s, NASA developed a series of infrared observatories with telescopes carried above Earth's atmospheric water vapor by jet aircraft. Such airborne observatories are also able to fly to remote parts of Earth to monitor astronomical events not observable by any other telescope. The modern successor to those earlier flying observatories is the Stratospheric Observatory for Infrared Astronomy, or SOFIA (Figure 6-18). SOFIA consists of a -m telescope looking out an opening with a rollback door in the left side of a modified Boeing aircraft

why you can't measure the diameters of stars by looking at them through a telescope

The diameter of the images in the telescope is set by diffraction and not by the actual diameters of the stars. That is, the stars are much smaller in diameter than the diameter of the diffraction images.

modern radio telescope

The dish reflector of a radio telescope, like the mirror of a reflecting telescope, collects and focuses radiation. Although a radio telescope's dish may be tens or hundreds of meters in diameter, the receiver antenna may be as small as your hand. Its function is to absorb the radio energy collected by the dish. Because radio wavelengths are in the range of a few millimeters to a few tens of meters, the dish only needs to be shaped to that level of accuracy, much less smooth than a good optical mirror. In fact, wire mesh works well as a mirror for all but the shortest-wavelength radio waves.

large gamma ray space telescope

The first large gamma-ray space telescope was the Compton Gamma Ray Observatory, launched in 1991. It mapped the entire sky at gamma-ray wavelengths. The European-built INTernational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite was launched in 2002 and has been very productive in the study of violent eruptions of stars and black holes. The Fermi Gamma-ray Space Telescope, launched in 2008 and operated by a consortium of nations led by the United States, is capable of making highly sensitive gamma-ray maps of large areas of the sky. Modern astronomy has come to depend on observations that cover the entire electromagnetic spectrum. More orbiting space telescopes are planned that will be even more versatile and sensitive than the ones operating now.

refracting telescopes problem

- suffer from a serious optical distortion that limits their use. When light is refracted through glass, shorter-wavelength light bends more than longer wavelengths; so, for example, blue light comes to a focus closer to the lens than does red light - if you focus the eyepiece on the blue image, the other colors are out of focus, and you see a colored blur around the image. If you focus instead on the red image, all the colors except red are blurred, and so on. This color separation is called chromatic aberration. - solution: can grind a telescope lens with two components made of different kinds of glass and thereby bring two different wavelengths to the same focus (Figure 6-7b). That improves the image, but these so-called achromatic lenses are not totally free of chromatic aberration. Even though two colors have been brought together, the others are still out of focus. - more expensive and difficult to build

The relationship among the wavelength, frequency, and speed of a wave

- wave length* frequency = speed of wave - The higher the frequency, the shorter the wavelength.

compare the relative light-gathering powers (LGP) of two telescopes A and B

- you can calculate the ratio of the areas of their primaries, which equals the ratio of the primaries' diameters squared - (LGPa)/(LGPb) = (Da/Db)^2 - Because the diameter ratio is squared, even a small increase in diameter produces a relatively large increase in light-gathering power and allows astronomers to study significantly fainter objects. This principle holds not just at visual wavelengths but also for telescopes collecting any kind of radiation.

space telescopes

- The most successful observatory in history, the Hubble Space Telescope (Figure 6-19a), is named after Edwin Hubble, the astronomer who discovered the expansion of the Universe. The Hubble telescope, also known as HST, was launched in 1990 and contains a -m mirror plus three instruments with which it can observe visible light plus some ultraviolet and infrared wavelengths. Its greatest advantage is the lack of seeing distortion, located completely above Earth's atmosphere. Hubble therefore can detect fine detail, and because it concentrates light into sharp images, it can detect extremely faint objects. It is controlled from a research center on Earth and observes almost continuously. Nevertheless, the telescope has time to complete only a fraction of the many projects proposed by astronomers from around the world. - The Hubble Space Telescope (HST) orbits Earth at an average altitude of km above the surface. In this image, the telescope is viewing toward the upper left. (b) Artist's conception of HST's eventual successor, the James Webb Space Telescope (JWST). JWST will be located in solar orbit almost million miles from Earth, four times as far away as the Moon. It will not have an enclosing tube, thus resembling a radio dish more than a conventional optical telescope. JWST will observe the Universe from behind a multilayered sunscreen larger than a tennis court. (c) Artist's conception of the Herschel infrared space telescope that carried a -m mirror and instruments cooled almost to absolute zero. - Hubble has been visited a number of times by the space shuttle so that astronauts could service its components and install new cameras and other instruments. Thanks to the work of the space shuttle crew who visited in 2009 and accomplished another refurbishment of the telescope's instruments, batteries, and gyroscopes, Hubble will almost certainly last until it can be replaced by the James Webb Space Telescope (JWST), which is expected to be ready in about the year 2018. JWST telescope will be launched into a solar orbit to avoid interference from Earth's strong infrared glow. Its primary mirror is a cluster of beryllium mirror segments that will open in space to form a -m mirror (Figure 6-19b). Telescopes carrying long-wavelength infrared detectors must carry coolant such as liquid helium to chill their optics to near absolute zero temperature ( or ) so that heat radiation from the insides of the telescope and instruments does not blind the detectors. Such observatories have limited lifetimes because the coolant eventually runs out. The European Space Agency's Herschel -meter infrared space telescope (Figure 6-19c), named after the scientist who discovered infrared radiation (Figure 6-4), was launched into solar orbit in 2009 together with the smaller Planck space observatory that studied millimeter-wavelength radiation. Herschel and Planck made important discoveries concerning distant galaxies, star formation, planets orbiting other stars, and the origin of the Universe during their -year lifetimes.

average wavelength of visible light

- .0005 mm - ranges from 400 to 700nm or 4000 to 7000 A - Light with wavelengths at the short-wavelength (400nm) end of the visible spectrum appears violet to your eyes, and light with wavelengths at the long-wavelength (700nm) end appears red

relationship between energy a photon carries and wavelength

- E = hc/wavelength - h is Planck's constant and = 6.63 *10^-34 joules - c = speed of light - The inverse proportion means that as wavelength gets smaller energy (E) gets larger: Shorter-wavelength photons carry more energy, and longer-wavelength photons carry less energy - short wavelength, high frequency, and large photon energy go together; long wavelength, low frequency, and small photon energy go together.

frequency of the wave

- The number of times a given event occurs in a given time; for a wave, the number of cycles that pass the observer in second. - represented by the Greek lowercase letter nu (ν). - If your favorite FM station is on the dial at 89.5 , that means the station's radio waves have a frequency v = 89.5 megahertz (c = 3.00 *10^8 m/s, wavelength = 3.35m)

reflecting telescope structure

- The primary mirrors of reflecting telescopes are much less expensive than lenses because the light reflects off the front surface of the mirror. This means that only the front surface needs to be made with a precise shape and that surface is coated with a highly reflective surface of aluminum or silver. Consequently, the glass of the mirror does not need to be transparent, and the mirror can be supported across its back surface to reduce sagging caused by its own weight. - Most important, reflecting telescopes do not suffer from chromatic aberration because the light does not pass through the glass, so reflection does not depend on wavelength.

sound

- a periodically repeating pressure disturbance that moves from source to ear. - requires a medium, meaning a substance such as air, water, or rock to travel through.

radiowaves

- a type of light (electromagnetic radiation) that your radio receiver transforms into sound so you can listen

stars in the Southern Hemisphere

- appear to circle around the south celestial pole that lies in the faint constellation of Octans (the Octant), not marked by any bright star.

Star twinkles

- are caused by turbulence in Earth's atmosphere, and a star near the horizon, where you look through more air, will twinkle and blur more than a star overhead.

electromagnetic radiation behavior

- can act as a wave phenomenon—that is, it is associated with a periodically repeating disturbance—a wave—that carries energy - can also behave as a stream of particles

2 most important powers of telescope

- light-gathering power and resolving power—depend on the diameter of the telescope that is essentially impossible to change - explains why astronomers describe telescopes by diameter and not by magnification

ground based telescopes

- limited by Earth's atmospheric turbulence and transparency - Most types of electromagnetic radiation arriving here from the Universe—gamma-rays, X-rays, ultraviolet, and much of the infrared—do not reach Earth's surface because they are partly or completely absorbed by Earth's atmosphere. To gather light with those blocked wavelengths, telescopes must go to high altitudes or into space

visible light

- makes up only a small part of the entire electromagnetic spectru

two most important factors of a telescope

- quality of the optics and the diameter of the primary lens or mirror.

which power of a large ground based optical telescope is severely limited by earths atmosphere on a cloudless night

- resolving power

The focal length

- the distance from the lens or mirror to the point where parallel rays of light from a very distant object come to a focus.

The Stratospheric Observatory for Infrared Astronomy (SOFIA),

a joint project of NASA and the German Aerospace Center (DLR), flies at altitudes up to km where it can collect infrared radiation with wavelengths that are unobservable even from high mountaintops. (b) A visual-wavelength image of the planet Jupiter (left) compared with a composite infrared image (right) using images at wavelengths of , , and microns made during SOFIA's "First Light" flight in 2010. The white stripe in the infrared image is a region of relatively transparent clouds through which the warm interior of the planet can be seen.

chromatic aberration

- A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by color fringes

photon

- A quantum of electromagnetic energy; carries an amount of energy that depends inversely on its wavelength. - A particle of electromagnetic radiation - The amount of energy a photon carries is inversely proportional to its wavelength

eyepiece

- A short-focal-length lens used to enlarge the image in a telescope; the lens nearest the eye.

Refracting telescopes

- A telescope that forms images by bending (refracting) light with a lens. - uses a primary lens to focus starlight into an image that is magnified by another lens called an eyepiece. The primary lens has a long focal length, and the eyepiece has a short focal length. - primary lens is main lens in a refracting telescope.

radio telescopes

- A telescope that gathers and focuses electromagnetic energy with microwave and radio wavelengths. - extremely difficult to make a lens that can focus radio waves, so all radio telescopes, including small ones, are reflecting telescopes; the dish is the primary mirror - affected by atmospheric seeing, but less than optical telescopes, so they do not benefit much in this respect by being located on mountains - don't have eyepieces, but they do have instruments that examine the radio waves focused by the telescope, and each such instrument would, in effect, have its own magnifying power.

nanometer (nm)

- A unit of length equal to 10^-9 m - used to measure wavelength of light

test resolving power

- measuring the angular distance between two stars that are just barely distinguishable as separate objects - The resolving power in arc seconds of a telescope with primary diameter D that is collecting light of wavelength equals: resolving power =(the conversion between radians and arc seconds) 2.06 *10^5 (wave legth/D) - resolving power (arc seconds) = .113/D

Earths atmosphere

- opaque to most electromagnetic radiation - Gamma-rays and X-rays are absorbed high in Earth's atmosphere, and a layer of ozone at altitudes of about 15 to 30 km ( 10 to 20 mi) absorbs most ultraviolet radiation - Water vapor in the lower atmosphere absorbs most long-wavelength infrared radiation and microwaves. Only visible light, some short-wavelength infrared radiation, and some radio waves reach Earth's surface through wavelength bands called atmospheric windows.

A ground-based telescope

normally operated by astronomers and technicians working in a control room in the same building, but some telescopes are now used by astronomers many miles, even thousands of miles, from the observatory.

High energy astronomy

- Like infrared-emitting objects, gamma-ray, X-ray, and ultraviolet sources in the Universe are difficult to observe because the telescopes must be located high in Earth's atmosphere or in space. Also, high-energy photons are difficult to bring to a focus. The first high-energy astronomy satellite, Ariel 1, was launched by the United Kingdom in 1962 and made solar observations in the ultraviolet and X-ray segments of the electromagnetic spectrum. Since then, many more space telescopes have followed Ariel's lead. Some high-energy astronomy satellites such as XMM-Newton, an X-ray observatory developed by a consortium of European and British astronomers, have been general-purpose telescopes that observe many different kinds of objects. In contrast, some space telescopes are designed to study a single question or a single object. For example, the Japanese satellite Hinode (pronounced, hee-no-day) studies the Sun continuously at visual, ultraviolet, and X-ray wavelengths, and the Kepler space observatory operated for years detecting planets orbiting stars other than the Sun. The largest X-ray telescope to date is the Chandra X-ray Observatory (CXO). Chandra operates in an orbit that extends a third of the way to the Moon so that it spends percent of the time above the belts of charged particles surrounding Earth that would produce electronic noise in its detectors. (Chandra is named for the late Indian American Nobel laureate Subrahmanyan Chandrasekhar, who was a pioneer in many branches of theoretical astronomy.) Focusing X-rays is difficult because they penetrate into most mirrors, so astronomers devised cylindrical mirrors in which the X-rays reflect at shallow angles from the polished inside of the cylinders to form images on X-ray detectors, as shown in Figure 6-20. The Chandra observatory has made important discoveries about everything from star formation to monster black holes in distant galaxies that will be described in later chapters. - X-rays that hit a mirror at grazing angles are reflected like a pebble skipping across a pond. Thus, X-ray telescope mirrors like the ones in Chandra are shaped like barrels rather than dishes.

magnifying power

- M = (Fp/Fe) - The magnifying power of a telescope equals the focal length of the primary mirror or lens divided by the focal length of the eyepiece.

modern telescopes

- Other telescopes are fully automated and operate without direct human supervision. That, plus continuous improvement in computer speed and storage capacity, has made possible huge surveys of the sky in which millions of objects have been observed or are planned for observation. For example, the Sloan Digital Sky Survey (SDSS) mapped the entire Northern Hemisphere sky, measuring the position and brightness of million stars and galaxies at five ultraviolet, optical, and infrared wavelengths.

For most of the th century, astronomers faced a serious limitation on the size of astronomical telescopes

- Telescope mirrors were made thick to avoid bending that would distort the reflecting surface, but those thick mirrors were heavy

Light-gathering power

- The ability of a telescope to collect light; proportional to the area of the telescope's objective lens or mirror. - A large-diameter telescope gathers more light and produces a brighter image than a smaller telescope of the same focal length. - proportional to the area of the telescope primary lens or mirror; a lens or mirror with a large area gathers a large amount of light. The area of a circular lens or mirror written in terms of its diameter D is (pi* d^2)/4 .

resolving power

- The ability of a telescope to reveal fine detail; depends on the diameter of the telescope objective. - Aside from diffraction, two other factors—optical quality and atmospheric conditions—limit resolving power. A telescope must have high-quality optics to achieve its full potential resolving power. Even a large telescope reveals little detail if its optical surfaces are marred by imperfections.

light pollution .

- The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects

primary mirror

- The main optical element in an astronomical telescope. The large lens at the top of the telescope tube or the large mirror at the bottom.

light

- The wavelike properties of light produce a rainbow, whereas the particle-like properties are involved in the operation of a digital camera. - used to refer to electromagnetic radiation that humans can see, but visible light is only one among many types of electromagnetic radiation that include X-rays and radio waves - Most forms of light (electromagnetic radiation) are absorbed in Earth's atmosphere. Light can reach Earth's surface only through the visual and radio "windows."

atmospheric windows

- Wavelength region in which Earth's atmosphere is transparent—at visual, infrared, and radio wavelengths.

telescope resolution

- ability to reveal fine detail, depends on the quality of the optics, but it also depends on the diameter of the telescope. Larger telescopes produce smaller diffraction fringes and sharper images. The resolving power of a telescope is the angular separation between two stars that are just barely visible through the telescope as separate images. For telescopes focusing visible light, the resolving power in arc seconds equals 0.113 divided by the diameter of the telescope in meters.

find sites with the best seeing,

- astronomers carefully select mountains where the airflow is measured to be smooth and not turbulent. Also, the air at high altitude is thin, dry, and more transparent, which is important not only for optical telescopes but also for other types of telescopes.

Radio astronomers

- face a problem of radio interference comparable to visible light pollution. Weak radio waves from the cosmos are easily drowned out by human-made radio noise—everything from automobiles with faulty spark plugs to poorly designed communication systems. A few narrow radio bands are reserved for astronomy research, but even those are often contaminated by stray signals. To avoid that noise and have the radio equivalent of a dark sky, astronomers locate radio telescopes as far from civilization as possible. Hidden in mountain valleys or in remote deserts, they are able to study the Universe protected from humanity's radio output.

Galileo

- first person to systematically record observations of celestial objects using a telescope, beginning a little more than years ago in 1610. - Galileo's telescope was a refractor.

modern telescopes

The four telescopes of the European Very Large Telescope (VLT) are housed in separate domes at Paranal Observatory in Chile. (b) The Large Binocular Telescope (LBT) in Arizona carries two -m mirrors. The light gathered by the two mirrors can be analyzed separately or combined. The entire building rotates as the telescope moves. (c) The Gran Telescopio Canarias (GTC) on La Palma in the Canary Islands contains hexagonal mirror segments in its -m primary mirror. - Other giant telescopes are being planned for completion in the 2020s, all with segmented or multiple mirrors (Figure 6-15). The Giant Magellan Telescope (GMT) will carry seven asymmetrically curved thin mirrors, each m in diameter, on a single mounting. It will be located in Chile and have the light-gathering power of a single -m telescope. The Thirty Meter Telescope (TMT), now under development by a consortium of countries, including the United States, Canada, Japan, China, and India, is planned to have a mirror up to m in diameter comprised of hexagonal segments and will be placed on Mauna Kea in Hawai'i. An international team is designing the European Extremely Large Telescope (E-ELT) to carry segments, making up a mirror m (nearly ft) in diameter. The E-ELT will be built on Cerro Armazones, a mountain in Chile's Atacama Desert.

largest radio dish

The largest single radio dish in the world at the time of this writing (mid-2014) is m in diameter. Such a large dish can't be supported easily, so it is built into a mountain valley in Arecibo, Puerto Rico. The primary mirror is a thin metallic surface supported above the valley floor by cables attached near the rim, and the antenna platform hangs above the dish on cables from towers built on three mountain peaks around the valley's rim (Figure 6-17). By moving the antenna above the dish, radio astronomers can point the telescope at any object that passes within degrees of the zenith as Earth rotates

primary lens

The main lens in a refracting telescope.

adaptive objects

You have already learned about active optics, which is a technique to adjust the shape of telescope optics slowly, compensating for effects of changing temperature as well as gravity bending the mirror when the telescope points at different locations in the sky. Adaptive optics is a more sophisticated technique that uses high-speed computers to monitor the distortion produced by turbulence in Earth's atmosphere and rapidly alter some optical components to correct the telescope image, sharpening a fuzzy blob into a crisp picture. The resolution of the image is still limited by diffraction in the telescope, but removing much of the seeing distortion produces a dramatic improvement in the detail that is visible - (a) In these images of the center of our galaxy, the adaptive optics system was turned "Off" for the left image and "On" for the right image. In the "On" image, the images of stars are sharper because the light is focused into smaller images; fainter stars are visible. (b) The laser beam shown leaving one of the Keck telescopes produces an artificial star in the field of view, and the adaptive optics system uses that laser guide star as a reference to reduce seeing distortion in the entire image.

The future Large Synoptic Survey Telescope (LSST)

has an -m primary mirror already completed; construction of facilities on Cerro Pachón in Chile began in 2014 (Figure 6-16). Using a -billion-pixel charge-coupled device (CCD) camera, LSST will be able to record the brightness at selected ultraviolet, visual, and infrared wavelengths of every object in one hemisphere of the sky brighter than magnitude every three nights. Astronomers and private citizens will be studying those data for decades to come.