Telescopes

Describe the four parts and order of a radio telescopes operation(s):

1. A dish reflector 2. antenna 3. amplifier 4. recorder

How much does the 5-m mirror weigh on Mount Palomar?

14.5 tons.

Hinode

A Japanese satellite that studies the sun continuously at visual, ultraviolet, and X-ray wavelengths.

What fills the space between a star(s)?

A glowing fog of cosmic rays.

Limitations of a Radio Telescope

A radio astronomer works under three handicaps: poor resolution, low intensity, and interference. Recall that the resolving power of an optical telescope depends on the diameter of the objective lens or mirror. It also depends on the wavelength of the radiation because, at very long wavelengths like those of radio waves, the diffraction fringes are very large. That means a radio map can't show fine detail. As with an optical telescope, there is no way to improve the resolving power without building a bigger telescope. Consequently, radio telescopes generally have large diameters to minimize the diffraction fringes. Even so, the resolving power of a radio telescope is not good. A dish 30 m in diameter receiving radiation with a wavelength of 21 cm has a resolving power of about 0.5°. Such a radio telescope would be unable to detect any details in the sky smaller than the moon. Fortunately, radio astronomers can combine two or more radio telescopes to form a radio interferometer capable of much higher resolution. Just as in the case of optical interferometers, the radio astronomer combines signals from two or more widely separated dishes and "fools the waves" into behaving as if they were collected by a much bigger radio telescope. Radio interferometers can be quite large. The Very Large Array (VLA) consists of 27 dish antennas spread in a Y-shape across the New Mexico desert. In combination, they have the resolving power of a radio telescope 36 km (22 mi) in diameter. The VLA can resolve details smaller than 1 arc second. Eight new dish antennas being added across New Mexico will give the VLA ten times better resolving power. A typical weather map uses contours with added color to show which areas are likely to receive precipitation. A false-color-image radio map of Tycho's supernova remnant, the expanding shell of gas produced by the explosion of a star in 1572. The radio contour map has been color-coded to show intensity. The Very Large Array uses 27 radio dishes, which can be moved to different positions along a Y-shaped set of tracks across the New Mexico desert. They are shown here in the most compact arrangement. Signals from the dishes are combined to create very-high- resolution radio maps of celestial objects. The proposed Square Kilometer Array will have a concentration of detectors and radio dishes near its center with more dishes scattered up to 3000 km away. (Xilostudios/SKA Program Development Office) Another large radio interferometer, the Very Long Baseline Array (VLBA), consists of matched radio dishes spread from Hawaii to the Virgin Islands and has an effective diameter almost as large as Earth. The Allen Telescope Array being built in California will eventually include 350 separate radio dishes. Radio astronomers are now planning the Square Kilometer Array, which will contain a huge number of radio dishes totaling a square kilometer of collecting area and spread to a diameter of at least 6000 kilometers. These giant radio interferometers depend on state-of-the-art, high-speed computers to combine signals and create radio maps. The second handicap radio astronomers face is the low intensity of the radio signals. You learned earlier 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. To get detectable signals focused on the antenna, the radio astronomer must build large collecting areas either as single large dishes or arrays of smaller dishes.

European Southern Observatory (ESO)

has built the Very Large Telescope (VLT) high in the remote Andes Mountains of northern Chile. The VLT actually consists of four telescopes, each with a computer-controlled mirror 8.2 m in diameter and only 17.5 cm (6.9 in.) thick. The four telescopes can work singly or can combine their light to work as one large telescope. Italian and American astronomers have built the Large Binocular Telescope, which carries a pair of 8.4-m mirrors on a single mounting. The Gran Telescopio Canarias, located atop a volcanic peak in the Canary Islands, carries a segmented mirror 10.4 meters in diameter and holds, for the moment, the record as the largest single telescope in the world.

The Two-Micron All Sky Survey

has mapped the entire sky at three wavelengths in the infrared.

Computer Memory

An electronic memory device. The temporary, internal storage within a computer. Any physical device capable of storing information temporarily or permanently: Because the radio energy from celestial objects is so weak, it must be strongly amplified before it can be measured and its strength is recorded in ______________?

Spectrograph

An instrument that separates light into colors and makes an image of the resulting spectrum. a device that breaks the light into colors and produces an image of the resulting spectrum. An instrument that separates light into colors and photographs the resulting spectrum.

The Powers of a Telescope

Astronomers build large telescopes because a telescope can aid your eyes in three ways—the three powers of a telescope—and the two most important of these powers depends on the diameter of the telescope. Nearly all of the interesting objects in the sky are faint sources of light, so astronomers need telescopes that can gather large amounts of light to produce bright images. Light-gathering power refers to the ability of a telescope to collect light. Catching light in a telescope is like catching rain in a bucket—the bigger the bucket, the more rain it catches. Light gathering power (LGP) is proportional to the area of the telescope objective. A lens or mirror with a large area gathers a large amount of light. Remember that the area of a circle is pi times the radius squared. To compare the relative light-gathering powers (LGP) of two telescopes A and B, you can calculate the ratio of the areas of their objectives, which reduces to the ratio of their diameters squared. For example, suppose you compared a telescope 24 cm in diameter with a telescope 4 cm in diameter. The ratio of the diameters is 24/4, or 6, but the larger telescope does not gather six times as much light. Light-gathering power increases as the ratio of diameters squared, so it gathers 36 times more light than the smaller telescope. This example shows the importance of diameter in astronomical telescopes. Even a small increase in diameter produces a large increase in light-gathering power and allows astronomers to study much fainter objects. (a) Stars are so far away that their images are points, but the wave nature of light surrounds each star image 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. (Computer model by M. A. Seeds) The second power, resolving power, refers to the ability of the telescope to reveal fine detail. Because light acts as a wave, it produces a small diffraction fringe around every point of light in the image, and you cannot see any detail smaller than the fringe. Astronomers can't eliminate diffraction fringes, but the larger a telescope is in diameter, the smaller the diffraction fringes are. That means the larger the telescope, the better its resolving power. If you consider only optical telescopes, you can estimate the resolving power by calculating the angular distance between two stars that are just barely visible through the telescope as two separate images. Astronomers say the two images are "resolved," meaning they are separated from each other. The resolving power, alpha , in arc seconds, equals 11.6 divided by the diameter of the telescope in centimeters. For example, the resolving power of a 25.0 cm telescope is 11.6 divided by 25.0, or 0.46 arc seconds. No matter how perfect the telescope optics, this is the smallest detail you can see through that telescope. This calculation gives you the best possible resolving power of a telescope of diameter d, but the actual resolution can be limited by two other factors—lens quality and atmospheric conditions. The left half of this photograph of a galaxy is from an image recorded on a night of poor seeing. Small details are blurred. The right half of the photo is from an image recorded on a night when Earth's atmosphere above the telescope was steady and the seeing was better. Much more detail is visible under good seeing conditions. (Courtesy William Keel) A telescope must contain high-quality optics to achieve its full potential resolving power. Even a large telescope reveals little detail if its optics are marred with imperfections. Also, when you look through a telescope, you are looking up through miles of turbulent air in Earth's atmosphere, which makes the image dance and blur, a condition called seeing. A related phenomenon is the twinkling of stars. The 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. On a night when the atmosphere is unsteady, the images are blurred, and the seeing is bad. Even under good seeing conditions, the detail visible through a large telescope is limited, not by its diffraction fringes but by the air through which the telescope must look. A telescope performs better on a high mountaintop where the air is thin and steady, but even there Earth's atmosphere limits the detail the best telescopes can reveal to about 0.5 arc seconds. You will learn later about telescopes that orbit above Earth's atmosphere and are not limited by seeing. Seeing and diffraction limit the amount of information in an image, and that limits the accuracy of any measurement made based on that image. Have you ever tried to magnify a newspaper photo to distinguish some detail? Newspaper photos are made up of tiny dots of ink, and no detail smaller than a single dot will be visible no matter how much you magnify the photo. In an astronomical image, the resolution is often limited by seeing. You can't see a detail in the image that is smaller than the resolution. That's why stars look like fuzzy points of light no matter how big your telescope. All measurements have some built-in uncertainty, and scientists must learn to work within those limitations.

What is one fact about radio telescopes and dust in space?

Astronomers observing at visual wavelengths can't see through the dusty clouds in space. Light waves are so short that they are scattered by the tiny dust grains and never get through the dust to reach optical telescopes on Earth. However, radio signals have wavelengths much longer than the diameters of dust grains, so radio waves from far across the galaxy pass unhindered, giving radio astronomers an unobscured view. (The same is also true, to a large extent, of infrared radiation.)

How to Calculate the Resolving Power

Astronomers say the two images are "resolved," meaning they are separated from each other. The resolving power, alpha , in arc seconds, equals 11.6 divided by the diameter of the telescope in centimeters.

Why did astronomers think it was useless to build bigger telescopes on the Earth's surface? How did they dodge this question?

Because of seeing distortion caused by the atmosphere. In the 1990s, computers became fast enough to allow astronomers to correct for some distortion, and that has made the new generation of giant telescopes possible.

The Seeing Details

Because the amount of detail you can see is limited by the seeing conditions and the resolving power, very high magnification does not necessarily show more detail. You can change the magnification by changing the eyepiece, but you cannot alter the telescope's light-gathering power or resolving power without changing the diameter of the objective lens or mirror, and that would be so expensive that you might as well build a whole new telescope.

Do thin mirrors cool faster or slower at night time? do they produce better images?

Faster; Yes

An Example of Calculating a Telescopes Magnification

For example, if a telescope has an objective with a focal length of 80 cm and you use an eyepiece whose focal length is 0.5 cm, the magnification is 80/0.5, or 160 times.

Magnification = FocalLengthObjective / FocalLengthEyepiece

For example, if a telescope has an objective with a focal length of 80 cm and you use an eyepiece whose focal length is 0.5 cm, the magnification is 80/0.5, or 160 times. Notice that the two most important powers of the telescope, light-gathering power and resolving power, depend on the diameter of the telescope. This explains why astronomers refer to telescopes by diameter and not by magnification. Astronomers will refer to a telescope as an 8-meter telescope or a 10-meter telescope, but they would never identify a research telescope as being a 200-power telescope. The quest for light-gathering power and high resolution explains why nearly all major observatories are located far from big cities and usually on high mountains. Astronomers avoid cities because light pollution, the brightening of the night sky by light scattered from artificial outdoor lighting, can make it impossible to see faint objects. In fact, many residents of cities are unfamiliar with the beauty of the night sky because they can see only the brightest stars. Even far from cities, nature's own light pollution, the moon, is sometimes so bright it drowns out fainter objects, and astronomers are often unable to observe on the nights near full moon. On such nights, faint objects cannot be detected even with the largest telescopes on high mountains. Astronomers prefer to place their telescopes on carefully selected high mountains for a number of reasons. The air there is thin, very dry, and more transparent, especially in the infrared. For the best seeing, astronomers select mountains where the air flows smoothly and is not turbulent. Building an observatory on top of a high mountain far from civilization is difficult and expensive, as you can imagine from the photo in Figure 6-10b, but the dark sky and steady seeing make it worth the effort.

An Example of the LGP formula

For example, suppose you compared a telescope 24 cm in diameter with a telescope 4 cm in diameter. The ratio of the diameters is 24/4, or 6, but the larger telescope does not gather six times as much light. Light-gathering power increases as the ratio of diameters squared, so it gathers 36 times more light than the smaller telescope. This example shows the importance of diameter in astronomical telescopes. Even a small increase in diameter produces a large increase in light-gathering power and allows astronomers to study much fainter objects.

An Example of the Resolving Power Formula

For example, the resolving power of a 25.0 cm telescope is 11.6 divided by 25.0, or 0.46 arc seconds. No matter how perfect the telescope optics, this is the smallest detail you can see through that telescope. This calculation gives you the best possible resolving power of a telescope of diameter d, but the actual resolution can be limited by two other factors—lens quality and atmospheric conditions.

New Generation Telescopes

For most of the 20th century, astronomers faced a serious limitation on the size of astronomical telescopes. Traditional telescope mirrors were made thick to avoid sagging that would distort the reflecting surface, but those thick mirrors were heavy. The 5-m (200-in.) mirror on Mount Palomar weighs 14.5 tons. These traditional telescopes were big, heavy, and expensive.

Are Optical (lower-wavelength) interferometers easier or harder to build?

Harder: Recall that the wavelength of light is very short, roughly 0.0005 mm, so building optical interferometers is one of the most difficult technical problems that astronomers face.

Turbulence

In the Earth's atmosphere, it constantly distorts the light, and high-speed computers must continuously adjust the light paths.

Stratospheric Observatory for Infrared Astronomy (SOFIA)

Infrared telescopes have flown to high altitudes under balloons and in airplanes to get above absorption by water vapor. A Boeing 747SP that will carry a 2.5-m telescope, control systems, and a team of astronomers, technicians, and educators into the dry fringes of the atmosphere. Once at that altitude, they can open a door above the telescope and make infrared observations for hours as the plane flies a precisely calculated path.

What can interferometers do to turbulence?

It can be partially averaged out.

How does new equipment get installed on the Hubble Telescope?

It has been visited a number of times by the space shuttle so that astronauts can maintain their equipment and install new cameras and spectrographs.

What happens if you reduce the weight of the telescope mirror?

It reduces the weight of the rest of the telescope and makes it stronger and less expensive.

Are scientists sure where cosmic rays come from

No: This area of astronomical research is just beginning to bear fruit. Astronomers can't be sure what produces cosmic rays. At present, cosmic rays largely remain an exciting mystery.

Do cosmic rays reach the ground?

No: They don't reach the ground, but they do smash gas atoms in the upper atmosphere, and fragments of those atoms shower down on you day and night over your entire life. These secondary cosmic rays are passing through you as you read this sentence.

What has modern astronomy depended on when observing with telescopes?

Observations that cover the entire electromagnetic spectrum. More orbiting space telescopes are planned that will be more versatile and more sensitive.

Why are there lasers on newer telescopes?

astronomers can point a laser at a spot in the sky very close to their target object, and where the laser excites gas in Earth's upper atmosphere, it produces a glowing artificial star in the field of view. The adaptive optics system can use the artificial star to correct the image of the fainter target. Today astronomers are planning huge optical telescopes composed of segmented mirrors tens of meters in diameter. Those telescopes would be almost useless without adaptive optics.

How would you describe these traditional telescopes? big, heavy, and expensive.

big, heavy, and expensive

How are Ultraviolet Waves Absorbed?

by the ozone layer extending from about 15 km to 30 km above the Earth's surface: No mountaintop is that high, and no airplane can fly to such an altitude. To observe in the far-ultraviolet or beyond at X-ray or gamma-ray wavelengths, telescopes must be in space above the atmosphere.

CHARA Array: Mt. Wilson

combines six 1-meter telescopes to create the resolving power equivalent of a telescope one-fifth of a mile in diameter.

Very Large Array (VLA)

consists of 27 dish antennas spread in a Y-shape across the New Mexico desert. In combination, they have the resolving power of a radio telescope 36 km (22 mi) in diameter. This can resolve details smaller than 1 arc second. Eight new dish antennas being added across New Mexico will give this ten times better resolving power.

Very Large Telescope (VLT)

consists of four telescopes, each with a computer-controlled mirror 8.2 m in diameter and only 17.5 cm (6.9 in.) thick. The four telescopes can work singly or can combine their light to work as one large telescope. The four telescopes of this telescope are housed in separate domes at Paranal Observatory in Chile. It consists of four 8.2-m telescopes that can operate separately but can also be linked together through underground tunnels with three 1.8-m telescopes on the same mountaintop. The resulting optical interferometer provides the resolution (but, of course, not the light-gathering power) of a telescope 200 meters in diameter.

altazimuth mountings

depend on computers to move the telescope so that it follows the westward motion of the stars as Earth rotates.

Active Optics

depend on computers to move the telescope so that it follows the westward motion of the stars as Earth rotates. control mirrors based on temperature and orientation. Collection of techniques used to increase the resolution of ground-based telescopes. Minute modifications are made to the overall configuration of an instrument as its temperature and orientation change; used to maintain the best possible focus at all times: computer control of the shape of telescope mirrors, allows the use of thin, lightweight mirrors—either 'floppy' mirrors or segmented mirrors.

How to Calculate the Telescope Magnification

dividing the focal length of the objective by the focal length of the eyepiece

Very Long Baseline Array (VLBA)

is a set of radio telescopes linked together electronically to provide very high resolution. consists of matched radio dishes spread from Hawaii to the Virgin Islands and has an effective diameter almost as large as Earth. The Allen Telescope Array being built in California will eventually include 350 separate radio dishes. Radio astronomers are now planning the Square Kilometer Array, which will contain a huge number of radio dishes totaling a square kilometer of collecting area and spread to a diameter of at least 6000 kilometers. These giant radio interferometers depend on state-of-the-art, highspeed computers to combine signals and create radio maps.

How Can Astronomers Observe Ultraviolet Waves and Under?

it can be recorded by specialized detectors: Beyond the other end of the visible spectrum, astronomers can observe in the near-ultraviolet at wavelengths of about 290 to 400 nm. Your eyes don't detect this radiation, but it can be recorded by specialized detectors. Wavelengths shorter than about 290 nm, the far-ultraviolet, are completely absorbed by the ozone layer extending from about 15 km to 30 km above Earth's surface. No mountaintop is that high, and no airplane can fly to such an altitude. To observe in the far-ultraviolet or beyond at X-ray or gamma-ray wavelengths, telescopes must be in space above the atmosphere.

What do traditional telescopes use?

large, solid, heavy mirrors to focus starlight to a prime focus, or, by using a secondary mirror, to a Cassegrain focus.

What limits the detail you can see in an image?

limited resolution: Example: You see this on your computer screen because images there are made up of picture elements, pixels. If your screen has large pixels, the resolution is low, and you can't see much detail. In an astronomical image, the size of a picture element is set by seeing and by diffraction in the telescope. You can't see detail smaller than that resolution limit.

Gran Telescopio Canarias (GTC)

located atop a volcanic peak in the Canary Islands, carries a segmented mirror 10.4 meters in diameter and holds, for the moment, the record as the largest single telescope in the world. contains 36 hexagonal mirror segments in its 10.4-m primary mirror.

What was used in the past instead of CCDs?

measurements of intensity and color were made using specialized light meters attached to a telescope or on photographic plates. Today, nearly all such measurements are made more easily and more accurately with CCD images.

Is the resolving power of radio telescope good?

not good: A dish 30 m in diameter receiving radiation with a wavelength of 21 cm has a resolving power of about 0.5°. Such a radio telescope would be unable to detect any details in the sky smaller than the moon. Fortunately, radio astronomers can combine two or more radio telescopes to form a radio interferometer capable of much higher resolution. Just as in the case of optical interferometers, the radio astronomer combines signals from two or more widely separated dishes and "fools the waves" into behaving as if they were collected by a much bigger radio telescope.

Thirty Meter Telescope (TMT)

now under development by American astronomers, will have a mirror 30 meters in diameter, composed of 492 hexagonal segments. The European Extremely Large Telescope is being planned by an international team. It will carry 906 segments, making up a mirror 42 meters in diameter.

Where do Astronomers Prefer to Place their Telescopes?

on carefully selected high mountains: The air there is thin, very dry, and more transparent, especially in the infrared. For the best seeing, astronomers select mountains where the air flows smoothly and is not turbulent. Building an observatory on top of a high mountain far from civilization is difficult and expensive

What are the two atmospheric windows the Earth has?

optical telescopes and radio telescopes

Interferometry

process that links separate telescopes so they act as one telescope, producing more detailed images as the distance between them increases. Process of linking separate radio telescopes to act as one. A technique that uses the images from several telescopes to produce a single image: astronomers have been able to achieve very high resolution by connecting multiple telescopes together to work as if they were a single telescope. This method of synthesizing a larger telescope is known as?

Light gathering power (LGP)

proportional to the area of the telescope objective. A lens or mirror with a large area gathers a large amount of light. Remember that the area of a circle is pi times the radius squared.

What do radio astronomers do to avoid interference with radio telescopes?

radio astronomers locate their telescopes as far from civilization as possible. Hidden deep in mountain valleys, they are able to listen to the sky protected from human-made radio noise.

Two Kinds of Optical Telescopes

reflecting and refracting: Optical telescopes can focus light into an image by using either a lens or a mirror. In a refracting telescope, the primary (or objective) lens bends (refracts) the light as it passes through the glass and brings it to a focus to form a small inverted image. In a reflecting telescope, the primary (or objective) mirror—a concave piece of glass with a reflective surface—forms an image by reflecting the light.

How have Adaptive Optics affected resolution?

still limited by diffraction in the telescope, but removing much of the seeing distortion produces a dramatic improvement in the detail visible.

Magnifying Power

the ability of a telescope to make an image larger. how much it magnifies. Each microscope lens is inscribed with a number signifying its ______: its ability to make the image bigger is the least important of the three powers.

What does a radio telescope measure?

the amount of radio energy coming from a specific point on the sky: So the radio telescope must be scanned over an object to produce a map of the radio intensity at different points. Such radio maps are usually represented using contours to mark areas of similar radio intensity, much like a weather map where contours filled with color indicate areas of precipitation.

focal length

the distance from the center of a lens to the focal point. the distance from a lens to its focus. the distance between the center of a lens or curved mirror and its focus.

Low Intensity

the energy of a photon depends on its wavelength. Photons of radio energy have such long wavelengths that their individual energies are quite low. To get detectable signals focused on the antenna, the radio astronomer must build large collecting areas either as single large dishes or arrays of smaller dishes.

Photographic Plate

the first device used by astronomers to record images of celestial objects. Fogging of a photographic emulsion viewed under a high-power microscope shows beta and gamma rays causing silver bromide grains to develop in a scattered fashion. In this, you can see the effect of radiation and even track an alpha particle's range with special ones for beta as well. The discovery of radioactivity by Henri Bequerel involved a... The original imaging device in astronomy was the_________? It could record images of faint objects in long time exposures and could be stored for later analysis. But they have been almost entirely replaced by electronic imaging systems.

chromatic aberration

the focusing of different colors of light at different distances behind a lens. the property of a lens whereby light of different colors is focused at different places a spherical lens defect in which light passing through a lens is focused at different points, causing an object viewed through a lens to seem to be ringed with color: Telescope designers can grind a telescope lens of two components made of different kinds of glass and so bring two different wavelengths to the same focus.

What Would Happen if the Atmosphere were Unsteady?

the images are blurred, and the seeing is bad. Even under good seeing conditions, the detail visible through a large telescope is limited, not by its diffraction fringes but by the air through which the telescope must look. A telescope performs better on a high mountaintop where the air is thin and steady, but even there Earth's atmosphere limits the detail the best telescopes can reveal to about 0.5 arc seconds. You will learn later about telescopes that orbit above Earth's atmosphere and are not limited by seeing.

In SOFIA's 747, How Does the NASA Team Reduce Noise?

the light-sensitive detectors in astronomical telescopes are cooled to very low temperatures, usually, with liquid nitrogen: This is especially necessary for a telescope observing at infrared wavelengths. Infrared radiation is emitted by heated objects, and if the telescope is warm it will emit many times more infrared radiation than that coming from a distant object. Imagine trying to look for something at night through binoculars that are themselves glowing.

This limitation on the detail in an image is related to?

the limited precision of any measurement: Example: Imagine a zoologist trying to measure the length of a live snake by holding it along a meter stick. The wriggling snake is hard to hold, so it is hard to measure accurately. Also, meter sticks are usually not marked finer than millimeters. Both factors limit the precision of the measurement. If the zoologist said the snake was 43.28932 cm long, you might be suspicious. The resolution of the measurement technique does not justify the accuracy implied by all those digits.

Where are cosmic rays deflected?

the magnetic fields spread through our galaxy: This means astronomers can't tell where their original sources are located.

Common Misconception

the purpose of an astronomical telescope is to magnify the image. In fact, the magnifying power of a telescope, its ability to make the image bigger, is the least important of the three powers.

What does High Energy Astrophysics refer to?

the use of X-ray and gamma ray observations of the sky. Making such observations is difficult but can reveal the secrets of processes such as the explosive deaths of massive stars and eruptions of supermassive black holes.

What plans are being made to put interferometers in space?

to avoid atmospheric turbulence altogether. The Space Interferometry Mission, for example, will work at visual wavelengths and study everything from the cores of erupting galaxies to planets orbiting nearby stars.

Why were traditional telescope mirrors made heavier and thicker?

to avoid sagging that would distort the reflecting surface. These mirrors were heavy.

What have higher speed computers allowed astronomers to do?

to build new, giant telescopes with unique designs.

Why do current Astronomers build bigger telescopes?

to increase resolving power, and astronomers have been able to achieve very high resolution by connecting multiple telescopes together to work as if they were a single telescope.

Radio Maps

usually represented using contours to mark areas of similar radio intensity, much like a weather map where contours filled with color indicate areas of precipitation.

What shouldn't you confuse Adaptive Optics with?

with the slower-speed active optics that controls the overall shape of a telescope mirror.

The Operation of a Radio Telescope

A radio telescope usually consists of four parts: a dish reflector, an antenna, an amplifier, and a recorder. These components, working together, make it possible for astronomers to detect radio radiation from celestial objects. The dish reflector of a radio telescope, like the mirror of a reflecting telescope, collects and focuses radiation. Because radio waves are much longer than light waves, the dish need not be as smooth as a mirror; wire mesh will reflect all but the shortest wavelength radio waves. Though a radio telescope's dish may be many meters in diameter, the antenna may be as small as your hand. Like the antenna on a TV set, its only function is to absorb the radio energy collected by the dish. Because the radio energy from celestial objects is so weak, it must be strongly amplified before it can be measured and its strength is recorded in computer memory. Radio telescopes do not produce images. A single observation with a radio telescope measures the amount of radio energy coming from a specific point on the sky. So the radio telescope must be scanned over an object to produce a map of the radio intensity at different points. Such radio maps are usually represented using contours to mark areas of similar radio intensity, much like a weather map where contours filled with color indicate areas of precipitation.

What Could Relate to Seeing?

A related phenomenon is the twinkling of stars. The 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.

Prism

A solid figure that has two congruent, parallel polygons as its bases. Its sides are parallelograms. A solid geometric figure whose two ends are similar, equal, and parallel rectilinear figures, and whose sides are parallelograms. A piece of glass that separates white light into colors of the spectrum: When Issac Newton placed a prism in the beam, it spread the light into a beautiful spectrum that splashed across his bedroom wall. From this and related experiments Newton concluded that white light is made of a mixture of all the colors.

The Hubble Space Telescope

A space telescope and camera named for a famous astronaut (Edward Hubble) used to study space elements. Large space telescope able to see farther than any other telescope at the end of the 20th century. This telescope has improved our knowledge of the Universe: Named after Edwin Hubble, the astronomer who discovered the expansion of the universe, the Hubble Space Telescope is the most successful telescope ever to orbit Earth. It was launched in 1990 and contains a 2.4-m (95-in.) mirror with which it can observe visible, near-ultraviolet, and near-infrared light. It is controlled from a research center on Earth and observes continuously. Nevertheless, the telescope has time to complete only a fraction of the projects proposed by astronomers from around the world. The telescope is as big as a large bus and has been visited a number of times by the space shuttle so that astronauts can maintain its equipment and install new cameras and spectrographs. It orbits Earth only 569 km (353 mi) above the surface.

Diameter

A straight line passing from side to side through the center of a circle or sphere. the distance across a circle through its center. a chord that passes through the center of the circle: Astronomers build large telescopes because a telescope can aid your eyes in three ways—the three powers of a telescope—and the two most important of these powers depends on the diameter of the telescope.

Adaptive Optics

A technique in which telescope mirrors flex rapidly to compensate for the bending of starlight caused by atmospheric turbulence. Rapid changes in mirror shape compensate for atmospheric turbulence. the technique in which computer-controlled mirrors constantly flex and bend to correct for atmospheric distortion: uses highspeed computers to monitor the distortion produced by turbulence in Earth's atmosphere and then correct the telescope image to sharpen a fuzzy blob into a crisp picture.

reflecting telescope

A telescope that uses a curved mirror to collect and focus light. a telescope that uses a curved mirror to gather and focus light from distant objects. a telescope in which a mirror is used to collect and focus light: the primary (or objective) mirror—a concave piece of glass with a reflective surface—forms an image by reflecting the light.

refracting telescope

A telescope that uses convex lenses to gather and focus light. a telescope that uses a set of lenses to gather and focus light from distant objects. a telescope that uses a converging lens to collect light: The primary (or objective) lens bends (refracts) the light as it passes through the glass and brings it to a focus to form a small inverted image.

Seeing

A term used to describe the ease with which good telescopic observations can be made from Earth's surface, given the blurring effects of atmospheric turbulence. A quantity measuring the stability of the Earth's atmosphere: A telescope must contain high-quality optics to achieve its full potential resolving power. Even a large telescope reveals little detail if its optics are marred with imperfections. Also, when you look through a telescope, you are looking up through miles of turbulent air in Earth's atmosphere, which makes the image dance and blur, a condition called?

Advantages of a Radio Telescope

Building large radio telescopes in isolated locations is expensive, but three factors make it all worthwhile. First, and most important, a radio telescope can reveal where clouds of cool hydrogen and other atoms and molecules are located. These clouds are important because, for one thing, they are the places where stars are born. Although cool clouds of gas are completely invisible to normal telescopes because they produce no visible light of their own and reflect too little to be detected in photographs, some gas atoms and molecules do emit radio photons. Cool hydrogen, for example, emits radio energy at the specific wavelength of 21 cm. Other gas molecules emit radio energy with their own characteristic wavelengths. The only way astronomers can detect these clouds is with a radio telescope. Another reason radio telescopes are important is related to dust in space. Astronomers observing at visual wavelengths can't see through the dusty clouds in space. Light waves are so short that they are scattered by the tiny dust grains and never get through the dust to reach optical telescopes on Earth. However, radio signals have wavelengths much longer than the diameters of dust grains, so radio waves from far across the galaxy pass unhindered, giving radio astronomers an unobscured view. (The same is also true, to a large extent, of infrared radiation.) Finally, radio telescopes are important because they can detect objects that are more luminous at radio wavelengths than at visible wavelengths. This includes, for example, intensely hot gas orbiting black holes. Some of the most violent events in the universe are detectable at radio wavelengths.

Special bulbs in spectrographs

Built into the spectrograph, these bulbs produce bright lines given off by such atoms as thorium, argon, or neon. The wavelengths of these spectral lines have been measured to high precision in the laboratory, so astronomers can use spectra of these light sources as guides to measure wavelengths and identify spectral lines in the spectrum of a star, galaxy, or planet.

What celestial objects emit radio energy?

Clouds of gas and erupting stars emit radio energy: astronomers on Earth can study such objects by observing at wavelengths in the radio window where Earth's atmosphere is transparent to radio waves.

Interferometer

Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes. separated telescopes combined to produce a virtual telescope with the resolution of a much larger-diameter telescope. several telescopes connected to act as one: the light from the separate telescopes must be combined as if it had been collected by a single large mirror. That means that a network of small, high-precision mirrors must bring the light beams together, and the path that each light beam travels must be controlled so that it does not vary more than a small fraction of the wavelength.

Radio Telescopes

Devices used to detect radio waves from objects in space. Detect radio waves. detect radio waves from objects in space: Celestial objects such as clouds of gas and erupting stars emit radio energy, and astronomers on Earth can study such objects by observing at wavelengths in the radio window where Earth's atmosphere is transparent to radio waves. You might think an erupting star would produce a strong radio signal, but the signals arriving on Earth are astonishingly weak—a million to a billion times weaker than the signal from an FM radio station. Detecting such weak signals calls for highly sensitive equipment.

Spitzer Space Telescope

It's the most sophisticated of the infrared telescopes put in orbit. Designed to detect infrared radiation to study objects ranging from our solar system to distant regions of the universe. A telescope in space that uses infrared rays. A large space telescope launched by the U.S. in 2003 and placed into a heliocentric solar orbit, it photographs the sky in Infrared wavelengths: like IRAS, is cooled to −269°C (−452°F). Launched in 2003, it observes from behind a sunscreen. In fact, it could not observe from Earth's orbit because Earth is such a strong source of infrared radiation, so the telescope was sent into an orbit around the sun that will carry it slowly away from Earth as its coolant is used up. Named after theoretical physicist Lyman Spitzer Jr. who originally suggested that space telescopes would be useful, it has made important discoveries concerning star formation, planets orbiting other stars, distant galaxies, and more.

Large Binocular Telescope (LBT)

Italian and American astronomers have built this telescope. It carries a pair of 8.4-m mirrors on a single mounting. It carries two 8.4-m mirrors that combine their light. This can be used as an interferometer.

Special Instruments

Just looking through a telescope doesn't tell you much. A star looks like a point of light. A planet looks like a little disk. A galaxy looks like a hazy patch. To use an astronomical telescope to learn about the universe, you must be able to analyze the light the telescope gathers. Special instruments attached to the telescope make that possible.

Green Bank Telescope:

Largest Fully Steerable Radio Telescope: Green Bank, West Virginia. It's at the National Radio Astronomy Observatory. The telescope has a reflecting surface 100 meters in diameter, big enough to hold an entire football field, and can be pointed anywhere in the sky. Its surface consists of 2004 computer-controlled panels that adjust to maintain the shape of the reflecting surface.

The European INTEGRAL Satellite

Launched in 2002 and has been very productive in the study of violent eruptions of stars and black holes.

GLAST (Gamma-Ray Large Area Space Telescope)

Launched in 2008, is capable of mapping large areas of the sky to high sensitivity.

What is the result of light bending?

Light passing through a prism is bent at an angle that depends on its wavelength. Violet (short wavelength) bends most, red (long wavelength) least, so the white light is spread into a spectrum (see figure below). You could build a simple spectrograph using a prism to spread the light and a lens to guide the light into a camera.

Antenna

Like a TV's version, its only function is to absorb the radio energy collected by the dish.

Dish reflector

Like the mirror of a reflecting telescope, this collects and focuses radiation. Because radio waves are much longer than light waves, the dish need not be as smooth as a mirror; wire mesh will reflect all but the shortest wavelength radio waves.

Compton Gamma Ray Observatory

Mission to study gamma-ray emissions in our galaxy and beyond. It burned up in Earth's atmosphere in 2000. It's no longer in space, but it detected gamma rays from objects, such as black holes. The first gamma-ray observatory; mapped the Milky Way (1991): When launched in 1991, It mapped the entire sky at gamma-ray wavelengths.

How have telescopes changed

Modern computers have revolutionized telescope design and operation.

What Will Happen if you Buy an Expensive Telescope?

More Diameter; and Less Chromatic Aberration: Not only will you get more diameter per dollar, but your telescope will not suffer from chromatic aberration. You can safely ignore magnification.

Facts about Reflecting Telescopes

Most important, reflecting telescopes do not suffer from chromatic aberration because the light is reflected before it enters the glass. For these reasons, every large astronomical telescope built since the beginning of the 20th century has been a reflecting telescope.

How are modern computers convenient for people and telescopes?

Nearly all large telescopes are operated by astronomers and technicians working at computers in a control room, and some telescopes can be operated by astronomers thousands of miles from the observatory. Some telescopes are fully automated and observe without direct human supervision.

Some small telescopes have a?

Newtonian or Schmidt-Cassegrain focus

Do radio telescopes manufacture images?

No

Diffraction

Occurs when an object causes a wave to change direction and bend around it. The bending of a wave as it moves around an obstacle or passes through a narrow opening. a change in the direction of a wave when the wave finds an obstacle or an edge, such as an opening: limit the amount of information in an image, and that limits the accuracy of any measurement made based on that image. Have you ever tried to magnify a newspaper photo to distinguish some detail? Newspaper photos are made up of tiny dots of ink, and no detail smaller than a single dot will be visible no matter how much you magnify the photo. In an astronomical image, the resolution is often limited by seeing. You can't see detail in the image that is smaller than the resolution. That's why stars look like fuzzy points of light no matter how big your telescope. All measurements have some built-in uncertainty, and scientists must learn to work within those limitations.

Arecibo Observatory

One of the largest radio telescopes in the world: The largest radio dish in the world is 300 m (1000 ft) in diameter. So large a dish can't be supported in the usual way, so it is built into a mountain valley in Puerto Rico. The reflecting dish is a thin metallic surface supported above the valley floor by cables attached near the rim, and the antenna hangs above the dish on cables from three towers built on three mountain peaks that surround the valley. By moving the antenna above the dish, radio astronomers can point the telescope at any object that passes within 20 degrees of the zenith as Earth rotates. Since its completion in 1963, the telescope has been an international center of radio astronomy research.

Giant Magellan Telescope

Other giant telescopes are being planned with segmented mirrors or with multiple mirrors. This telescope will carry 7 thin mirrors, each 8.4 meters in diameter, on a single mounting. It will be located in the Chilean Andes and is planned to have the light-gathering power of a 22-m telescope.

What three handicaps do radio astronomers work under?

Poor resolution, low intensity, and interference.

What can make a radio telescope have higher resolution?

Radio Interferometry radio astronomers can combine two or more radio telescopes to form a radio interferometer capable of much higher resolution. Just as in the case of optical interferometers, the radio astronomer combines signals from two or more widely separated dishes and "fools the waves" into behaving as if they were collected by a much bigger radio telescope. Radio interferometers can be quite large.

What is a fact about radio telescopes detecting luminous radio waves?

Radio telescopes can detect objects that are more luminous at radio wavelengths than at visible wavelengths. This includes, for example, intensely hot gas orbiting black holes. Some of the most violent events in the universe are detectable at radio wavelengths.

Some space telescopes are designed to study a?

Single problem or a single object.

What is a general purpose of satellite telescope?

Some of these satellites have been general-purpose telescopes that can observe many different kinds of objects.

Are Radio and Infrared interferometers easier to build?

Somewhat easier: Infrared- and radio-wavelength interferometers are slightly easier to build because the wavelengths are longer. In fact, as you will discover later, the first astronomical interferometers were built by radio astronomers.

The Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) is one of the most extensive and ambitious astronomical surveys undertaken by modern astronomers. In its first two stages, lasting from 2000 to 2008, SDSS mapped almost 30 percent of the northern sky using a dedicated 2.5 meter telescope at the Apache Point Observatory in New Mexico. The survey used a 120-megapixel camera to image over 350 million objects, and collected the spectra of hundreds of thousands of galaxies, quasars, and stars. Notable SDSS discoveries include some of the oldest known quasars and stars moving fast enough to escape from our galaxy. SDSS data has also been used to map the distribution of dark matter around galaxies through observations of weak gravitational lensing and to study the evolution of structure in the universe through observations of how both galaxies and quasars are distributed at different redshifts. The third phase of the survey is scheduled to end in 2014, and is expected to yield many exciting scientific discoveries.

Where do some lower-frequency cosmic rays come from?

The Sun

Cosmic Rays

Subatomic particles traveling through space at tremendous velocities. Extremely powerful radiation that comes from deep space. A highly energetic atomic nucleus or other particle traveling through space at a speed approaching that of light. High-energy radiation that originates from outside the Solar System: All of the radiation you have read about has been electromagnetic radiation, but there is another form of energy raining down from space, and scientists aren't sure where it comes from. They are subatomic particles traveling through space at tremendous velocities. Almost no cosmic rays reach the ground, but they do smash gas atoms in the upper atmosphere, and fragments of those atoms shower down on your day and night over your entire life. These secondary cosmic rays are passing through you as you read this sentence. Some of this research can be done from high mountains or high-flying aircraft; but, to study cosmic rays in detail, detectors must go into space. A number of these detectors have been carried into orbit, but this area of astronomical research is just beginning to bear fruit. Astronomers can't be sure what produces these. Because they are atomic particles with electric charges, they are deflected by the magnetic fields spread through our galaxy, and that means astronomers can't tell where their original sources are located. The space between the stars is a glowing fog of______________. Some lower-energy versions of these come from the sun, and observations show that at least some high-energy versions are produced by the violent explosions of dying stars and by supermassive black holes at the centers of galaxies. At present, they largely remain an exciting mystery.

Chandra X-ray Observatory

Telescope in space designed to detect x-ray emissions from very hot regions of the universe. The major NASA x-ray observatory, the former Advanced X-ray Astrophysics Facility, launched in 1999 to make high-resolution observations in the x-ray part of the spectrum. A space-based telescope that detects x-rays: launched in 1999. It orbits a third of the way to the moon. It's named for the late Indian-American Nobel laureate Subrahmanyam 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 from the polished inside of the cylinders and form images on special detectors. The telescope has made important discoveries about everything from star formation to monster black holes in distant galaxies.

Observing Beyond the Ends of the Visible Spectrum

Telescopes in mountain-top observatories usually observe at visual wavelengths, but important observations also can be made from Earth at some infrared and ultraviolet wavelengths. Beyond the red end of the visible spectrum, some infrared radiation leaks through the atmosphere in narrow, partially open atmospheric windows ranging from wavelengths of 1200 nm to about 20,000 nm. Infrared astronomers usually measure wavelength in micrometers (10−6 meters), so they refer to this wavelength range as 1.2 to 30 micrometers (or microns for short). Even in this range, much of the radiation from celestial sources is absorbed by water vapor, carbon dioxide, and ozone molecules. Nevertheless, some infrared observations can be made from mountaintops where the air is thin and dry. For example, a number of important infrared telescopes observe from the 4200-m (13,800-ft) summit of Mauna Kea in Hawaii. At this altitude, the telescopes are above much of the water vapor in Earth's atmosphere. Infrared telescopes have flown to high altitudes under balloons and in airplanes to get above absorption by water vapor. NASA is now testing the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747SP that will carry a 2.5-m telescope, control systems, and a team of astronomers, technicians, and educators into the dry fringes of the atmosphere. Once at that altitude, they can open a door above the telescope and make infrared observations for hours as the plane flies a precisely calculated path. You can see the door in the photo in Figure 6-11. To reduce internal noise, the light-sensitive detectors in astronomical telescopes are cooled to very low temperatures, usually with liquid nitrogen, as shown in Figure 6-11. This is especially necessary for a telescope observing at infrared wavelengths. Infrared radiation is emitted by heated objects, and if the telescope is warm it will emit many times more infrared radiation than that coming from a distant object. Imagine trying to look for something at night through binoculars that are themselves glowing. Beyond the other end of the visible spectrum, astronomers can observe in the near-ultraviolet at wavelengths of about 290 to 400 nm. Your eyes don't detect this radiation, but it can be recorded by specialized detectors. Wavelengths shorter than about 290 nm, the far-ultraviolet, are completely absorbed by the ozone layer extending from about 15 km to 30 km above Earth's surface. No mountaintop is that high, and no airplane can fly to such an altitude. To observe in the far-ultraviolet or beyond at X-ray or gamma-ray wavelengths, telescopes must be in space above the atmosphere.

How do telescopes follow the stars? How do they provide that motion?

Telescopes must have a sidereal drive to follow the stars, and an equatorial mounting with easy motion around a polar axis is the traditional way to provide that motion.

Infrared Astronomy from Orbit

Telescopes that observe in the far-infrared must be protected from heat and must get above Earth's absorbing atmosphere. They have limited lifetimes because they must carry coolant to chill their optics. The Infrared Astronomical Satellite (IRAS) was a joint project of the United Kingdom, the United States, and the Netherlands. IRAS was launched in January of 1983 and carried liquid helium coolant to keep its telescope cold. It made 250,000 observations and, for example, discovered disks of dust around stars where planets are now thought to have formed. Its coolant ran out after 300 days of observation. The most sophisticated of the infrared telescopes put in orbit, the Spitzer Space Telescope, like IRAS, is cooled to −269°C (−452°F). Launched in 2003, it observes from behind a sunscreen. In fact, it could not observe from Earth orbit because Earth is such a strong source of infrared radiation, so the telescope was sent into an orbit around the sun that will carry it slowly away from Earth as its coolant is used up. Named after theoretical physicist Lyman Spitzer Jr. who originally suggested that space telescopes would be useful, it has made important discoveries concerning star formation, planets orbiting other stars, distant galaxies, and more.

Why are Telescopes Sometimes Useless?

The Mount: A good telescope on a poor mounting is almost useless. You might be buying a telescope to put in your backyard, but you must think about the same issues astronomers consider when they design giant telescopes to go on mountaintops.

Interference

The combination of two or more waves that results in a single wave. The amount of overlap that one part has with another when assembled. The interaction between waves that meet: A radio telescope is an extremely sensitive radio receiver listening to faint radio signals. Such weak signals are easily drowned out by interference that includes everything from poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems. A few narrow radio bands in the electromagnetic spectrum are reserved for radio astronomy, but even those are often contaminated by radio noise. To avoid interference, radio astronomers locate their telescopes as far from civilization as possible. Hidden deep in mountain valleys, they are able to listen to the sky protected from human-made radio noise.

What happens when these modern telescopes move around?

The entire building rotates as the telescope moves.

Ariel 1

The first astronomical satellite, ____________, was launched by British astronomers in 1962 and made solar observations in the ultraviolet and X-ray part of the spectrum. Since then, many space telescopes have made high-energy observations from orbit. Some of these satellites have been general-purpose telescopes that can observe many different kinds of objects.

How are diffraction fringes affected in larger and smaller telescopes?

The images from such a virtual telescope are not limited by the diffraction fringes of the individual small telescopes but rather by the diffraction fringes of the much larger virtual telescope.

What is the Hubble Telescope's greatest advantage above the Earth's atmosphere?

The lack of seeing distortion. It can detect fine detail, and, because it concentrates light into sharp images, it can see faint objects.

Imaging Systems

The original imaging device in astronomy was the photographic plate. It could record images of faint objects in long time exposures and could be stored for later analysis. But photographic plates have been almost entirely replaced by electronic imaging systems. Most modern astronomers use charge-coupled devices (CCDs) to record images. A CCD is a specialized computer chip containing millions of microscopic light detectors arranged in an array about the size of a postage stamp. Although CCDs for astronomy are extremely sensitive and therefore expensive, less sophisticated CCDs are used in video and digital cameras. Not only can CCD chips replace photographic plates, but they have some dramatic advantages. They can detect both bright and faint objects in a single exposure, are much more sensitive than photographic plates, and can be read directly into computer memory for later analysis. You can easily sharpen and enhance images from your digital camera because the image from a CCD is stored as numbers in computer memory. Astronomers can also manipulate images to bring out otherwise invisible details. Astronomers can produce false-color images in which different colors represent different levels of intensity and are not related to the true colors of the object. Or they can use false colors to represent different wavelengths not otherwise visible to the human eye, as in the adjacent figure. False-color images are so useful they are commonly used in many other fields, such as medicine and meteorology. In the past, measurements of intensity and color were made using specialized light meters attached to a telescope or on photographic plates. Today, nearly all such measurements are made more easily and more accurately with CCD images.

Why are Observatories Built Away from Cities?

The quest for light-gathering power, high resolution, and Staying away from light pollution. The quest for light-gathering power and high resolution explains why nearly all major observatories are located far from big cities and usually on high mountains. Astronomers avoid cities because light pollution, the brightening of the night sky by light scattered from artificial outdoor lighting, can make it impossible to see faint objects. In fact, many residents of cities are unfamiliar with the beauty of the night sky because they can see only the brightest stars. Even far from cities, nature's own light pollution, the moon, is sometimes so bright it drowns out fainter objects, and astronomers are often unable to observe on the nights near full moon. On such nights, faint objects cannot be detected even with the largest telescopes on high mountains.

High-Energy Astrophysics

The study of x-rays, gamma rays, and cosmic rays, and of the processes that make them: High-energy astrophysics refers to the use of X-ray and gamma ray observations of the sky. Making such observations is difficult but can reveal the secrets of processes such as the explosive deaths of massive stars and eruptions of supermassive black holes. The first astronomical satellite, Ariel 1, was launched by British astronomers in 1962 and made solar observations in the ultraviolet and X-ray part of the spectrum. Since then many space telescopes have made high-energy observations from orbit. Some of these satellites have been general-purpose telescopes that can observe many different kinds of objects. ROSAT, for example, was an X-ray observatory developed by an international consortium of European astronomers. Some space telescopes are designed to study a single problem or a single object. The Japanese satellite Hinode, for example, studies the sun continuously at visual, ultraviolet, and X-ray wavelengths. The largest X-ray telescope to date, the Chandra X-ray Observatory, was launched in 1999. Chandra orbits a third of the way to the moon and 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 from the polished inside of the cylinders and form images on special detectors. The telescope has made important discoveries about everything from star formation to monster black holes in distant galaxies. One of the first gamma-ray observatories was the Compton Gamma Ray Observatory, launched in 1991. It mapped the entire sky at gamma-ray wavelengths. The European INTEGRAL satellite was launched in 2002 and has been very productive in the study of violent eruptions of stars and black holes. The GLAST (Gamma-Ray Large Area Space Telescope), launched in 2008, is capable of mapping large areas of the sky to high sensitivity. Modern astronomy has come to depend on observations that cover the entire electromagnetic spectrum. More orbiting space telescopes are planned that will be more versatile and more sensitive.

James Webb Space Telescope

The successor the Hubble Space Telescope; It's planned to study the evolution of galaxies, the production of elements by stars, and the process of star and planet formation. It will launch in 2018, will use infrared wavelengths. It will give scientists the opportunity to study many things: Astronomers hope that it will last until it is replaced by the ______________? It's expected to launch no sooner than 2013. The Webb telescope will carry a cluster of beryllium segments that will open to form a 6.5-m (256-in.) mirror once in space. It will be over six times larger in the collecting area. It will not have a tube but will observe from behind a sun screen. The infrared Spitzer Space Telescope orbits the sun slightly more slowly than Earth and gradually falls behind as it uses up its liquid helium coolant.

Spectral Lines

The wavelengths where a specific element can absorb or emit light. Called the element's bright lines spectrum used to identify an element. Dark lines or bright lines observed in the spectra of stars: The spectrum of an astronomical object can contain hundreds of__________—dark or bright lines that cross the spectrum at specific wavelengths. The sun's spectrum, for instance, contains hundreds of dark spectral lines produced by the atoms in the sun's hot gases. To measure the precise wavelengths of individual lines and identify the atoms that produced them, astronomers use a comparison spectrum as a calibration.

Where can cosmic ray research be done from?

They can be done from high mountains or high-flying aircraft. To study cosmic rays in detail, detectors must go into space. A number of cosmic-ray detectors have been carried into orbit, but this area of astronomical research is just beginning to bear fruit.

What are Common Characteristics of Seeing and Diffraction?

They limit the amount of information in an image, and that limits the accuracy of any measurement made based on that image. Have you ever tried to magnify a newspaper photo to distinguish some detail? Newspaper photos are made up of tiny dots of ink, and no detail smaller than a single dot will be visible no matter how much you magnify the photo. In an astronomical image, the resolution is often limited by seeing. You can't see a detail in the image that is smaller than the resolution. That's why stars look like fuzzy points of light no matter how big your telescope. All measurements have some built-in uncertainty, and scientists must learn to work within those limitations.

What's significant about infrared telescopes and heat?

They must be protected from heat and must get above Earth's absorbing atmosphere. They have limited lifetimes because they must carry coolant to chill their optics.

Why Should you be Careful When Looking at Telescopes in Department Stores or Camera Shops?

They talk about the magnification of a telescope: Department stores and camera shops may advertise telescopes by quoting their magnification, but it is not an important number. What you can see is determined by light-gathering power, optical quality, and Earth's atmosphere.

Describe cosmic rays

They're atomic particles with electric charges.

Where do some higher-frequency cosmic rays come from?

They're produced by the violent explosions of dying stars and by supermassive black holes at the centers of galaxies.

Buying a Telescope

Thinking about how to shop for a new telescope will not only help you if you decide to buy one but will also illustrate some important points about astronomical telescopes. Assuming you have a fixed budget, you should buy the highest-quality optics and the largest-diameter telescope you can afford. You can't make the atmosphere less turbulent, but you can choose good optics. If you buy a telescope from a toy store and it has plastic lenses, you shouldn't expect to see very much. Also, you want to maximize the light-gathering power of your telescope, so you want to purchase the largest-diameter telescope you can afford. Given a fixed budget, that means you should buy a reflecting telescope rather than a refracting telescope. Not only will you get more diameter per dollar, but your telescope will not suffer from chromatic aberration. You can safely ignore magnification. Department stores and camera shops may advertise telescopes by quoting their magnification, but it is not an important number. What you can see is determined by light-gathering power, optical quality, and Earth's atmosphere. You can change the magnification simply by changing eyepieces. Other things being equal, you should choose a telescope with a solid mounting that will hold the telescope steady and allow you to point it at objects easily. Computer-controlled pointing systems are available for a price on many small telescopes. A good telescope on a poor mounting is almost useless. You might be buying a telescope to put in your backyard, but you must think about the same issues astronomers consider when they design giant telescopes to go on mountaintops.

Facts about Refracting Telescopes

This does improve the image, but these achromatic lenses are not totally free of chromatic aberration. Even though two colors have been brought together, the other wavelengths still blur. Telescopes made with achromatic lenses were popular until the end of the 19th century. The primary lens of a refracting telescope is more expensive than a mirror of the same size. The lens must be achromatic, so it must be made of two different kinds of glass with four precisely ground surfaces. Also, the glass must be pure and flawless because the light passes through it. The largest refracting telescope in the world was completed in 1897 at Yerkes Observatory in Wisconsin. Its lens is 1 m (40 in.) in diameter and weighs half a ton. Larger refracting telescopes are prohibitively expensive. The primary mirrors of reflecting telescopes are much less expensive because the light reflects off the front surface of the mirror. This means that only the front surface needs to be ground to precise shape. This front surface is coated with a highly reflective surface of an aluminum alloy, and the light reflects from this front surface without entering the glass. Consequently, the glass of the mirror need not be perfectly transparent, and the mirror can be supported over its back surface to reduce sagging.

How have the computers changed the way people view the sky?

This has made possible huge surveys of the sky in which millions of objects are observed: The Sloan Digital Sky Survey, for example, mapped the sky, measuring the position and brightness of 100 million stars and galaxies at a number of wavelengths. The Two-Micron All-Sky Survey (2MASS) has mapped the entire sky at three wavelengths in the infrared. Other surveys are being made at other wavelengths. Astronomers will study those data banks for decades to come.

The two Keck 10 m Telescope

This telescope, separated by a distance of 85 m, can operate as an optical interferometer. What is its resolution when it observes in the infrared at a wavelength of 2 microns? Other telescopes, such as this one, can work as interferometers.

Space Interferometry Mission

This will work at visual wavelengths and study everything from the cores of erupting galaxies to planets orbiting nearby stars.

The Spectrograph

To analyze light in detail, astronomers need to spread the light out according to wavelength to form a spectrum, a task performed by a spectrograph. You can understand how this instrument works if you imagine reproducing an experiment performed by Isaac Newton in 1666. Newton bored a small hole in the window shutter of his bedroom to admit a thin beam of sunlight. When he placed a prism in the beam, it spread the light into a beautiful spectrum that splashed across his bedroom wall. From this and related experiments Newton concluded that white light is made of a mixture of all the colors. Light passing through a prism is bent at an angle that depends on its wavelength. Violet (short wavelength) bends most, red (long wavelength) least, so the white light is spread into a spectrum (see figure below). You could build a simple spectrograph using a prism to spread the light and a lens to guide the light into a camera. Nearly all modern spectrographs use a grating in place of a prism. A grating is a piece of glass with thousands of microscopic parallel grooves scribed onto its surface. Different wavelengths of light reflect from the grating at slightly different angles, so white light is spread into a spectrum. You have probably noticed this effect when you look at the closely spaced lines etched onto a compact disk; as you tip the disk, different colors flash across its surface. You could build a modern spectrograph by using a high quality grating to spread light into a spectrum and a CCD camera to record the spectrum. The spectrum of an astronomical object can contain hundreds of spectral lines—dark or bright lines that cross the spectrum at specific wavelengths. The sun's spectrum, for instance, contains hundreds of dark spectral lines produced by the atoms in the sun's hot gases. To measure the precise wavelengths of individual lines and identify the atoms that produced them, astronomers use a comparison spectrum as a calibration. Special bulbs built into the spectrograph produce bright lines given off by such atoms as thorium, argon, or neon. The wavelengths of these spectral lines have been measured to high precision in the laboratory, so astronomers can use spectra of these light sources as guides to measure wavelengths and identify spectral lines in the spectrum of a star, galaxy, or planet.

Comparing the Light Gathering Power (LGP)

To compare the relative light-gathering powers (LGP) of two telescopes A and B, you can calculate the ratio of the areas of their objectives, which reduces to the ratio of their diameters squared.

Two Kinds of Optical Telescopes

Two Kinds of Optical Telescopes Optical telescopes can focus light into an image by using either a lens or a mirror. In a refracting telescope, the primary (or objective) lens bends (refracts) the light as it passes through the glass and brings it to a focus to form a small inverted image. In a reflecting telescope, the primary (or objective) mirror—a concave piece of glass with a reflective surface—forms an image by reflecting the light. A refracting telescope uses a primary lens to focus starlight into an image that is magnified by a lens called an eyepiece. The primary lens has a long focal length, and the eyepiece has a short focal length. (b) A reflecting telescope uses a primary mirror to focus the light by reflection. A small secondary mirror reflects the starlight back down through a hole in the middle of the primary mirror to the eyepiece. In either case, the focal length is the distance from the lens or mirror to the image of a distant light source such as a star. Short-focal-length lenses and mirrors must be strongly curved, and long-focal length lenses and mirrors are less strongly curved. Grinding the proper shape on a lens or mirror is a delicate, time-consuming, and expensive process. The image formed by the primary lens or primary mirror of a telescope is small, inverted, and difficult to view directly. Astronomers use a small lens called the eyepiece to magnify the image and make it convenient to view. Refracting telescopes suffer from a serious optical distortion that limits their usefulness. When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, so blue light, for example, having shorter wavelengths, comes to a focus closer to the lens than does red light. That means 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 on the red image, all the other colors blur.

What type of observations can the Hubble Space Telescope make?

Visual wavelengths

Are radio signals strong or weak?

Weak: the signals arriving on Earth are astonishingly weak—a million to a billion times weaker than the signal from an FM radio station. Detecting such weak signals calls for highly sensitive equipment.

Resolution and Precision

What limits the detail you can see in an image? All images have limited resolution. You see this on your computer screen because images there are made up of picture elements, pixels. If your screen has large pixels, the resolution is low, and you can't see much detail. In an astronomical image, the size of a picture element is set by seeing and by diffraction in the telescope. You can't see detail smaller than that resolution limit. This limitation on the detail in an image is related to the limited precision of any measurement. Imagine a zoologist trying to measure the length of a live snake by holding it along a meter stick. The wriggling snake is hard to hold, so it is hard to measure accurately. Also, meter sticks are usually not marked finer than millimeters. Both factors limit the precision of the measurement. If the zoologist said the snake was 43.28932 cm long, you might be suspicious. The resolution of the measurement technique does not justify the accuracy implied by all those digits. Whenever you make a measurement you should ask yourself how accurate that measurement can be. The accuracy of the measurement is limited by the resolution of the measurement technique, just as the amount of detail in a photograph is limited by its resolution. If you photographed a star, you would not be able to see details on its surface for the same reason the zoologist can't measure the snake to high precision.

What's a Lesson When Measuring Things?

Whenever you make a measurement you should ask yourself how accurate that measurement can be. The accuracy of the measurement is limited by the resolution of the measurement technique, just as the amount of detail in a photograph is limited by its resolution. If you photographed a star, you would not be able to see details on its surface for the same reason the zoologist can't measure the snake to high precision.

Is it expensive to build radio telescopes?

Yes

Are other surveys measuring different (other) wavelengths?

Yes: Astronomers will study those data banks for decades to come.

Can Infrared Waves Leak through the Atmosphere?

Yes: In narrow windows: Beyond the red end of the visible spectrum, some infrared radiation leaks through the atmosphere in narrow, partially open atmospheric windows ranging from wavelengths of 1200 nm to about 20,000 nm. Infrared astronomers usually measure wavelength in micrometers (10−6 meters), so they refer to this wavelength range as 1.2 to 30 micrometers (or microns for short).

Can Infrared Astronomers Observe with the Dome Lights on

Yes: Often. The materials aren't often sensitive to visible light.

Are radio telescopes sensitive?

Yes: Very Such weak signals are easily drowned out by interference that includes everything from poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems. A few narrow radio bands in the electromagnetic spectrum are reserved for radio astronomy, but even those are often contaminated by radio noise.

How do astronomers enhance images using a CCD?

You can easily sharpen and enhance images from your digital camera because the image from a CCD is stored as numbers in computer memory. Astronomers can also manipulate images to bring out otherwise invisible details. Astronomers can produce false-color images in which different colors represent different levels of intensity and are not related to the true colors of the object. Or they can use false colors to represent different wavelengths not otherwise visible to the human eye, as in the adjacent figure. False-color images are so useful they are commonly used in many other fields, such as medicine and meteorology.

How did Issac Newton influence the establishment of spectrographs?

You can understand how this instrument works if you imagine reproducing an experiment performed by Isaac Newton in 1666. Newton bored a small hole in the window shutter of his bedroom to admit a thin beam of sunlight. When he placed a prism in the beam, it spread the light into a beautiful spectrum that splashed across his bedroom wall. From this and related experiments Newton concluded that white light is made of a mixture of all the colors. Light passing through a prism is bent at an angle that depends on its wavelength. Violet (short wavelength) bends most, red (long wavelength) least, so the white light is spread into a spectrum (see figure below). You could build a simple spectrograph using a prism to spread the light and a lens to guide the light into a camera.

Astronomy from Space

You have learned about the observations that ground-based telescopes can make through the two atmospheric windows in the visible and radio parts of the electromagnetic spectrum. Most of the rest of the electromagnetic radiation—infrared, ultraviolet, X-ray, and gamma ray—never reaches Earth's surface; it is absorbed high in Earth's atmosphere. To observe at these wavelengths, telescopes must go above the atmosphere.

Infrared Astronomical Satellite

a joint project of the United Kingdom, the United States, and the Netherlands. This was launched in January of 1983 and carried liquid helium coolant to keep its telescope cold. It made 250,000 observations and, for example, discovered disks of dust around stars where planets are now thought to have formed. Its coolant ran out after 300 days of observation.

Amplifier

an electronic device for increasing the amplitude of electrical signals, used chiefly in sound reproduction. An electric device that boots sound to a level that everyone can hear clearly. Electronic equipment that increases the strength of signals passing through it. Because the radio energy from celestial objects is so weak, it must be strongly amplified before it can be measured and its strength is recorded in computer memory.

Charge Coupled Devices (CCDs)

a photodetector - a device that is capable of converting visible light into an electric charge and storing it in a sequential pattern. This stored charge can be released line by line to the ADC (analog to digital converter) Typically, the light from a scintillator is "coupled" to the device using a focusing lens. Considered indirect acquisition because it uses a scintillator. Doesn't use a TFT An electronic device used for data acquisition; composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it. devices that convert light signals into electric signals in digital format. Most modern astronomers use these to record images. This is a specialized computer chip containing millions of microscopic light detectors arranged in an array about the size of a postage stamp. Although they, for astronomy, are extremely sensitive and therefore expensive, less sophisticated versions are used in video and digital cameras. Not only can these chips replace photographic plates, but they have some dramatic advantages. They can detect both bright and faint objects in a single exposure, are much more sensitive than photographic plates, and can be read directly into computer memory for later analysis.

Grating

a piece of glass with thousands of microscopic parallel grooves scribed onto its surface: Nearly all modern spectrographs use a grating in place of a prism. A grating is a piece of glass with thousands of microscopic parallel grooves scribed onto its surface. Different wavelengths of light reflect from the grating at slightly different angles, so a white light is spread into a spectrum. You have probably noticed this effect when you look at the closely spaced lines etched onto a compact disk; as you tip the disk, different colors flash across its surface. You could build a modern spectrograph by using a high-quality grating to spread light into a spectrum and a CCD camera to record the spectrum.

Can radio telescopes show much detail?

a radio map can't show fine detail: Recall that the resolving power of an optical telescope depends on the diameter of the objective lens or mirror. It also depends on the wavelength of the radiation because, at very long wavelengths like those of radio waves, the diffraction fringes are very large. That means a radio map can't show fine detail. As with an optical telescope, there is no way to improve the resolving power without building a bigger telescope. Consequently, radio telescopes generally have large diameters to minimize the diffraction fringes.

What is one fact about radio telescopes locating molecules, atoms, and cool hydrogen clouds?

a radio telescope can reveal where clouds of cool hydrogen and other atoms and molecules are located. These clouds are important because, for one thing, they are the places where stars are born. Although cool clouds of gas are completely invisible to normal telescopes because they produce no visible light of their own and reflect too little to be detected in photographs, some gas atoms and molecules do emit radio photons. Cool hydrogen, for example, emits radio energy at the specific wavelength of 21 cm. Other gas molecules emit radio energy with their own characteristic wavelengths. The only way astronomers can detect these clouds is with a radio telescope.

Optical Telescopes

a telescope that uses lenses or mirrors to collect and focus visible light. Telescopes that collect only visible light. use light to produce magnified images. Earth has two atmospheric windows, so there are two main types of ground-based telescopes—optical telescopes and radio telescopes. You can start with optical telescopes, which gather light and focus it into sharp images. This requires sophisticated optical and mechanical designs, and it leads astronomers to build gigantic telescopes on the tops of high mountains.

Resolving Power

ability to show detail. A measure of the clarity of an image; the ability of an optical instrument to show two objects as separate. a measure of the clarity of an image: refers to the ability of the telescope to reveal fine detail. Because light acts as a wave, it produces a small diffraction fringe around every point of light in the image, and you cannot see any detail smaller than the fringe. Astronomers can't eliminate diffraction fringes, but the larger a telescope is in diameter, the smaller the diffraction fringes are. That means the larger the telescope, the better this power. If you consider only optical telescopes, you can estimate this power by calculating the angular distance between two stars that are just barely visible through the telescope as two separate images. Astronomers say the two images are "set on," meaning they are separated from each other. This power, alpha , in arc seconds, equals 11.6 divided by the diameter of the telescope in centimeters.

Monitoring the distortion in an image

adaptive optics systems must look at a fairly bright star in the field of view, and there isn't always such a star properly located near a target object such as a faint galaxy. In that case, astronomers can point a laser at a spot in the sky very close to their target object, and where the laser excites gas in Earth's upper atmosphere, it produces a glowing artificial star in the field of view.

ROSAT

an X-ray observatory developed by an international consortium of European astronomers.

How have current astronomers fixed these problems

•Traditional telescopes use large, solid, heavy mirrors to focus starlight to a prime focus, or, by using a secondary mirror, to a Cassegrain focus. Some small telescopes have a Newtonian focus or a Schmidt-Cassegrain focus. •Telescopes must have a sidereal drive to follow the stars, and an equatorial mounting with easy motion around a polar axis is the traditional way to provide that motion. Today, astronomers can build simpler, lighter-weight telescopes on altazimuth mountings that depend on computers to move the telescope so that it follows the westward motion of the stars as Earth rotates. •Active optics, computer control of the shape of telescope 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 and makes it stronger and less expensive. Also, thin mirrors cool faster at nightfall and produce better images.