Chapter 4 Astronomical Telescopes and Instruments: Extending Humanity’s Vision

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Chapter 4
Astronomical Telescopes and Instruments: Extending Humanity’s Vision

4.1 Radiation: Information from Space

  • This chapter’s discussion of astronomical research concentrates on large telescopes and the special instruments and techniques used to analyze light.

Radiation: Information from Space

  • Modern astronomers analyze light using sophisticated instruments and techniques to investigate the __________________, _________________, internal processes, and evolution of celestial objects.

    • To understand this, you must learn about the nature of ___________.

Light as a Wave and as a Particle

  • If you have noticed the colors in a _______ bubble, then you have seen one effect of light behaving as a wave.

  • When that same light enters the light meter on a camera, it behaves as a particle.

    • How light ____________ depends on how you treat it.

    • Light has both ______________ and ________________ properties.

  • Light is composed of a combination of ________ and ____________ waves that can travel through empty space.

    • Unlike sound, light waves do not require a __________ and thus can travel through a ________.

  • As light is made up of both electric and magnetic fields, it is referred to as ________________ radiation.

    • _______________ light is only one form of electromagnetic radiation.

  • The changing electric and magnetic fields of electromagnetic waves travel through space at about _______________ kilometers per second (_________________ miles per second).

    • That is commonly referred to as the __________ of light.

    • It is, however, the speed of all electromagnetic radiation.

  • The electromagnetic _________________ is simply the types of electromagnetic radiation arranged in order of increasing wavelength.

    • ___________________ are spectra of visible light.

The Electromagnetic Spectrum

  • The colors of visible light have different ________________________.

    • ________ has the longest wavelength.

    • _____________ has the shortest.

  • The average wavelength of visible light is about _________________ mm.

    • ________ light waves would fit end-to-end across the thickness of a sheet of paper.

  • It is too awkward to measure such short distances in millimeters.

  • So, physicists and astronomers describe the wavelength of light using either of two units:

    • _______________ (nm), one billionth of a meter (10-9 m)

    • _________________ (Å), named after the Swedish astronomer Anders Ångström, equal to 10-10 m or 0.1 nm

  • The wavelength of visible light ranges between about ________ and ________, or, equivalently, ________ Å and __________ Å.

    • ______________ astronomers often refer to wavelengths using units of microns (10-6 m).

    • ____________astronomers use millimeters, centimeters, or meters.

  • Beyond the________ end of the visible range lies ______________ (IR) radiation—with wavelengths ranging from _________ nm to about _____ mm.

  • _______ waves have even longer wavelengths than IR radiation.

    • The radio radiation used for _______ radio transmissions has wavelengths of a few hundred meters.

    • _______, television, and also ___________, governmental, and amateur radio transmissions have wavelengths from a few tens of centimeters to a few tens of meters.

  • Microwave transmissions, used for ______________and long-distance telephone communications, have wavelengths from about 1 millimeter to a few _____________________.

  • Electromagnetic waves with wavelengths ___________ than violet light are called ultraviolet (UV).

  • Shorter-wavelength electromagnetic waves than UV are called _________.

  • The shortest are _____________ rays.

  • Although light behaves as a wave, under certain conditions, it also behaves as a particle.

    • A particle of light is called a ___________.

    • You can think of a photon as a minimum-sized bundle of electromagnetic waves.

  • The amount of _______________a photon carries depends on its wavelength.

    • Shorter-wavelength photons carry ____________ energy.

    • Longer-wavelength photons carry ____________ energy.

    • A photon of visible light carries a _________ amount of energy.

    • An X-ray photon carries ____________ more energy.

    • A radio photon carries _______________ less.

  • Astronomers are interested in electromagnetic radiation because it carries almost all available ___________ to the nature of planets, stars, and other celestial objects.

  • Only visible light, some short-wavelength infrared radiation, and some radio waves reach Earth’s surface—through what are called atmospheric ________________.

  • To study the sky from ____________ surface, you must look out through one of these ‘windows’ in the electromagnetic spectrum.

4.2 Telescopes

  • Astronomers build optical telescopes to __________ light and focus it into __________ images.

    • This requires careful __________and mechanical designs.

    • It leads astronomers to build very ____________ telescopes.

    • To understand that, you need to learn the terminology of telescopes—starting with the different types of telescopes and why some are better than others.

Two Kinds of Telescopes

  • Astronomical telescopes __________ light into an image in one of two ways.

    • A _________ bends (refracts) the light as it passes through the glass and brings it to a focus to form an image.

    • A _________—a curved piece of glass with a reflective surface—forms an image by bouncing light.

  • Thus, there are two types of astronomical telescopes.

    • _________________ telescopes use a lens to gather and focus the light.

    • _________________ telescopes use a mirror.

  • The main lens in a refracting telescope is called the ____________ lens.

  • The main mirror in a reflecting telescope is called the primary ________________.

  • Both kinds of telescopes form a small, inverted image that is difficult to observe directly.

  • So, a lens called the _____________ is used to magnify the image and make it convenient
    to view.

  • The focal length is the ________________ from a lens or mirror to the image it forms of a distant light source such as a star.

  • Creating the proper optical shape to produce a good focus is an ________________ process.

    • The surfaces of lenses and mirrors must be ______________ and polished to have no irregularities larger than the wavelength of light.

    • Creating the optics for a large telescope can take _________ or years; involve huge, precision machinery; and employ several expert optical engineers and scientists.

  • Refracting telescopes have ______________ disadvantages.

  • Most importantly, they suffer from an optical _____________________ that limits their usefulness.

    • When light is refracted through glass, shorter wavelengths ____________ more than longer wavelengths.

    • As a result, you see a __________ blur around every image.

    • This color separation is called ________________ aberration and it can be only partially corrected.

  • Astronomers also build __________telescopes to gather radio radiation.

    • Radio waves from ______________ objects—like visible light waves—penetrate Earth’s atmosphere and reach the ground.

  • You can see how the dish reflector of a typical radio telescope focuses the radio waves so their intensity can be measured.

    • As radio wavelengths are so __________, the disk reflector does not have to be as perfectly __________________as the mirror of a reflecting optical telescope.

The Powers of a Telescope

  • Astronomers struggle to build large telescopes because a telescope can help human eyes in three important ways.

  • These are called the ___________ powers of a telescope.

    • The two most important of these three powers depend on the ____________ of the telescope.

  • Light-gathering power refers to the ability of a telescope to ______________ light.

    • Catching light in a telescope is like catching rain in a bucket—the ________ the bucket, the __________ rain it catches.

  • The light-gathering power is proportional to the _______of the primary mirror—that is, proportional to the square of the primary’s diameter.

    • A telescope with a diameter of ___ meters has ______ times (___X) the light-gathering power of a 1-meter telescope.

    • That is why astronomers use large telescopes and why telescopes are ranked by their __________________.

  • One reason radio astronomers build big radio dishes is to collect enough radio ______________—which have low energies—and concentrate them for measurement.

  • ____________________ power refers to the ability of the telescope to reveal fine detail.

  • One consequence of the wavelike nature of light is that there is an inevitable small blurring called a ___________________ fringe around every point of light in the image.

    • You cannot see any detail __________________ than the fringe.

  • Astronomers __________ eliminate diffraction fringes.

  • However, the fringes are smaller in larger telescopes.

    • That means they have better resolving power and can reveal finer detail.

    • For example, a ____-meter telescope has diffraction fringes___ as large, and thus ___X better resolving power, than a ____-meter telescope.

  • One way to improve resolving power is to connect two or more telescopes in an _________________.

    • This has a resolving power equal to that of a telescope as large as the maximum separation between the individual telescopes.

  • The first interferometers were built by radio astronomers connecting radio dishes _____________apart.

  • Modern technology has allowed astronomers to connect optical telescopes to form interferometers with very high ____________________.

  • Aside from diffraction fringes, two other factors limit resolving power:

    • __________ quality

    • __________________ conditions

  • Also, when you look through a telescope, you look through miles of __________________ in Earth’s atmosphere, which makes images dance and blur—a condition astronomers call _______________.

  • A related phenomenon is the __________________ of a star.

    • The twinkles are caused by turbulence in Earth’s atmosphere.

    • A star near the____________—where you look through more air—will twinkle more than a star overhead.

    • On a night when the atmosphere is unsteady, the stars _____________, the images are blurred, and the seeing is bad.

  • A telescope performs best on __ ______________ mountaintop—where the air is thin and steady.

  • However, even at good sites, atmospheric turbulence spreads star images into blobs 0.5 to 1 arc seconds in diameter.

  • That situation can be improved by a difficult and expensive technique called ___________ optics.

    • By this technique,____________ computer calculations adjust the telescope optics and partly compensate for seeing distortions.

  • A telescope’s primary function is to ___________ light and thus make faint things appear brighter,

    • so the light-gathering power is the most important power and the diameter of the telescope is its most important characteristic.

  • Light-gathering power and ________________ power are fundamental properties of a telescope that cannot be altered,

    • Whereas magnifying power can be changed simply by changing the _________________.

4.3 Observatories on Earth-Optical and Radio

  • Optical astronomers avoid cities because light ______________—the brightening of the night sky by light scattered from artificial outdoor lighting—can make it impossible to see __________ objects.

    • In fact, many residents of cities are unfamiliar with the beauty of the night sky because they can see only the __________________stars.

  • Radio astronomers face a problem of __________ interference analogous to light pollution.

    • __________ radio signals from the cosmos are easily drowned out by human radio interference—everything from automobiles with faulty ignition systems to poorly designed transmitters in communication.

  • To avoid that, radio astronomers locate their telescopes as far from civilization as possible.

    • Hidden deep in mountain ____________, they are able to listen to the sky protected from human-made radio noise.

There are two important points to notice about modern astronomical telescopes.

  • One, research telescopes must focus their light to positions at which ___________ and other ___________________ can be placed.

  • Two, small telescopes can use other ___________ arrangements that would be inconvenient in larger telescopes.

  • Telescopes located on the surface of Earth, whether optical or radio, must ______________ continuously to stay pointed at a celestial object as Earth turns on its axis.

    • This is called _________________ tracking (‘sidereal’ refers to the stars).

  • High-speed computers have allowed astronomers to build new, giant telescopes with unique designs.

  • The European Southern Observatory has built the _________ Large Telescope (VLT) high in the remote Andes Mountains of northern Chile.

  • The _________ actually consists of four telescopes, each with a computer-controlled mirror _____ m in diameter and only _____ cm (_______ in.) thick.

  • The __________ telescopes can work singly or can combine their light to work as one large telescope.

  • Italian and American astronomers have built the Large __________ Telescope, which carries a pair of ____ -m mirrors on a single mounting.

  • The Gran Telescopio Canarias, located atop a volcanic peak in the Canary Islands, carries a segmented mirror. ______ meters in diameter.

  • It holds, for the moment, the record as the largest single telescope in the world.

  • Other giant telescopes are being planned with innovative designs.

  • The largest fully steerable radio telescope in the world is at the National Radio Astronomy Observatory in _______________ ______________________.

  • The telescope has a reflecting surface _______meters in diameter made of ________ computer-controlled panels that adjust to maintain the shape of the reflecting surface.

  • The largest radio dish in the world is ______ m (________ ft) in diameter, and is built into a mountain valley in Arecibo, Puerto Rico.

  • The antenna hangs on cables above the dish, and, by moving the antenna, astronomers can point the telescope at any object that passes within _______ degrees of the zenith as Earth rotates.

  • The ________ ________ __________ (VLA) consists of ___ dishes spread in a ___-pattern across the New Mexico desert.

  • Operated as an interferometer, the VLA has the resolving power of a radio telescope up to ____km (____ mi) in diameter.

  • Such arrays are very powerful, and radio astronomers are now planning the Square Kilometer Array

    • It will consist of radio dishes spanning _______ km (almost __________ mi) and having a total collecting area of one square kilometer.

4.4 Astronomical Instruments and Techniques

  • The photographic _______ was the first image-recording device used with telescopes.

    • Brightness of objects imaged on a photographic plate can be measured with a lot of hard work—yielding only moderate precision.

  • Astronomers also build _______________________.

    • These are sensitive light meters that can be used to measure the _________________ of individual objects very precisely.

  • Most modern astronomers use ___________- ___________ devices (CCDs) as both image-recording devices and photometers.

    • A CCD is a specialized computer chip containing as many as a ____________or more microscopic light detectors arranged in an array about the size of a _________________stamp.

    • These array detectors can be used like a small photographic plate.

  • CCDs have dramatic advantages over both photometers and photographic plates.

    • They can detect both ________ and _______ objects in a single exposure and are much more sensitive than a photographic plate.

    • CCD images are digitized—converted to numerical data—and can be read directly into a computer memory for later ________________.

  • Although CCDs for astronomy are extremely sensitive and thus expensive, __________sophisticated CCDs are now used in commercial video and ___________ cameras.

  • _____________ astronomers use array detectors similar in operation to optical CCDs.

    • At other wavelengths, photometers are still used for measuring brightness of celestial objects.

  • For example, astronomical images are often reproduced as negatives—with the sky __________ and the stars ___________.

    • This makes the faint parts of the image easier to see.

  • Astronomers also manipulate images to produce __________-_________ images.

    • The colors represent different levels of intensity and are _________ related to the true colors of the object.

  • One way is to measure the strength of the radio signal at various places in the sky and draw a map in which contours mark areas of uniform radio intensity.

  • _____________ maps are very common in radio astronomy and are often reproduced using false colors.

  • To analyze light in detail, you need to spread the light out according to wavelength into a _________________—a task performed by a spectrograph.

    • You can understand how this works by reproducing an experiment performed by Isaac Newton in 1666.

  • Newton didn’t think in terms of wavelength. You, however, can use that modern concept to see that the light passing through the prism is bent at an angle that depends on the wavelength.

    • ______________ (______-wavelength) light bends most, and ______ (_____ wavelength) light least.

  • Thus, the ___________light entering the prism is spread into what is called a spectrum.

  • Most modern spectrographs use a grating in place of a prism.

    • A ___________is a piece of glass with thousands of microscopic parallel lines scribed onto its surface.

    • Different wavelengths of light reflect from the grating at slightly different angles.

    • So, white light is spread into a spectrum and can be recorded—often by a CCD camera.

4.5 Airborne and Space Observatories

  • Most of the rest of the __________—infrared, ultraviolet, X-ray, and gamma-ray radiation—never reaches Earth’s surface.

    • To observe at these wavelengths, telescopes must fly above the atmosphere in high-flying aircraft, rockets, balloons, and satellites.

  • For example, a number of important infrared telescopes are located on the summit of Mauna Kea in Hawaii—at an altitude of ____________ m (______________ ft).

  • For many years, the NASA Kuiper Airborne Observatory (KAO) carried a _____-cm infrared telescope and crews of astronomers to altitudes of over ________km (_____________ ft).

    • This was in order to get above ______ percent or more of the water vapor in Earth’s atmosphere.

  • Now retired from service, the________ will soon be replaced by the Stratospheric Observatory for Infrared Astronomy (SOFIA).

    • This is a Boeing 747-P aircraft that will carry a _____-m (_____-in.) telescope to the fringes of the atmosphere.

  • No mountain is that high, and no balloon or airplane can fly that high.

  • So, astronomers cannot observe far-UV, X-ray, and gamma-ray radiation—without going into space.

Telescopes in Space

___________________ _____________________ _________________ ______________________

  • The largest X-ray telescope to date, the ______________________ Observatory, was launched in 1999 and orbits a third of the way to the moon.

  • _______________ is named for the late Indian-American Nobel Laureate Subrahmanyan Chandrasekhar, who was a pioneer in many branches of theoretical astronomy.

  • The telescope has made important discoveries about everything from star formation to monster black holes in distant galaxies.

  • 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 ____________ (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 ____________ space telescopes are planned that will be more versatile and more sensitive.



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