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Chapter 4.2-4.5

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Shohini Ghose

Chapter 4.2-4.5 Astronomers build optical telescopes to gather light and focus it into sharp images - Requires careful optical and mechanical designs - Leads astronomers to build very large telescopes Two kinds of Telescopes Astronomical telescopes focus light into an image in one of two ways 1) A lens bends (refracts) the light as it passes through the glass and brings it to a focus to form an image 2) A mirror (a curved piece of glass with a reflective surface) forms an image by bouncing light Thus results in two different types of telescopes.. 1) Refracting Telescopes use a lens to gather and focus the light 2) Reflecting Telescopes use a mirror Main lens in a refracting telescope is called the primary lens, and the main mirror in a reflecting telescope is called the primary mirror Both kinds of telescopes form a small, inverted image that is difficult to observe directly, so a lens called the eyepiece is used to magnify the image and make it convenient to view Focal length is the distance from a lens of mirror to the image it forms of a distant light source such as a star Surfaces of lenses and mirrors must be shaped and polished to have no irregularities larger than a wavelength of light Creating the optics for a large telescope can take moths, of even years; involve huge, precision machinery; and employ several expert optical engineers and scientists. Refracting telescopes have serious disadvantages They suffer from an optical distortion that limits their usefulness When light is refracted through glass, shorter wavelengths bend more than longer wavelengths You see a colour blur around every image Called chromatic aberration and it can be only particularly corrected The glass in primary lenses must be pure and flawless because the light passes all the way through it The weight of the lens can be supported only around its outer edge Light reflects from the front surface of a reflecting telescope’s primary mirror but does not pass through it, so reflecting telescopes have no chromatic aberration Mirrors are less expensive to make than similarly sized lenses and the weight of telescope mirrors is easily supported Every large astronomical telescope built since 1900 has been a reflecting telescope Optical telescopes gather visible light, but astronomers also build radio telescopes to gather radio radiation Radio Waves from celestial objects, like visible light waves, penetrate Earth’s atmosphere and reach the ground The dish reflector of a typical radio telescope focuses the radio waves so their intensity can be measured Because radio wave lengths are so long, the disk reflector does not have to be as perfectly smooth as the mirror of a reflecting optical telescope Figure 4.4 In most radio telescopes, a dish reflector concentrates the radio signal on the antenna The signal is then amplified and recorded For all but the shortest radio waves, wire mesh is an adequate reflector 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 three powers of a telescope) - Two most important of these three powers depend on the diameter of the telescope Most celestial objects of interest to astronomers are faint, so you need a telescope that can gather large amounts of light to produce a bright image. 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) The light-gathering power is proportional to the area of the primary mirror, that is, proportional to the square of the primary’s diameter A telescope with a diameter of 2 meters has four times (4x) the light-gathering power of a 1-meter telescope One reason radio astronomers build big radio dishes is to collect enough radio photons, which have low energies, and concentrate them for measurement The resolving 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 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 fringes are smaller in larger telescopes, meaning they have better resolving power and can reveal finer detail - ie. A 2-meter telescope has diffraction fringes ½ as large, and thus 2x better resolving power, than a 1-meter telescope The size of the diffraction fringes also depends on wavelength, and at the long wavelengths of radio waves, the fringes are large and the resolving power is poor, which is another reason radio telescopes need to be larger than optical telescopes One way to improve resolving power is to connect two or more telescopes in an interferometer, which 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 kilometers apart Modern technology has allowed astronomers to connect optical telescopes to form interferometers with very high resolution Aside from diffraction fringes, two other factors (optical quality and atmospheric conditions) limit resolving power A telescope must contain high-quality optics to achieve its full potential resolving power Even a large telescope shows little detail if its optical surfaces have imperfections When you look through a telescope, you look through miles of turbulence in Earth’s atmosphere, which makes images dance and blur, a condition that astronomers call seeing A related phenomenon is the twinkling of a star The twinkles are caused by turbulence in Earth’s atmosphere, and a star near the horizon, where you look through more air, will twinkle more than a star overhead Even with good seeing, the detail visible through a large telescope is limited, not just by its diffraction fringes, but by the steadiness of the air through which the observer must look A telescope performs best on a high mountaintop where the air is thin and steady, but 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 adaptive optics, in which rapid computer calculations adjust the telescope optics and partly compensate for seeing distortions All measurements have some built-in uncertainty, and scientists must learn to work within those limitations The third, least-important power of a telescope is magnifying power, the ability to make an image large The magnifying power of a telescope equals the focal length of the primary mirror or lens divided by the focal length of the eyepiece - ie. A telescope with a primary mirror that has a focal length of 700 mm and an eyepiece with a focal length of 14 mm had a magnifying power of 503 Higher magnifying power does not necessarily show you more detail, because the amount of detail you can see in practice is limited by a combination of the seeing conditions and the telescope’s resolving power and optical quality A telescope’s primary function is to gather 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 resolving power are fundamental properties of a telescope that cannot be altered, whereas magnifying power can be changed simply by changing the eyepiece Observatories on Earth – Optical and Radio Most major observations are located far from big cities and usually on high mountains Optical 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 Radio astronomers face a problem of radio interference analogous to light pollution Weak 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 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 Astronomers prefer to place optical telescopes on mountains because the air there is thin and more transparent, but, most importantly they carefully select mountains where the airflow is usually not turbulent so the seeing is good Building an observatory on top of a high mountain far from civilization is difficult and expensive but the dark sky and good seeing make it worth the effort 1) Research telescopes must focus their light to positions at which cameras and other instruments can be placed 2) Small telescopes can use other focal arrangements that would be inconvenient in larger telescopes Telescopes located on the surface of Earth, whether optical or radio, must move continuously to stay pointed at a celestial object as Earth turns on its axis, which is called sidereal tracking High-speed computers have allowed astronomers to build new, giant telescopes with unique designs The European Southern Observatory 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 m in diameter and holds, for the moment, the record as the largest single telescope in the world The largest fully steerable radio telescope in the world is at the National Radio Astronomy Observatory in West Virginia The telescope had a reflecting surface 100 meters in diameter made of 2,004 computer controlled panels that adjust to maintain the shape of the reflecting surface The largest radio dish in the world is 300 m (1,000 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 20 degrees of the zenith as Earth rotates The Very Large Array (VLA) consists of 27 dishes spread in a Y-pattern across the New Mexico desert Operated as an interferometer, the VLA has the resolving power of a radio telescope up to 36 km in diameter Modern Astronomical Telescopes 1) Large astronomical telescopes must gather light and guide it to locations where it can be recorded with cameras or other instruments o in larger telescopes, the light can be focused to a prime focus position high in the telescope tube, as shown at the right o although it is a good place to image faint objects, the prime focus is inconvenient for large instruments o a secondary mirror can reflect the light through a hole in the primary mirror to a Cassegrain focus o this focal arrangement may be the most common form or astronomical telescope 2) Smaller telescopes can use other focal arrangements. The Newtonian focus that Isaac Newton used in his first reflecting telescope is awkward for large telescopes, but is common for small telescopes Imaging Systems and Photometers The photographic plate 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 photometers, sensitive light meters to measure the brightness of individual objects very precisely Most modern astronomers use charge-coupled devices (CCDs), as both image-recording devices and photometers A CCD is a specialized computer chip containing as many as a million or more microscopic light detectors arranged in an array about the size of a postage stamp These array detectors can be used like a small photographic plate, but they have dramatic advantages over both photometers and photographic plates CCDs can detect both bright and faint objects in a single exposure and are much more se
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