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Astronomical Instruments

Astronomical Instruments. Early Telescopes. Ancient cultures built special sites, called observatories , for observing the sky At these observatories , they were able to measure the positions of celestial objects that were visible to the naked eye

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Astronomical Instruments

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  1. Astronomical Instruments AST 2010: Chapter 5

  2. Early Telescopes • Ancient cultures built special sites, called observatories, for observing the sky • At these observatories, they were able to measure the positions of celestial objects that were visible to the naked eye • Telescopes were first used to observe the sky by Galileo Galilei, and so they are a relatively recent addition to the tools astronomers use • The use of telescopes, however, completely revolutionized our ideas about the universe AST 2010: Chapter 5

  3. Galileo’s Telescopes • Galileo first used a telescope to observe the sky in 1610 • His telescopes were simple tubes held by hand • They were also small in comparison to the telescopes in use today • The use of these small telescopes allowed Galileo to revolutionize the field of astronomy AST 2010: Chapter 5

  4. Why Use Telescopes to Observe the Sky? • Celestial objects — planets, stars, galaxies, etc. — emit (or reflect) light in many different directions • Only a minuscule fraction of the light emitted (or reflected) by celestial objects is captured by the human eye, with its tiny opening  • The light not shining into the eye is “wasted” • Most objects of interest to astronomers are extremely faint • The more light from such objects we can collect, the better we can study them • A telescope is a very important tool because it • has a much larger opening than the human eye and, therefore, captures much more light • focuses all the light collected into an imagemuch better than the naked eye can AST 2010: Chapter 5

  5. Telescopes of All Kinds • Stars and other celestial objects emit all types of electromagnetic waves, not only visible light • Nowadays, there are types of telescopes that collect not visible light, but other forms of EM radiation, such as radio waves, infrared, ultraviolet, X-rays, and even gamma rays • Such telescopes may use collecting devices that look very different from the lenses and mirrors used in visible-light telescopes, but those devices serve the same function AST 2010: Chapter 5

  6. Aperture • In telescopes of all types, the light-gathering ability is determined by the area of the device acting as the main collector of light (or other forms of electromagnetic radiation) • The aperture of a telescope with round lenses and mirrors corresponds to the diameter of its primary lens or primary reflector (mirror) • The light-gathering power of a telescope is determined by its aperture • The amount of light a telescope can collect increases with the square of the aperture • For example, an aperture with a 4-meter diameter can collect 16 times as much light as an aperture with a 1-meter diameter AST 2010: Chapter 5

  7. Need for Images • The study of astronomical objects requires the formation of their images • Once formed, each of the images can be • looked at directly with the naked eye • imprinted on a photographic film • detected and recorded with various light-sensitive devices AST 2010: Chapter 5

  8. Telescope Images in History • Before the 20th century, telescope images were simply looked at with the naked eye • This was a rather inefficient and unreliable way of gathering/collecting and preserving the information • In the 20th century, before the arrival of computers, images were imprinted/recorded on photographic films • Nowadays, astronomers actually rarely look through the larger telescopes • Most images are recorded electronically on computers AST 2010: Chapter 5

  9. Formation of Image by Lens • A convex lens is a transparent piece of material that bends parallel rays of light passing through it and brings them to a focus or focal point

  10. Eyepiece • Telescopes use a combination of lenses and mirrors to produce images • An image formed by the primary lens of a telescope can be viewed, and magnified, by using a second, smaller lens called an eyepiece • Nowadays, the eyepiece of a telescope is usually replaced by a camera or electronic light detector AST 2010: Chapter 5

  11. Magnification • The eyepiece can magnify the image • Stars are typically so distant that they appear as points of light, and consequently magnification does not do much • Planets, however, are much closer, and galaxies much bigger, than stars so that magnification is actually quite useful to see the shape and structure of planetsand galaxies AST 2010: Chapter 5

  12. Concave Mirrors • A telescope can also be built using a concave mirror to form an image • Such a mirror reflects incoming parallel rays through its focus • Thus images can be produced by a concave mirror exactly as they are by a convex lens Convex lens Concave mirror AST 2010: Chapter 5

  13. Refracting telescopes Reflecting telescopes Basic Designs of Visible-Light Telescopes AST 2010: Chapter 5

  14. Refracting Telescopes • In a refractor (refracting telescope), the primary light-gathering device is a convex lens • Galileo's telescopes were all refractors, as are today’s binoculars and opera glasses • Refractors are not good for most astronomical applications • It is very difficult to make a large lens without flaws and support it without causing it to become distorted AST 2010: Chapter 5

  15. Reflecting Telescopes Newton’s telescope • Telescopes designed with mirrors avoid the problems of refractors with large lenses • The first successful reflecting telescope (reflector ) was built by Newton in 1668 • A concave mirror is placed at the bottom of the reflecting telescope • The mirror reflects the light back up the tube to form an image near the front end at a location called the prime focus • Images can be observed directly at the prime focus, or additional mirrors can be used to redirect the light to a more convenient location AST 2010: Chapter 5

  16. Focus Arrangements for Reflecting Telescopes • Different options for where the light is brought to a focus AST 2010: Chapter 5

  17. Some Large-Aperture Telescopes (1) • The Hale telescope on Palomar Mountain in southern California • is a reflector • was built in 1948 • has a mirror that is 5 meters (200 in) in diameter • was the world’s largest visible-light telescope for 45 years • Website Keck

  18. Some Large-Aperture Telescopes (2) • Two Keck telescopes on (dormant) Mauna Kea volcano in Hawaii • became operational in 1993-1996 • each have a mirror that is 10 meters in diameter and composed of 36 hexagonal sections • are sensitive to both visible and infrared wavelengths • Website

  19. Resolution of Telescope • In addition to collecting as much light as they can, astronomers also seek to get the sharpest images possible • Sharper images provide more details about the objects observed • The resolution of a telescope refers to the fineness of detail present in the images it produces • The resolution of an image is measured in units of angle on the sky • The angular size is typically expressed in arcseconds • One arcsec is 1/3600 degree — a very tiny angle! • One arcsec is how a quarter would look like when seen from a distance of 5 km (3 mi) • One of the factors that determine resolution is the telescope’s aperture • Larger apertures result in sharper images AST 2010: Chapter 5

  20. Important External Factor Limiting Resolution • External factors, however, can also affect the resolution • Turbulence in the Earth's atmosphere above a telescope results in the blurring of the images it produces • The “twinkling” of stars as seen with the naked eye from Earth is a result of atmospheric turbulence • In the absence of the atmosphere, the light of stars appears steady • Therefore, it is important to place telescopes at high altitudes where atmospheric blurring is minimized • Telescopes mounted in outer space, above the Earth's atmosphere, are not affected by atmospheric blurring AST 2010: Chapter 5

  21. Adaptive Optics • The technique of adaptive optics can make corrections for atmospheric blurring AST 2010: Chapter 5

  22. Additional Factors Affecting Performance of Telescopes • The weather — clouds, wind, rain, etc — is the most obvious limitation • At the best sites, it is clear as much as 75% of the time • The atmosphere filters out certain amount of starlight • Water vapor absorbs much of the infrared • The preferred sites are dry, at high altitudes • “Light pollution” in the sky often occurs near cities • This is the scattering by air of the glare from city lights producing an illumination that hides the faintest stars • The best sites have dark sky, far away from large cities • The air is often unsteady • Light passing turbulent air is disturbed, resulting in blurred images — an effect called “bad seeing” • Sites with steady atmosphere are preferred AST 2010: Chapter 5

  23. Visible-Light Detecting (1) • After capturing the radiation from celestial objects, astronomers sort it according to wavelength • The instruments used may be as simple as colored filters or a complicated spectrometer • A spectrometer is an instrument designed to disperse light into a spectrum to be recorded for detailed analysis • Spectroscopy is one of the most powerful techniques used in astronomy • After the radiation passes through the sorting instruments, its properties are recorded and measured using detectors AST 2010: Chapter 5

  24. A Prism Spectometer • Nowadays, the prism is replaced by a diffraction grating, which is a piece of transparent material with thousands of grooves in its surface that also cause the light waves to spread out into a spectrum AST 2010: Chapter 5

  25. Visible-Light Detecting (2) • Throughout most of the 20th century, photographic films, or plates, served as the main astronomical detectors • In a plate, a light-sensitive chemical coating is applied to a piece of glass which, when developed, provides a lasting record of the image • Although photographic films represent a large improvement over the human eye, they are inefficient, only recording about 1% of the light incident on them • Astronomers now use much more efficient electronic detectors to record images • Most often, these are charge-coupled devices (CCDs), which are similar to the detectors used in video camcorders or in digital cameras • CCDs record up to 70% of the photons that strike them, resulting in much sharper images, and also provide output that can go directly to a computer for analysis AST 2010: Chapter 5

  26. Infrared Observations • Observing the sky in the infrared band presents additional challenges • The infrared extends from wavelengths near 1 micrometer out to 100 micrometer or longer • Infrared radiation is basically heat radiation • The human body emits heat in the infrared range • A big challenge: at typical temperatures on Earth’s surface, the telescope being used and the atmosphere are all emitting infrared radiation! • To infrared “eyes”, everything on Earth is brightly aglow • The challenge is to detect faint cosmic sources against this sea of infrared • The solution is to isolate the detector in very cold surroundings, often held near absolute zero temperature (1-3 kelvin), by immersing them in liquid helium AST 2010: Chapter 5

  27. Radio Telescopes • Radio emission was discovered by Karl G. Jansky, an engineer of the Bell Telephone Laboratories, in 1931 • In 1936, Grote Reber built from galvanized iron and wood the first antenna specifically designed receive cosmic radio waves • Over the years, he built several antennas and conducted pioneering surveys of the sky for celestial radio sources • In commercial radio broadcasting, sound information is encoded at the source and decoded at the receiving ends, the listener's radios where they are played into headphones or speakers • Radio waves from space do not contain music or other types of human information • The waves nonetheless carry some information about the chemistry and physical conditions of their sources AST 2010: Chapter 5

  28. Radio Astronomy • Radio waves can produce an electric current in conductors (such as metals) • An antenna is such a conductor • It intercepts the path of waves which in turn induce a small current in it • The current is then amplified in a radio receiver and recorded • Receivers, like our TV or radio sets, can be tuned to select a single frequency (channel) • An astronomical radio telescope provides radio spectra, giving information about how much radiation we receive at each wavelength or frequency AST 2010: Chapter 5

  29. Radio Telescopes • A radio-reflecting telescope consists of a concave metal reflector, called a dish, quite analogous to an optical telescope mirror • The radio waves collected by the dish are reflected to the focal point of the reflector where a receiver detects the waves and record them  • Astronomers often construct a pictorial representation of the radio sources they observe in order to communicate and visualize their data more easily AST 2010: Chapter 5

  30. A Radio Image Colors have been added to help the eye sort out regions of different intensities. Red regions are the most intense, blue the least AST 2010: Chapter 5

  31. Green Bank Telescope • This is the world's largest fully steerable radio telescope • located at the National Radio Astronomy Observatory in West Virginia • with a dish about 100 meters across • that can be pointed to any direction in the sky AST 2010: Chapter 5

  32. Arecibo Telescope (1) • It is the world’s largest radar telescope • consisting of a 305-m fixed reflecting surface, made up of 40,000 individual panels • It is suspended in a natural limestone sinkhole in northwestern Puerto Rico • Incoming rays are reflected back from the surface to two additional reflectors located 450 feet above on the “platform”, a 500-ton structure supported by cables from three towers AST 2010: Chapter 5

  33. Arecibo Telescope (2) AST 2010: Chapter 5

  34. Radio Interferometry (1) • A telescope resolution depends primarily on its aperture • It also depends on the wavelength of the wave being detected • The longer the waves, the harder it becomes to detect fine details • Radio waves have very large wavelengths • Substantial challenges for astronomers who need good resolution • The largest radio dishes cannot have poorer resolution than small optical telescopes • To overcome this difficulty, astronomers have learned to link two or more radio telescopes together electronically, and succeeded in greatly sharpening the images they get AST 2010: Chapter 5

  35. Radio Interferometry (2) • An array of telescopes linked together in this way is called an interferometer • The word indicates that these devices operate via a measurement of the degree of interference between different waves • Interference is a technical term for the way waves that arrive in a detector at slightly different times interact with each other • The resolution of an interferometer depends on the separation of the telescopes, not on their individual apertures • Even better resolution can be achieved by combining more than two reflectors into an interferometer array AST 2010: Chapter 5

  36. VLA • The most extensive such instrument is the National Radio Astronomical Observatory's Very Large Array (VLA) near Socorro, New Mexico • It consists of 27 movable radio telescopes, each having an aperture of 25 m, spread over a total span of about 36 km • The telescopes signals are combined electronically and permit astronomers to obtain pictures of the sky with a resolution comparable to those obtained with an optical telescope with a resolution of about 1 arcsecond AST 2010: Chapter 5

  37. Observations Outside Earth's Atmosphere • Earth's atmosphere blocks most radiation at wavelengths shorter than those of visible light • It is thus possible to make astronomical observation at these wavelengths only from space • Getting around the disturbing effects of the atmosphere is also a great advantage at visible and infrared wavelengths • Since stars do not twinkle in empty space, the resolution is far superior than that on Earth • The resolution thus becomes solely limited by the size and quality of the instrument used to collect the light AST 2010: Chapter 5

  38. Hubble Space Telescope (1) • Launched in April 1990, it permitted a giant leap forward in astronomy • It has an aperture of 2.4 m • the largest of those put in space to date • limited by the payload of the space shuttle used to put it in orbit • It was named after Edwin Hubble, the astronomer who discovered the expansion of the universe in the 1920s AST 2010: Chapter 5

  39. Hubble Space Telescope (2) • It is operated jointly by NASA’s Goddard Space-Flight Center and the Space Telescope Science Institute in Baltimore • It was the first orbiting observatory designed to be serviced by shuttle astronauts • Visits by astronauts in 1993, 1997, and 1999 allowed improvements and replacements of the initial instruments AST 2010: Chapter 5

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