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Astronomical Instruments

Astronomical Instruments. Early Telescopes. Ancient cultures used special sites, observatories , to observe the sky At these observatories, they were able to measure the position of celestial objects visible with 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 used special sites, observatories, to observe the sky • At these observatories, they were able to measure the position of celestial objects visible with the naked eye • Telescopes were first used by Galileo Galilei and are a relatively recent addition to the tools used by astronomers • Their use however completely revolutionized our views and understanding of 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 (and get into a lot of trouble with church authorities) AST 2010: Chapter 5

  4. Why Use Telescopes to Observe the Sky? • Celestial objects, such as stars, planets, and galaxies, emit light in all directions • Only a minuscule fraction emitted (or reflected) by celestial objects is perceived by the human eye, with its tiny opening  • Telescopes enable astronomers to gather the light emitted by a distant object over a much larger surface • The mount of light gathered by a telescope is much, much larger than what can be gathered by a naked eye • Also, they provide for much higher quality images AST 2010: Chapter 5

  5. Telescopes of all kinds… • Stars and other celestial objects emit all wavelengths of radiations, not only visible light • Nowadays telescopes enable astronomers to observe electromagnetic radiations that are not visible such as radio-waves, infrared, ultraviolet, x-rays, and even gamma rays • Telescopes enable us • to see more light • to see waves that one could not otherwise perceive with our human senses AST 2010: Chapter 5

  6. Aperture • The light-gathering ability of a telescope is determined by its aperture • The aperture of a telescopes corresponds to the width of its primary lens or reflector • Basically, the larger the aperture of a telescope, the larger its ability to gather light AST 2010: Chapter 5

  7. Need for Images • The study of astronomical objects requires the formation of their images • 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 is a rather inefficient and unreliable way of gathering/collecting and preserving the information • Nowadays astronomers actually rarely look through the larger telescopes • Images are recorded electronically on computers AST 2010: Chapter 5

  9. Focal length Parallel light rays focus lens Formation of Image by Lens or Mirror • A lens is a transparent piece of material that bends parallel rays of light passing through it - bringing them to a focus or focal point. AST 2010: Chapter 5

  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 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 so magnification does not do much • Planets are however much closer and galaxies much bigger than stars so that magnification is actually quite useful to see the shape and structure of planets AST 2010: Chapter 5

  12. Focal length Parallel light rays focus Concave mirror Concave Mirrors • Rays can also be focused to form an image with a concave mirror, which • Has a curved surface where the light is reflected has a parabolic shape • Has a surface coated with a thin layer of metal to make if light reflecting • Reflects incoming rays parallel through its focus • Thus images are produced by a mirror exactly as they are by a lens AST 2010: Chapter 5

  13. Telescope Types • Refracting telescopes • Reflecting telescopes AST 2010: Chapter 5

  14. Refracting Telescopes • They use a lens as the primary light gathering device • Galileo's telescopes were all refractors • Binoculars and opera glasses are refractors • Refractors not good for most astronomical applications • Difficult to support large telescope structure • Quite difficult to make a large piece of glass with the exact right surface shape to reflect light without distortions • Not much in used by modern astronomers AST 2010: Chapter 5

  15. Reflecting Telescopes • Conceived by James Gregory in 1663 • First successful model was built by Newton in 1668 • Concave mirror placed at the bottom of a tube or open framework • 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 at the prime focus directly or via various systems of auxiliary mirrors and lenses to bring it to a more convenient location AST 2010: Chapter 5

  16. Parallel light rays Parallel light rays Reflecting Telescope Variations • Newtonian mode • A small secondary mirror is angled to reflect the light off to the side of the telescope tube • Cassegrain mode • The secondary mirror reflects the light back down the tube, through the primary mirror, and to a focus below the telescope tube AST 2010: Chapter 5

  17. Brightness • A measure of the amount of light that is concentrated in a unit area • Depends on the amount of light focused by the primary mirror • Doubling the aperture of the primary mirror • Increases its area by a factor of four (4) • Results in images that are 4 times brighter AST 2010: Chapter 5

  18. Large-Aperture Telescopes • Two Keck telescopes at Mauna Kea, Hawaii, both have apertures of 10 m • 33 feet long • twice the aperture of the older 5-m Hale telescope at Mt. Palomar • have images four times brighter than those of the Hale telescope Palomar Keck Keck AST 2010: Chapter 5

  19. Resolution • Resolution refers to the fineness of detail present in the image • Astronomers seek to get as many details about the object they observe as possible • The larger the telescope aperture, the sharper the image will be • The resolution of images is however also limited by external factors AST 2010: Chapter 5

  20. Important Factor Limiting Resolution • Fluctuations of the Earth's atmosphere above the telescope introduce a blurring of the images produced by a telescope • Important to place telescopes at high altitudes where such distortions are minimized • Distortions observable with the naked eye • twinkling of the stars • Telescopes mounted in outer space, above the Earth's atmosphere do not have this twinkle • light of the stars is steady AST 2010: Chapter 5

  21. Resolution Measurement Units • The resolution of an astronomical image is measured by the angular size of a point source such as  a star • This  size is expressed in seconds of arc, or arcsec • One arcsec is 1/3600 degree • One arcsec is how a quarter would look like seen from a distance of 5 km AST 2010: Chapter 5

  22. Telescope Fabrication (1) • Purpose: collect light from very faint sources • Use as large an aperture as achievable • Fabricated out of one or multiple large disk or piece of glass • Glass is ground and polished to produce a concave mirror of high optical quality • Shape is critical • All parallel rays, no matter where they hit the mirror, must be reflected into a single focus Keck Keck AST 2010: Chapter 5

  23. Telescope Fabrication (2) • Primary mirror mounted in a support structure • Strong, complex, and steady support structure to provide • a steady support • ability to move the telescope rapidly yet accurately to any desired direction in the sky 5-m Hale telescope at Mt. Palomar very heavy, with 14.5 tons of glass AST 2010: Chapter 5

  24. Telescope Fabrication (3) • Since Earth is constantly rotating • Use motorized system • Move telescope backward at the exact same rate the Earth is moving so that it continuously points to the designated direction • Sophisticated instruments needed • to analyze and record the light collected at the telescope's focus • All machinery housed in a dome to protect it from weather AST 2010: Chapter 5

  25. Telescope Usages • Astronomers use large telescopes in three basic ways: • Imaging • Consists in photographing or recording the appearance of a small portion of the sky • Brightness and color measurements • Consist in a determination of the intensity and the dominant color of the light received from an object • Spectroscopy • Consists in a measurement of the spectra of astronomical sources AST 2010: Chapter 5

  26. Photographic and Electronic Detectors • Photographic films or plates used throughout  most of the 20th century for direct imaging and for spectroscopy • 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 still present serious limitations • films are inefficient: only about one per cent of the light incident on the plates contributes to an image, the rest is wasted • Astronomers now use much more efficient electronic detectors to record images • These are most often charge-coupled devices or CCDs AST 2010: Chapter 5

  27. CCDs • The photons of light incident on a CCD generate a stream of charged particles (electrons) • The electrons are accumulated (stored) and counted at the end of the exposures • CCDs record as much as 60 to 70 percent of the photons that strike them : this allow to detect objects that are more than 60 times fainter (with a given exposure time) than would be possible with films • It also enables more accurate measurement of the brightness of the objects AST 2010: Chapter 5

  28. Infrared Observations • Pose many additional challenges • The infrared extends from wavelength near 1 micrometer out to 100 mm or longer • Infrared radiation is basically heat radiation • The human body emits heat in the infrared range • A big challenge to observe! Why? AST 2010: Chapter 5

  29. The Infrared Challenge • Typical temperature near the Earth's surface ~ 300 kelvin • According to Wien's law, the telescope, the observatory, and surrounding sky are radiating infrared energy with a peak wavelength of about 10 micrometers  • The challenge is to detect/distinguish faint cosmic sources from this background • It is like trying to observe the star (in the visible spectrum) during day time AST 2010: Chapter 5

  30. Infrared Observations • Astronomers must protect the infrared detector from nearby sources of radiation, just as one would shield photographic films from exposure to ambient bright day light • The detector must be isolated in cold surroundings and are often held near absolute zero temperature (1-3 kelvin) by immersing them in liquid helium • Astronomers also attempt to reduce the radiation emitted by the telescope structure and optics and block their radiation AST 2010: Chapter 5

  31. Spectroscopy • Spectroscopy is one of the most powerful techniques used in astronomy • More than half of the observation time at large observatories  is used for spectroscopy • The many wavelength present in light can be separated by passing it through a prism to form a spectrum • A spectrometer is an instrument designed to produce and record such a spectrum AST 2010: Chapter 5

  32. Principle of the Spectrometer • Light from the source (or its image) enters the instrument through a small hole or narrow slit and is collimated by a lens • Light passes through a prism and produces a spectrum • Different wavelengths directed in different directions • A second lens placed behind the prism to focus the many different images of the slits of a CCD • Nowadays, prism replaced by diffraction grating • A grating is a piece of transparent material with thousands of grooves in its surface • The grooves cause the light waves to interfere with each other with the result that the light also spreads out in to a spectrum AST 2010: Chapter 5

  33. A Prism Spectometer AST 2010: Chapter 5

  34. A Simple Spectrometer AST 2010: Chapter 5

  35. Factors Considered When Choosing Observation Site • Weather • Cloud, rain, etc • Best sites have clear weather as much as 75% of the time • Atmosphere • Dust & Humidity • Atmosphere filters out a fraction of starlight • Absorption in infrared due principally to humidity • Prefer dry sites, at high altitudes • Dark Sky - Light Pollution • Near cities, the air scatters the glare from lights producing an illumination that hides the faintest stars • Unsteady air - Bad Seeing • Light passing turbulent air is disturbed • Produces images distortions AST 2010: Chapter 5

  36. Funding • Telescopes built during the 1st half of the 20th century were funded by private donations and were essentially available to but a few astronomers • In recent years, the National Science Foundation (NSF) provides funds to support astronomy research – built various facilities around the world in collaboration with other countries AST 2010: Chapter 5

  37. Radio Telescopes • Radio emission was discovered by Karl G. Jansky, an engineer of the Bell Telephone Laboratories, in 1931 • Grote Reber, built in 1936, the first amateur radio ham, using galvanized iron and wood • He was able to receive cosmic radio waves • Over the years, he built several antennas and conducted a pioneering survey of the sky for celestial radio sources AST 2010: Chapter 5

  38. Radio Waves • 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 head phones or speakers • Radio waves from space do not contain music or other types of human information • The waves nonetheless carry some elementary information about the chemistry and physical conditions of their source AST 2010: Chapter 5

  39. 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 can be used similarly to TVs or Radios, that is by tuning on single frequency • An astronomical radio telescope provide radio spectra, giving information about how much radiation we receive at each wavelength or frequency AST 2010: Chapter 5

  40. 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 more easily communicate and visualize their data AST 2010: Chapter 5

  41. A Radio Image AST 2010: Chapter 5

  42. Green Bank Telescope • World's largest fully steerable radio telescope under construction at the National Radio Astronomy Observatory's site in Green Bank, Pocahontas County, West Virginia • Dimensions of the surface are 100 by 110 meters • The overall structure of the GBT is a wheel-and-track design that allows the telescope to view the entire sky above 5 degrees elevation AST 2010: Chapter 5

  43. Arecibo Telescope (1) • It consists of a 305m fixed reflecting surface, made up of 40,000 individual panels • 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

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

  45. 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 wavelength • 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 succeed in greatly sharpening the images they get AST 2010: Chapter 5

  46. 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 AST 2010: Chapter 5

  47. Interferometer Resolution • The resolution of an interferometer depends on the separation of the telescopes - not on their individual apertures • Two small telescopes separated by 1 km provide the same resolution as would a single dish 1 km wide • However they clearly cannot gather the same quantity of waves • Even better resolution can be achieved by combining more than two reflectors into an interferometer array AST 2010: Chapter 5

  48. 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 resolution comparable to those obtained with an optical telescope with a resolution of about 1 arc second AST 2010: Chapter 5

  49. Observations Outside Earth's Atmosphere • Earth's atmosphere blocks most radiation at wavelength shorter than visible light • It is thus possible to make astronomical observation at these wavelength only from space • Getting above 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

  50. Hubble Space Telescope (1) • Launched in April 1990 • Permitted a giant leap forward in astronomy • It has an aperture of 2.4 m • Largest telescope put in space to date • Aperture limited by the payload of the space shuttle, used to put it in orbit • Named after Edwin Hubble, the astronomer who discovered the expansion of the Universe in the 1920s AST 2010: Chapter 5

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