1 / 53

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

jvanhouten
Download Presentation

Astronomical Instruments

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related