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Astronomical Instruments 8 Jul 2005 AST 2010: Chapter 5 1 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 8 Jul 2005 AST 2010: Chapter 5 2 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 8 Jul 2005 AST 2010: Chapter 5 3 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 image much better than the naked eye can 8 Jul 2005 AST 2010: Chapter 5 4 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 8 Jul 2005 AST 2010: Chapter 5 5 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 8 Jul 2005 AST 2010: Chapter 5 6 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 lightsensitive devices 8 Jul 2005 AST 2010: Chapter 5 7 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 8 Jul 2005 AST 2010: Chapter 5 8 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 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 8 Jul 2005 AST 2010: Chapter 5 10 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 planets and galaxies 8 Jul 2005 AST 2010: Chapter 5 11 Concave Mirrors A telescope can also be built using a concave Convex lens mirror to form an image Such a mirror reflects incoming parallel rays its focus Thus images can be produced by a concave mirror exactly as they are by a convex lens 8 Jul 2005 AST 2010: Chapter 5 through Concave mirror 12 Basic Designs of Visible-Light Telescopes Refracting telescopes 8 Jul 2005 AST 2010: Chapter 5 Reflecting telescopes 13 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 8 Jul 2005 AST 2010: Chapter 5 14 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 8 Jul 2005 AST 2010: Chapter 5 15 Focus Arrangements for Reflecting Telescopes Different options for where the light is brought to a focus 8 Jul 2005 AST 2010: Chapter 5 16 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 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 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 8 Jul 2005 AST 2010: Chapter 5 19 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 8 Jul 2005 AST 2010: Chapter 5 20 Adaptive Optics The technique of adaptive optics can make corrections for atmospheric blurring 8 Jul 2005 AST 2010: Chapter 5 21 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 8 Jul 2005 AST 2010: Chapter 5 22 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 8 Jul 2005 AST 2010: Chapter 5 23 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 8 Jul 2005 AST 2010: Chapter 5 24 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 8 Jul 2005 AST 2010: Chapter 5 25 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 8 Jul 2005 AST 2010: Chapter 5 26 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 8 Jul 2005 AST 2010: Chapter 5 27 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 8 Jul 2005 AST 2010: Chapter 5 28 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 8 Jul 2005 AST 2010: Chapter 5 29 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 8 Jul 2005 AST 2010: Chapter 5 30 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 8 Jul 2005 AST 2010: Chapter 5 31 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 8 Jul 2005 AST 2010: Chapter 5 32 Arecibo Telescope (2) 8 Jul 2005 AST 2010: Chapter 5 33 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 8 Jul 2005 AST 2010: Chapter 5 34 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 8 Jul 2005 AST 2010: Chapter 5 35 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 8 Jul 2005 AST 2010: Chapter 5 36 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 8 Jul 2005 AST 2010: Chapter 5 37 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 8 Jul 2005 AST 2010: Chapter 5 38 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 8 Jul 2005 AST 2010: Chapter 5 39