{"id":197,"date":"2017-08-08T13:12:15","date_gmt":"2017-08-08T17:12:15","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/6-4-radio-telescopes\/"},"modified":"2021-05-01T00:48:20","modified_gmt":"2021-05-01T04:48:20","slug":"6-4-radio-telescopes","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/6-4-radio-telescopes\/","title":{"raw":"6.4 Radio Telescopes","rendered":"6.4 Radio Telescopes"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\n<p id=\"fs-id1168584044152\">By the end of this section, you will be able to:<\/p>\r\n\r\n<ul id=\"fs-id1167470696550\">\r\n \t<li>Describe how radio waves from space are detected<\/li>\r\n \t<li>Identify the world\u2019s largest radio telescopes<\/li>\r\n \t<li>Define the technique of interferometry and discuss the benefits of interferometers over single-dish telescopes<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-id1167470614672\">In addition to visible and infrared radiation, radio waves from astronomical objects can also be detected from the surface of Earth. In the early 1930s, Karl G. <span class=\"no-emphasis\">Jansky<\/span>, an engineer at Bell Telephone Laboratories, was experimenting with antennas for long-range radio communication when he encountered some mysterious static\u2014radio radiation coming from an unknown source as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_RTelescope\">Figure 1<\/a>. He discovered that this radiation came in strongest about four minutes earlier on each successive day and correctly concluded that since Earth\u2019s sidereal rotation period (how long it takes us to rotate relative to the stars) is four minutes shorter than a solar day, the radiation must be originating from some region fixed on the celestial sphere. Subsequent investigation showed that the source of this radiation was part of the <span class=\"no-emphasis\">Milky Way Galaxy<\/span>; Jansky had discovered the first source of cosmic radio waves.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_RTelescope\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>First Radio Telescope.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_RTelescope-1.jpg\" alt=\"Photograph of Jensky and his model.\" width=\"487\" height=\"360\" \/> <strong>Figure 1.<\/strong> This rotating radio antenna was used by Jansky in his serendipitous discovery of radio radiation from the Milky Way.[\/caption]<\/figure>\r\n<p id=\"fs-id1167470932642\">In 1936, Grote <span class=\"no-emphasis\">Reber<\/span>, who was an amateur astronomer interested in radio communications, used galvanized iron and wood to build the first antenna specifically designed to receive cosmic radio waves. Over the years, Reber built several such antennas and used them to carry out pioneering surveys of the sky for celestial radio sources; he remained active in radio astronomy for more than 30 years. During the first decade, he worked practically alone because professional astronomers had not yet recognized the vast potential of radio astronomy.<\/p>\r\n\r\n<section id=\"fs-id1167471052220\">\r\n<h1>Detection of Radio Energy from Space<\/h1>\r\n<p id=\"fs-id1167470637865\">It is important to understand that radio waves cannot be \u201cheard\u201d: they are not the sound waves you hear coming out of the radio receiver in your home or car. Like light, radio waves are a form of electromagnetic radiation, but unlike light, we cannot detect them with our senses\u2014we must rely on electronic equipment to pick them up. In commercial radio broadcasting, we encode sound information (music or a newscaster\u2019s voice) into radio waves. These must be decoded at the other end and then turned back into sound by speakers or headphones.<\/p>\r\n<p id=\"fs-id1167470950841\">The radio waves we receive from space do not, of course, have music or other program information encoded in them. If cosmic radio signals were translated into sound, they would sound like the static you hear when scanning between stations. Nevertheless, there is information in the radio waves we receive\u2014information that can tell us about the chemistry and physical conditions of the sources of the waves.<\/p>\r\n<p id=\"fs-id1167470897710\">Just as vibrating charged particles can produce electromagnetic waves (see the <a class=\"target-chapter\" href=\"\/astronomy1105\/chapter\/5-0-thinking-ahead\/\">Radiation and Spectra<\/a> chapter), electromagnetic waves can make charged particles move back and forth. Radio waves can produce a current in conductors of electricity such as metals. An antenna is such a conductor: it intercepts radio waves, which create a feeble current in it. The current is then amplified in a radio receiver until it is strong enough to measure or record. Like your television or radio, receivers can be tuned to select a single frequency (channel). In astronomy, however, it is more common to use sophisticated data-processing techniques that allow thousands of separate frequency bands to be detected simultaneously. Thus, the astronomical radio receiver operates much like a spectrometer on a visible-light or infrared telescope, providing information about how much radiation we receive at each wavelength or frequency. After computer processing, the radio signals are recorded on magnetic disks for further analysis.<\/p>\r\n<p id=\"fs-id1167470602398\">Radio waves are reflected by conducting surfaces, just as light is reflected from a shiny metallic surface, and according to the same laws of optics. A radio-reflecting telescope consists of a concave metal reflector (called a <em>dish<\/em>), analogous to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed. Because humans are such visual creatures, radio astronomers often construct a pictorial representation of the radio sources they observe. <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_RadioImage\">Figure 2<\/a> shows such a radio image of a distant galaxy, where radio telescopes reveal vast jets and complicated regions of radio emissions that are completely invisible in photographs taken with light.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_RadioImage\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Radio Image of a Galaxy in Cygnus A<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_RadioImage-1.jpg\" alt=\"False color radio image of galaxy Cygnus A. This image shows two huge, diffuse clouds (lobes) of hot gas on either side of the galaxy. Thin jets of material are also seen, one on each side, connecting the galaxy to the lobes. The lobes are shown in three colors corresponding to the intensity of the radio energy detected. Blue is least intense and is concentrated in the regions of the lobes closest to the galaxy. Green is next and is located near the center and far edges of the lobes. Finally red is the most intense and is found at the edges of the lobes farthest from the galaxy.\" width=\"487\" height=\"279\" \/> <strong>Figure 2.<\/strong> This image has been constructed of radio observations at the Very Large Array of a galaxy called Cygnus A. Colours have been added to help the eye sort out regions of different radio intensities. Red regions are the most intense, blue the least. The visible galaxy would be a small dot in the centre of the image. The radio image reveals jets of expelled material (more than 160,000 light-years long) on either side of the galaxy. (credit: NRAO\/AUI)[\/caption]<\/figure>\r\n<p id=\"fs-id1167470679884\">Radio astronomy is a young field compared with visible-light astronomy, but it has experienced tremendous growth in recent decades. The world\u2019s largest radio reflectors that can be pointed to any direction in the sky have apertures of 100 meters. One of these has been built at the US National Radio Astronomy Observatory in West Virginia as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_GreenBank\">Figure 3<\/a>. <a class=\"autogenerated-content\" href=\"#fs-id1167470936824\">Figure 4<\/a> lists some of the major radio telescopes of the world.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_GreenBank\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Robert C. Byrd Green Bank Telescope.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_GreenBank-1.jpg\" alt=\"Photograph of the Robert C. Byrd radio telescope at Green Bank, West Virginia.\" width=\"975\" height=\"418\" \/> <strong>Figure 3.<\/strong> This fully steerable radio telescope in West Virginia went into operation in August 2000. Its dish is about 100 meters across. (credit: modification of work by \u201cb3nscott\u201d\/Flickr)[\/caption]<\/figure>\r\n<table id=\"fs-id1167470936824\" class=\"span-all\" summary=\"This table has 4 columns and 22 rows. The first row is a header with the values, \u201cObservatory\u201d, \u201cLocation\u201d, \u201cDescription\u201d, and \u201cWebsite\u201d. The second row is another header which spans all 4 columns, and has the value, \u201cIndividual Radio Dishes\u201d. Under the column labeled \u201cObservatory\u201d are the values, \u201cArecibo Observatory\u201d, \u201cGreen Bank Telescope (GBT)\u201d, \u201cEffelsberg 100-m Telescope\u201d, \u201dLovell Telescope\u201d, \u201cCanberra Deep Space Communication Complex (CDSCC)\u201d, \u201cGoldstone Deep Space Communications Complex (GDSCC)\u201d, and \u201cParkes Observatory\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cObservatory\u201d column continues with the values, \u201cSquare Kilometre Array (SKA)\u201d, \u201cAtacama Large Millimeter\/submillimeter Array (ALMA)\u201d, \u201cVery Large Array (VLA)\u201d, \u201cWesterbork Synthesis Radio Telescope (WSRT)\u201d, \u201cVery Long Baseline Array (VLBA)\u201d, \u201cAustralia Telescope Compact Array (ATCA)\u201d, and \u201cMulti-Element Radio Linked Interferometer Network (MERLIN)\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cObservatory\u201d column continues with the values, \u201cIRAM\u201d, \u201cJames Clerk Maxwell Telescope (JCMT)\u201d, \u201cNobeyama Radio Observatory (NRO)\u201d, and \u201cHat Creek Radio Observatory (HCRO)\u201d. Under the column labeled \u201cLocation\u201d are the values, \u201cArecibo, Puerto Rico\u201d, \u201cGreen Bank, WV\u201d, \u201cBonn, Germany\u201d, \u201cManchester, England\u201d, \u201cTidbinbilla, Australia\u201d, \u201cBarstow, CA\u201d, and \u201cParkes, Australia\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cLocation\u201d column continues with the values, \u201cSouth Africa and Western Australia\u201d, \u201cAtacama desert, Northern Chile\u201d, \u201cSocorro, New Mexico\u201d, \u201cWesterbork, the Netherlands\u201d, \u201cTen US sites, HI to the Virgin Islands\u201d, \u201cSeveral sites in Australia\u201d, and \u201cCambridge, England, and other British sites\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cLocation\u201d column continues with the values, \u201cGranada, Spain\u201d, \u201cMauna Kea, HI\u201d, \u201cMinamimaki, Japan\u201d, and \u201cCassel, CA\u201d. Under the column labeled \u201cDescription\u201d are the values, \u201c305-m fixed dish\u201d, \u201c110\u00d7100-m steerable dish\u201d, \u201c100-m steerable dish\u201d, \u201c76-m steerable dish,\u201d \u201c70-m steerable dish\u201d, \u201c70-m steerable dish\u201d, and \u201c64 m steerable dish\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cDescription\u201d column continues with the values, \u201cThousands of dishes, km2 collecting area, partial array in 2020\u201d, \u201c66 7-m and 12-m dishes\u201d, \u201c27-element array of 25-m dishes (36-km baseline)\u201d, \u201c12-element array of 25-m dishes (1.6-km baseline)\u201d, \u201c10-element array of 25-m dishes (9000-km baseline)\u201d, \u201c8-element array (seven 22-m dishes plus Parkes 64-m)\u201d, and \u201cNetwork of seven dishes (the largest is 32-m)\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cDescription\u201d column continues with the values, \u201c30-m steerable mm-wave dish\u201d, \u201c15 m steerable mm-wave dish\u201d, \u201c6-element array of 10-m wave dishes\u201d, and \u201c6-element array of 5-m wave dishes\u201d. Under the column labeled \u201cWebsite\u201d are the values, \u201cwww.naic.edu\u201d, \u201cwww.science.nrao.edu\/facilities\/gbt\u201d, \u201cwww.mpifr-bonn.mpg.de\/en\/effelsberg\u201d, \u201cwww.jb.man.ac.uk\/aboutus\/lovell\u201d, \u201cwww.cdscc.nasa.gov\u201d, \u201cwww.gdscc.nasa.gov\u201d, and \u201cwww.parkes.atnf.csiro.au\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cWebsite\u201d column continues with the values, www.skatelescope.org, \u201cwww.almaobservatory.org\u201d, \u201cwww.science.nrao.edu\/facilities\/vla\u201d, \u201cwww.astron.nl\/radio-observatory\/public\/public-0\u201d, \u201cwww.science.nrao.edu\/facilities\/vlba\u201d, \u201cwww.narrabri.atnf.csiro.au\u201d, and \u201cwww.e-merlin.ac.uk\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cWebsite\u201d column continues with the values, \u201cwww.iram-institute.org\u201d, \u201cwww.eaobservatory.org\/jcmt\u201d, \u201cwww.nro.nao.ac.jp\/en\u201d, and \u201cwww.sri.com\/research-development\/specialized-facilities\/hat-creek-radio-observatory\u201d.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"4\">Figure 4. Major Radio Observatories of the World<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Observatory<\/th>\r\n<th>Location<\/th>\r\n<th>Description<\/th>\r\n<th>Website<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td colspan=\"4\">Individual Radio Dishes<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Arecibo Observatory<\/span><\/td>\r\n<td>Arecibo, Puerto Rico<\/td>\r\n<td>305-m fixed dish<\/td>\r\n<td>www.naic.edu<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Green Bank Telescope<\/span> (GBT)<\/td>\r\n<td>Green Bank, WV<\/td>\r\n<td>110 \u00d7 100-m steerable dish<\/td>\r\n<td>www.science.nrao.edu\/facilities\/gbt<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Effelsberg 100-m Telescope<\/span><\/td>\r\n<td>Bonn, Germany<\/td>\r\n<td>100-m steerable dish<\/td>\r\n<td>www.mpifr-bonn.mpg.de\/en\/effelsberg<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Lovell Telescope<\/span><\/td>\r\n<td>Manchester, England<\/td>\r\n<td>76-m steerable dish<\/td>\r\n<td>www.jb.man.ac.uk\/aboutus\/lovell<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Canberra Deep Space Communication Complex<\/span> (CDSCC)<\/td>\r\n<td>Tidbinbilla, Australia<\/td>\r\n<td>70-m steerable dish<\/td>\r\n<td>www.cdscc.nasa.gov<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Goldstone Deep Space Communications Complex<\/span> (GDSCC)<\/td>\r\n<td>Barstow, CA<\/td>\r\n<td>70-m steerable dish<\/td>\r\n<td>www.gdscc.nasa.gov<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Parkes Observatory<\/span><\/td>\r\n<td>Parkes, Australia<\/td>\r\n<td>64-m steerable dish<\/td>\r\n<td>www.parkes.atnf.csiro.au<\/td>\r\n<\/tr>\r\n<tr>\r\n<td colspan=\"4\">Arrays of Radio Dishes<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Square Kilometre Array<\/span> (SKA)<\/td>\r\n<td>South Africa and Western Australia<\/td>\r\n<td>Thousands of dishes, km<sup>2<\/sup> collecting area, partial array in 2020<\/td>\r\n<td>www.skatelescope.org<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Atacama Large Millimeter\/submillimeter Array<\/span> (ALMA)<\/td>\r\n<td>Atacama desert, Northern Chile<\/td>\r\n<td>66 7-m and 12-m dishes<\/td>\r\n<td>www.almaobservatory.org<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Very Large Array<\/span> (VLA)<\/td>\r\n<td>Socorro, New Mexico<\/td>\r\n<td>27-element array of 25-m dishes (36-km baseline)<\/td>\r\n<td>www.science.nrao.edu\/facilities\/vla<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Westerbork Synthesis Radio Telescope<\/span> (WSRT)<\/td>\r\n<td>Westerbork, the Netherlands<\/td>\r\n<td>12-element array of 25-m dishes (1.6-km baseline)<\/td>\r\n<td>www.astron.nl\/radio-observatory\/public\/public-0<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Very Long Baseline Array<\/span> (VLBA)<\/td>\r\n<td>Ten US sites, HI to the Virgin Islands<\/td>\r\n<td>10-element array of 25-m dishes (9000 km baseline)<\/td>\r\n<td>www.science.nrao.edu\/facilities\/vlba<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Australia Telescope Compact Array<\/span> (ATCA)<\/td>\r\n<td>Several sites in Australia<\/td>\r\n<td>8-element array (seven 22-m dishes plus Parkes 64 m)<\/td>\r\n<td>www.narrabri.atnf.csiro.au<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Multi-Element Radio Linked Interferometer Network<\/span> (MERLIN)<\/td>\r\n<td>Cambridge, England, and other British sites<\/td>\r\n<td>Network of seven dishes (the largest is 32 m)<\/td>\r\n<td>www.e-merlin.ac.uk<\/td>\r\n<\/tr>\r\n<tr>\r\n<td colspan=\"4\">Millimeter-wave Telescopes<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">IRAM<\/span><\/td>\r\n<td>Granada, Spain<\/td>\r\n<td>30-m steerable mm-wave dish<\/td>\r\n<td>www.iram-institute.org<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">James Clerk Maxwell Telescope<\/span> (JCMT)<\/td>\r\n<td>Mauna Kea, HI<\/td>\r\n<td>15-m steerable mm-wave dish<\/td>\r\n<td>www.eaobservatory.org\/jcmt<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Nobeyama Radio Observatory<\/span> (NRO)<\/td>\r\n<td>Minamimaki, Japan<\/td>\r\n<td>6-element array of 10-m wave dishes<\/td>\r\n<td>www.nro.nao.ac.jp\/en<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><span class=\"no-emphasis\">Hat Creek Radio Observatory<\/span> (HCRO)<\/td>\r\n<td>Cassel, CA<\/td>\r\n<td>6-element array of 5-m wave dishes<\/td>\r\n<td>www.sri.com\/research-development\/specialized-facilities\/hat-creek-radio-observatory<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/section><section id=\"fs-id1167470853879\">\r\n<h1>Radio Interferometry<\/h1>\r\n<p id=\"fs-id1167470615520\">As we discussed earlier, a telescope\u2019s ability to show us fine detail (its resolution) depends upon its aperture, but it also depends upon the wavelength of the radiation that the telescope is gathering. The longer the waves, the harder it is to resolve fine detail in the images or maps we make. Because radio waves have such long wavelengths, they present tremendous challenges for astronomers who need good resolution. In fact, even the largest radio dishes on Earth, operating alone, cannot make out as much detail as the typical small visible-light telescope used in a college astronomy lab. To overcome this difficulty, radio astronomers have learned to sharpen their images by linking two or more radio telescopes together electronically. Two or more telescopes linked together in this way are called an interferometer.<\/p>\r\n<p id=\"fs-id1167470755666\">\u201cInterferometer\u201d may seem like a strange term because the telescopes in an interferometer work cooperatively; they don\u2019t \u201cinterfere\u201d with each other. Interference, however, is a technical term for the way that multiple waves interact with each other when they arrive in our instruments, and this interaction allows us to coax more detail out of our observations. The resolution of an interferometer depends upon the separation of the telescopes, not upon their individual apertures. Two telescopes separated by 1 kilometer provide the same resolution as would a single dish 1 kilometer across (although they are not, of course, able to collect as much radiation as a radio-wave bucket that is 1 kilometer across).<\/p>\r\nTo get even better resolution, astronomers combine a large number of radio dishes into an interferometer array. In effect, such an array works like a large number of two-dish interferometers, all observing the same part of the sky together. Computer processing of the results permits the reconstruction of a high-resolution radio image. The most extensive such instrument in the United States is the National Radio Astronomy Observatory\u2019s Very Large Array (VLA) near Socorro, New Mexico. It consists of 27 movable radio telescopes (on railroad tracks), each having an aperture of 25 meters, spread over a total span of about 36 kilometers. By electronically combining the signals from all of its individual telescopes, this array permits the radio astronomer to make pictures of the sky at radio wavelengths comparable to those obtained with a visible-light telescope, with a resolution of about 1 arcsecond.\r\n<p id=\"fs-id1167471083541\">The Atacama Large Millimeter\/submillimeter array (ALMA) in the Atacama Desert of Northern Chile as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_ALMA\">Figure 5<\/a>, at an altitude of 16,400 feet, consists of 12 7-meter and 54 12-meter telescopes, and can achieve baselines up to 16 kilometers. Since it became operational in 2013, it has made observations at resolutions down to 6 milliarcseconds (0.006 arcseconds), a remarkable achievement for radio astronomy.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_ALMA\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Atacama Large Millimeter\/Submillimeter Array (ALMA).<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_ALMA-1.jpg\" alt=\"Photograph of the Atacama Large Millimeter Array in Chile, taken at night. Many of the telescopes are seen pointing in various directions, with the Moon and Milky Way prominent in the background sky.\" width=\"975\" height=\"273\" \/> <strong>Figure 5.<\/strong> Located in the Atacama Desert of Northern Chile, ALMA currently provides the highest resolution for radio observations. (credit: ESO\/S. Guisard)[\/caption]<\/figure>\r\n<div id=\"fs-id1167470625259\" class=\"note astronomy link-to-learning\">\r\n<div class=\"textbox shaded\">Watch this <a href=\"https:\/\/youtu.be\/_Ryctl1Gij4\">documentary<\/a>\u00a0from the European Space Agency that explains the work that went into designing and building ALMA, discusses some of its first images, and explores its future.\u00a0 The URL is:<a href=\"https:\/\/youtu.be\/_Ryctl1Gij4\">\u00a0https:\/\/youtu.be\/_Ryctl1Gij4\u00a0<\/a><\/div>\r\n<\/div>\r\n<p id=\"fs-id1167470963381\">Initially, the size of interferometer arrays was limited by the requirement that all of the dishes be physically wired together. The maximum dimensions of the array were thus only a few tens of kilometres. However, larger interferometer separations can be achieved if the telescopes do not require a physical connection. Astronomers, with the use of current technology and computing power, have learned to time the arrival of electromagnetic waves coming from space very precisely at each telescope and combine the data later. If the telescopes are as far apart as California and Australia, or as West Virginia and Crimea in Ukraine, the resulting resolution far surpasses that of visible-light telescopes.<\/p>\r\n<p id=\"fs-id1167471061686\">The United States operates the Very Long Baseline Array (VLBA), made up of 10 individual telescopes stretching from the Virgin Islands to Hawaii as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_Baseline\">Figure 6<\/a>. The VLBA, completed in 1993, can form astronomical images with a resolution of 0.0001 arcseconds, permitting features as small as 10 astronomical units (AU) to be distinguished at the center of our Galaxy.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_Baseline\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Very Long Baseline Array.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_Baseline-1.jpg\" alt=\"Diagram of the Very Long Baseline Array. The image shows the Northern Hemisphere of Earth centered on North America. Icons of radio antennas are shown distributed throughout the continental United States, as well as on Hawai\u2019i and Puerto Rico.\" width=\"975\" height=\"216\" \/> <strong>Figure 6.<\/strong> This map shows the distribution of 10 antennas that constitute an array of radio telescopes stretching across the United States and its territories.[\/caption]<\/figure>\r\n<p id=\"fs-id1167470602103\">Recent advances in technology have also made it possible to do interferometry at visible-light and infrared wavelengths. At the beginning of the twenty-first century, three observatories with multiple telescopes each began using their dishes as interferometers, combining their light to obtain a much greater resolution. In addition, a dedicated interferometric array was built on Mt. Wilson in California. Just as in radio arrays, these observations allow astronomers to make out more detail than a single telescope could provide.<\/p>\r\n\r\n<table id=\"fs-id1167470667239\" class=\"span-all\" summary=\"This table has 5 columns and 4 rows. The first row is a header with the values \u201cLongest Baseline (m)\u201d, \u201cTelescope Name\u201d, \u201cLocation\u201d, \u201cMirrors\u201d, and \u201cStatus\u201d. Under the \u201cLongest Baseline (m)\u201d column are the values, \u201c400\u201d, \u201c200\u201d, \u201c85\u201d, and \u201c22.8\u201d. Under \u201cTelescope Name\u201d are the values, \u201c\u201cCHARA Array(Center for High Angular Resolution Astronomy)\u201d, \u201cVery Large Telescope\u201d, \u201cKeck I and II telescopes\u201d, and \u201cLarge Binocular Telescope\u201d. Under \u201cLocation\u201d are the values, Mount Wilson, CA\u201d, \u201cCerro Paranal, Chile\u201d, \u201cMauna Kea, HI\u201d, and \u201cMount Graham, AZ\u201d. Under the \u201cMirrors\u201d column are the values, \u201cSix 1-m telescopes\u201d, \u201cFour 8.2-m telescopes\u201d, \u201cTwo 10-m telescopes\u201d, and \u201cTwo 8.4-m telescopes\u201d. Under \u201cStatus\u201d are the values, \u201cOperational since 2004\u201d, \u201cCompleted 2000\u201d, \u201cOperated from 2001 to 2012\u201d, and \u201cFirst light 2004\u201d.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"5\">Visible-Light Interferometers<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Longest Baseline (m)<\/th>\r\n<th>Telescope Name<\/th>\r\n<th>Location<\/th>\r\n<th>Mirrors<\/th>\r\n<th>Status<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>400<\/td>\r\n<td><span class=\"no-emphasis\">CHARA<\/span> Array (Center for High Angular Resolution Astronomy)<\/td>\r\n<td>Mount Wilson, CA<\/td>\r\n<td>Six 1-m telescopes<\/td>\r\n<td>Operational since 2004<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>200<\/td>\r\n<td><span class=\"no-emphasis\">Very Large Telescope<\/span><\/td>\r\n<td>Cerro Paranal, Chile<\/td>\r\n<td>Four 8.2-m telescopes<\/td>\r\n<td>Completed 2000<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>85<\/td>\r\n<td><span class=\"no-emphasis\">Keck I and II<\/span> telescopes<\/td>\r\n<td>Mauna Kea, HI<\/td>\r\n<td>Two 10-m telescopes<\/td>\r\n<td>Operated from 2001 to 2012<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>22.8<\/td>\r\n<td><span class=\"no-emphasis\">Large Binocular Telescope<\/span><\/td>\r\n<td>Mount Graham, AZ<\/td>\r\n<td>Two 8.4-m telescopes<\/td>\r\n<td>First light 2004<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/section><section id=\"fs-id1167470658378\">\r\n<h1>CHIME Radio Telescope in British Columbia, Canada<\/h1>\r\n<\/section>The\u00a0<b>Canadian Hydrogen Intensity Mapping Experiment<\/b>\u00a0(<b>CHIME<\/b>) is an\u00a0<a class=\"mw-redirect\" title=\"Interferometer\" href=\"https:\/\/en.wikipedia.org\/wiki\/Interferometer\">interferometric<\/a>\u00a0<a title=\"Radio telescope\" href=\"https:\/\/en.wikipedia.org\/wiki\/Radio_telescope\">radio telescope<\/a>\u00a0at the\u00a0<a title=\"Dominion Radio Astrophysical Observatory\" href=\"https:\/\/en.wikipedia.org\/wiki\/Dominion_Radio_Astrophysical_Observatory\">Dominion Radio Astrophysical Observatory<\/a>\u00a0in\u00a0<a title=\"British Columbia\" href=\"https:\/\/en.wikipedia.org\/wiki\/British_Columbia\">British Columbia<\/a>,\u00a0<a title=\"Canada\" href=\"https:\/\/en.wikipedia.org\/wiki\/Canada\">Canada<\/a>\u00a0which consists of four\u00a0<a title=\"Antenna (radio)\" href=\"https:\/\/en.wikipedia.org\/wiki\/Antenna_(radio)\">antennas<\/a>\u00a0consisting of 100 x 20\u00a0<a title=\"Metre\" href=\"https:\/\/en.wikipedia.org\/wiki\/Metre\">metre<\/a>\u00a0cylindrical\u00a0<a title=\"Parabolic reflector\" href=\"https:\/\/en.wikipedia.org\/wiki\/Parabolic_reflector\">parabolic reflectors<\/a>\u00a0(roughly the size and shape of snowboarding\u00a0<a title=\"Half-pipe\" href=\"https:\/\/en.wikipedia.org\/wiki\/Half-pipe\">half-pipes<\/a>) with 1024 dual-polarization radio receivers suspended on a support above them. The antenna receives radio waves from\u00a0<a title=\"Hydrogen\" href=\"https:\/\/en.wikipedia.org\/wiki\/Hydrogen\">hydrogen<\/a>\u00a0in space at\u00a0<a title=\"Frequency\" href=\"https:\/\/en.wikipedia.org\/wiki\/Frequency\">frequencies<\/a>\u00a0in the 400\u2013800\u00a0<a title=\"Hertz\" href=\"https:\/\/en.wikipedia.org\/wiki\/Hertz\">MHz<\/a>\u00a0range. The telescope's\u00a0<a title=\"Low-noise amplifier\" href=\"https:\/\/en.wikipedia.org\/wiki\/Low-noise_amplifier\">low-noise amplifiers<\/a>\u00a0are built with components adapted from the cellphone industry and its data are processed using a custom-built\u00a0<a class=\"mw-redirect\" title=\"FPGA\" href=\"https:\/\/en.wikipedia.org\/wiki\/FPGA\">FPGA<\/a>\u00a0electronic system and 1000-processor high-performance\u00a0<a title=\"General-purpose computing on graphics processing units\" href=\"https:\/\/en.wikipedia.org\/wiki\/General-purpose_computing_on_graphics_processing_units\">GPGPU<\/a>\u00a0cluster.<sup id=\"cite_ref-nature\/news_1-0\" class=\"reference\"><a href=\"https:\/\/en.wikipedia.org\/wiki\/Canadian_Hydrogen_Intensity_Mapping_Experiment#cite_note-nature\/news-1\">[1]<\/a><\/sup>The telescope has no moving parts and observes half of the sky each day as the Earth turns. It has also turned out to be a superior instrument for observing the recently discovered phenomenon of\u00a0<a title=\"Fast radio burst\" href=\"https:\/\/en.wikipedia.org\/wiki\/Fast_radio_burst\">fast radio bursts<\/a>\u00a0(FRBs).\r\n\r\nCHIME is a partnership between the\u00a0<a title=\"University of British Columbia\" href=\"https:\/\/en.wikipedia.org\/wiki\/University_of_British_Columbia\">University of British Columbia<\/a>,\u00a0<a title=\"McGill University\" href=\"https:\/\/en.wikipedia.org\/wiki\/McGill_University\">McGill University<\/a>, the\u00a0<a title=\"University of Toronto\" href=\"https:\/\/en.wikipedia.org\/wiki\/University_of_Toronto\">University of Toronto<\/a>\u00a0and the Canadian\u00a0<a title=\"National Research Council (Canada)\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Research_Council_(Canada)\">National Research Council<\/a>'s\u00a0<a title=\"Dominion Radio Astrophysical Observatory\" href=\"https:\/\/en.wikipedia.org\/wiki\/Dominion_Radio_Astrophysical_Observatory\">Dominion Radio Astrophysical Observatory<\/a>. A\u00a0<a title=\"First light (astronomy)\" href=\"https:\/\/en.wikipedia.org\/wiki\/First_light_(astronomy)\">first light<\/a>\u00a0ceremony was held on 7 September 2017 to inaugurate the commissioning phase.\r\n\r\n<section id=\"fs-id1167470658378\">\r\n\r\n[caption id=\"attachment_1668\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-1668 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-1024x683.jpg\" alt=\"large radio telescope \" width=\"1024\" height=\"683\" \/> CHIME telescope in British Columbia, Canada. Photo credit: Image from Wikipedia CC BY-SA 4.0 Z22 This telescope has found a large number of FRBs = Fast Radio Bursts as of 2019.[\/caption]\r\n\r\n&nbsp;\r\n<h1>Radar Astronomy<\/h1>\r\n<p id=\"fs-id1167470691541\">Radar is the technique of transmitting radio waves to an object in our solar system and then detecting the radio radiation that the object reflects back. The time required for the round trip can be measured electronically with great precision. Because we know the speed at which radio waves travel (the speed of light), we can determine the distance to the object or a particular feature on its surface (such as a mountain).<\/p>\r\n<p id=\"fs-id1167470613198\">Radar observations have been used to determine the distances to planets and how fast things are moving in the solar system (using the Doppler effect, discussed in the <a class=\"target-chapter\" href=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/5-0-thinking-ahead\/\">Radiation and Spectra<\/a> chapter). Radar waves have played important roles in navigating spacecraft throughout the solar system. In addition, as will be discussed in later chapters, radar observations have determined the rotation periods of Venus and Mercury, probed tiny Earth-approaching asteroids, and allowed us to investigate the mountains and valleys on the surfaces of Mercury, Venus, Mars, and the large moons of Jupiter.<\/p>\r\n<p id=\"fs-id1167470586562\">Any radio dish can be used as a radar telescope if it is equipped with a powerful transmitter as well as a receiver. The most spectacular facility in the world for radar astronomy is the 1000-foot (305-meter) telescope at Arecibo in Puerto Rico (<a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_Largest\">[link]<\/a>). The Arecibo telescope is too large to be pointed directly at different parts of the sky. Instead, it is constructed in a huge natural \u201cbowl\u201d (more than a mere dish) formed by several hills, and it is lined with reflecting metal panels. A limited ability to track astronomical sources is achieved by moving the receiver system, which is suspended on cables 100 meters above the surface of the bowl. An even larger (500-meter) radar telescope is currently under construction. It is the <span class=\"no-emphasis\">Five-hundred-meter Aperture Spherical Telescope<\/span> (FAST) in China and is expected to be completed in 2016.<\/p>\r\n\r\n<figure id=\"OSC_Astro_06_04_Largest\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Largest Radio and Radar Dish.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_Largest-1.jpg\" alt=\"Photograph of Arecibo Observatory in Puerto Rico, seen from above. The huge 1000-ft metal dish is built into a natural depression in the mountains.\" width=\"487\" height=\"364\" \/> <strong>Figure 6.<\/strong> The Arecibo Observatory, with its 1000-foot radio dish-filling valley in Puerto Rico, is part of the National Astronomy and Ionosphere Center, operated by SRI International, USRA, and UMET under a cooperative agreement with the National Science Foundation. (credit: National Astronomy and Ionosphere Center, Cornell U., NSF)[\/caption]<\/figure>\r\n<\/section><section id=\"fs-id1167470881669\" class=\"summary\">\r\n<p id=\"fs-id1167470732320\">In the 1930s, radio astronomy was pioneered by Karl G. Jansky and Grote Reber. A radio telescope is basically a radio antenna (often a large, curved dish) connected to a receiver. Significantly enhanced resolution can be obtained with interferometers, including interferometer arrays like the 27-element VLA and the 66-element ALMA. Expanding to very long baseline interferometers, radio astronomers can achieve resolutions as precise as 0.0001 arcsecond. Radar astronomy involves transmitting as well as receiving. The largest radar telescope currently in operation is a 305-meter bowl at Arecibo.<\/p>\r\n\r\n<\/section>\r\n<div>\r\n<h2>Glossary<\/h2>\r\n<dl id=\"fs-id1167470606241\" class=\"definition\">\r\n \t<dt>interference<\/dt>\r\n \t<dd id=\"fs-id1167470759046\">process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1167470662948\" class=\"definition\">\r\n \t<dt>interferometer<\/dt>\r\n \t<dd id=\"fs-id1167470680988\">instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1167470585101\" class=\"definition\">\r\n \t<dt>interferometer array<\/dt>\r\n \t<dd id=\"fs-id1167470607777\">combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1167470746741\" class=\"definition\">\r\n \t<dt>radar<\/dt>\r\n \t<dd id=\"fs-id1167470709119\">technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it<\/dd>\r\n<\/dl>\r\n<\/div>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p id=\"fs-id1168584044152\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1167470696550\">\n<li>Describe how radio waves from space are detected<\/li>\n<li>Identify the world\u2019s largest radio telescopes<\/li>\n<li>Define the technique of interferometry and discuss the benefits of interferometers over single-dish telescopes<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-id1167470614672\">In addition to visible and infrared radiation, radio waves from astronomical objects can also be detected from the surface of Earth. In the early 1930s, Karl G. <span class=\"no-emphasis\">Jansky<\/span>, an engineer at Bell Telephone Laboratories, was experimenting with antennas for long-range radio communication when he encountered some mysterious static\u2014radio radiation coming from an unknown source as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_RTelescope\">Figure 1<\/a>. He discovered that this radiation came in strongest about four minutes earlier on each successive day and correctly concluded that since Earth\u2019s sidereal rotation period (how long it takes us to rotate relative to the stars) is four minutes shorter than a solar day, the radiation must be originating from some region fixed on the celestial sphere. Subsequent investigation showed that the source of this radiation was part of the <span class=\"no-emphasis\">Milky Way Galaxy<\/span>; Jansky had discovered the first source of cosmic radio waves.<\/p>\n<figure id=\"OSC_Astro_06_04_RTelescope\">\n<div class=\"title\" style=\"text-align: center\"><strong>First Radio Telescope.<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_RTelescope-1.jpg\" alt=\"Photograph of Jensky and his model.\" width=\"487\" height=\"360\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1.<\/strong> This rotating radio antenna was used by Jansky in his serendipitous discovery of radio radiation from the Milky Way.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1167470932642\">In 1936, Grote <span class=\"no-emphasis\">Reber<\/span>, who was an amateur astronomer interested in radio communications, used galvanized iron and wood to build the first antenna specifically designed to receive cosmic radio waves. Over the years, Reber built several such antennas and used them to carry out pioneering surveys of the sky for celestial radio sources; he remained active in radio astronomy for more than 30 years. During the first decade, he worked practically alone because professional astronomers had not yet recognized the vast potential of radio astronomy.<\/p>\n<section id=\"fs-id1167471052220\">\n<h1>Detection of Radio Energy from Space<\/h1>\n<p id=\"fs-id1167470637865\">It is important to understand that radio waves cannot be \u201cheard\u201d: they are not the sound waves you hear coming out of the radio receiver in your home or car. Like light, radio waves are a form of electromagnetic radiation, but unlike light, we cannot detect them with our senses\u2014we must rely on electronic equipment to pick them up. In commercial radio broadcasting, we encode sound information (music or a newscaster\u2019s voice) into radio waves. These must be decoded at the other end and then turned back into sound by speakers or headphones.<\/p>\n<p id=\"fs-id1167470950841\">The radio waves we receive from space do not, of course, have music or other program information encoded in them. If cosmic radio signals were translated into sound, they would sound like the static you hear when scanning between stations. Nevertheless, there is information in the radio waves we receive\u2014information that can tell us about the chemistry and physical conditions of the sources of the waves.<\/p>\n<p id=\"fs-id1167470897710\">Just as vibrating charged particles can produce electromagnetic waves (see the <a class=\"target-chapter\" href=\"\/astronomy1105\/chapter\/5-0-thinking-ahead\/\">Radiation and Spectra<\/a> chapter), electromagnetic waves can make charged particles move back and forth. Radio waves can produce a current in conductors of electricity such as metals. An antenna is such a conductor: it intercepts radio waves, which create a feeble current in it. The current is then amplified in a radio receiver until it is strong enough to measure or record. Like your television or radio, receivers can be tuned to select a single frequency (channel). In astronomy, however, it is more common to use sophisticated data-processing techniques that allow thousands of separate frequency bands to be detected simultaneously. Thus, the astronomical radio receiver operates much like a spectrometer on a visible-light or infrared telescope, providing information about how much radiation we receive at each wavelength or frequency. After computer processing, the radio signals are recorded on magnetic disks for further analysis.<\/p>\n<p id=\"fs-id1167470602398\">Radio waves are reflected by conducting surfaces, just as light is reflected from a shiny metallic surface, and according to the same laws of optics. A radio-reflecting telescope consists of a concave metal reflector (called a <em>dish<\/em>), analogous to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed. Because humans are such visual creatures, radio astronomers often construct a pictorial representation of the radio sources they observe. <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_RadioImage\">Figure 2<\/a> shows such a radio image of a distant galaxy, where radio telescopes reveal vast jets and complicated regions of radio emissions that are completely invisible in photographs taken with light.<\/p>\n<figure id=\"OSC_Astro_06_04_RadioImage\">\n<div class=\"title\" style=\"text-align: center\"><strong>Radio Image of a Galaxy in Cygnus A<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_RadioImage-1.jpg\" alt=\"False color radio image of galaxy Cygnus A. This image shows two huge, diffuse clouds (lobes) of hot gas on either side of the galaxy. Thin jets of material are also seen, one on each side, connecting the galaxy to the lobes. The lobes are shown in three colors corresponding to the intensity of the radio energy detected. Blue is least intense and is concentrated in the regions of the lobes closest to the galaxy. Green is next and is located near the center and far edges of the lobes. Finally red is the most intense and is found at the edges of the lobes farthest from the galaxy.\" width=\"487\" height=\"279\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 2.<\/strong> This image has been constructed of radio observations at the Very Large Array of a galaxy called Cygnus A. Colours have been added to help the eye sort out regions of different radio intensities. Red regions are the most intense, blue the least. The visible galaxy would be a small dot in the centre of the image. The radio image reveals jets of expelled material (more than 160,000 light-years long) on either side of the galaxy. (credit: NRAO\/AUI)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1167470679884\">Radio astronomy is a young field compared with visible-light astronomy, but it has experienced tremendous growth in recent decades. The world\u2019s largest radio reflectors that can be pointed to any direction in the sky have apertures of 100 meters. One of these has been built at the US National Radio Astronomy Observatory in West Virginia as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_GreenBank\">Figure 3<\/a>. <a class=\"autogenerated-content\" href=\"#fs-id1167470936824\">Figure 4<\/a> lists some of the major radio telescopes of the world.<\/p>\n<figure id=\"OSC_Astro_06_04_GreenBank\">\n<div class=\"title\" style=\"text-align: center\"><strong>Robert C. Byrd Green Bank Telescope.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_GreenBank-1.jpg\" alt=\"Photograph of the Robert C. Byrd radio telescope at Green Bank, West Virginia.\" width=\"975\" height=\"418\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 3.<\/strong> This fully steerable radio telescope in West Virginia went into operation in August 2000. Its dish is about 100 meters across. (credit: modification of work by \u201cb3nscott\u201d\/Flickr)<\/figcaption><\/figure>\n<\/figure>\n<table id=\"fs-id1167470936824\" class=\"span-all\" summary=\"This table has 4 columns and 22 rows. The first row is a header with the values, \u201cObservatory\u201d, \u201cLocation\u201d, \u201cDescription\u201d, and \u201cWebsite\u201d. The second row is another header which spans all 4 columns, and has the value, \u201cIndividual Radio Dishes\u201d. Under the column labeled \u201cObservatory\u201d are the values, \u201cArecibo Observatory\u201d, \u201cGreen Bank Telescope (GBT)\u201d, \u201cEffelsberg 100-m Telescope\u201d, \u201dLovell Telescope\u201d, \u201cCanberra Deep Space Communication Complex (CDSCC)\u201d, \u201cGoldstone Deep Space Communications Complex (GDSCC)\u201d, and \u201cParkes Observatory\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cObservatory\u201d column continues with the values, \u201cSquare Kilometre Array (SKA)\u201d, \u201cAtacama Large Millimeter\/submillimeter Array (ALMA)\u201d, \u201cVery Large Array (VLA)\u201d, \u201cWesterbork Synthesis Radio Telescope (WSRT)\u201d, \u201cVery Long Baseline Array (VLBA)\u201d, \u201cAustralia Telescope Compact Array (ATCA)\u201d, and \u201cMulti-Element Radio Linked Interferometer Network (MERLIN)\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cObservatory\u201d column continues with the values, \u201cIRAM\u201d, \u201cJames Clerk Maxwell Telescope (JCMT)\u201d, \u201cNobeyama Radio Observatory (NRO)\u201d, and \u201cHat Creek Radio Observatory (HCRO)\u201d. Under the column labeled \u201cLocation\u201d are the values, \u201cArecibo, Puerto Rico\u201d, \u201cGreen Bank, WV\u201d, \u201cBonn, Germany\u201d, \u201cManchester, England\u201d, \u201cTidbinbilla, Australia\u201d, \u201cBarstow, CA\u201d, and \u201cParkes, Australia\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cLocation\u201d column continues with the values, \u201cSouth Africa and Western Australia\u201d, \u201cAtacama desert, Northern Chile\u201d, \u201cSocorro, New Mexico\u201d, \u201cWesterbork, the Netherlands\u201d, \u201cTen US sites, HI to the Virgin Islands\u201d, \u201cSeveral sites in Australia\u201d, and \u201cCambridge, England, and other British sites\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cLocation\u201d column continues with the values, \u201cGranada, Spain\u201d, \u201cMauna Kea, HI\u201d, \u201cMinamimaki, Japan\u201d, and \u201cCassel, CA\u201d. Under the column labeled \u201cDescription\u201d are the values, \u201c305-m fixed dish\u201d, \u201c110\u00d7100-m steerable dish\u201d, \u201c100-m steerable dish\u201d, \u201c76-m steerable dish,\u201d \u201c70-m steerable dish\u201d, \u201c70-m steerable dish\u201d, and \u201c64 m steerable dish\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cDescription\u201d column continues with the values, \u201cThousands of dishes, km2 collecting area, partial array in 2020\u201d, \u201c66 7-m and 12-m dishes\u201d, \u201c27-element array of 25-m dishes (36-km baseline)\u201d, \u201c12-element array of 25-m dishes (1.6-km baseline)\u201d, \u201c10-element array of 25-m dishes (9000-km baseline)\u201d, \u201c8-element array (seven 22-m dishes plus Parkes 64-m)\u201d, and \u201cNetwork of seven dishes (the largest is 32-m)\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cDescription\u201d column continues with the values, \u201c30-m steerable mm-wave dish\u201d, \u201c15 m steerable mm-wave dish\u201d, \u201c6-element array of 10-m wave dishes\u201d, and \u201c6-element array of 5-m wave dishes\u201d. Under the column labeled \u201cWebsite\u201d are the values, \u201cwww.naic.edu\u201d, \u201cwww.science.nrao.edu\/facilities\/gbt\u201d, \u201cwww.mpifr-bonn.mpg.de\/en\/effelsberg\u201d, \u201cwww.jb.man.ac.uk\/aboutus\/lovell\u201d, \u201cwww.cdscc.nasa.gov\u201d, \u201cwww.gdscc.nasa.gov\u201d, and \u201cwww.parkes.atnf.csiro.au\u201d. At this point is another header row which spans all 4 columns with the value, \u201cArrays of Radio Dishes\u201d. The \u201cWebsite\u201d column continues with the values, www.skatelescope.org, \u201cwww.almaobservatory.org\u201d, \u201cwww.science.nrao.edu\/facilities\/vla\u201d, \u201cwww.astron.nl\/radio-observatory\/public\/public-0\u201d, \u201cwww.science.nrao.edu\/facilities\/vlba\u201d, \u201cwww.narrabri.atnf.csiro.au\u201d, and \u201cwww.e-merlin.ac.uk\u201d. At this point is the last header row which spans all 4 columns with the value, \u201cMillimeter-wave Telescopes\u201d. The \u201cWebsite\u201d column continues with the values, \u201cwww.iram-institute.org\u201d, \u201cwww.eaobservatory.org\/jcmt\u201d, \u201cwww.nro.nao.ac.jp\/en\u201d, and \u201cwww.sri.com\/research-development\/specialized-facilities\/hat-creek-radio-observatory\u201d.\">\n<thead>\n<tr>\n<th colspan=\"4\">Figure 4. Major Radio Observatories of the World<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Observatory<\/th>\n<th>Location<\/th>\n<th>Description<\/th>\n<th>Website<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td colspan=\"4\">Individual Radio Dishes<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Arecibo Observatory<\/span><\/td>\n<td>Arecibo, Puerto Rico<\/td>\n<td>305-m fixed dish<\/td>\n<td>www.naic.edu<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Green Bank Telescope<\/span> (GBT)<\/td>\n<td>Green Bank, WV<\/td>\n<td>110 \u00d7 100-m steerable dish<\/td>\n<td>www.science.nrao.edu\/facilities\/gbt<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Effelsberg 100-m Telescope<\/span><\/td>\n<td>Bonn, Germany<\/td>\n<td>100-m steerable dish<\/td>\n<td>www.mpifr-bonn.mpg.de\/en\/effelsberg<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Lovell Telescope<\/span><\/td>\n<td>Manchester, England<\/td>\n<td>76-m steerable dish<\/td>\n<td>www.jb.man.ac.uk\/aboutus\/lovell<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Canberra Deep Space Communication Complex<\/span> (CDSCC)<\/td>\n<td>Tidbinbilla, Australia<\/td>\n<td>70-m steerable dish<\/td>\n<td>www.cdscc.nasa.gov<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Goldstone Deep Space Communications Complex<\/span> (GDSCC)<\/td>\n<td>Barstow, CA<\/td>\n<td>70-m steerable dish<\/td>\n<td>www.gdscc.nasa.gov<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Parkes Observatory<\/span><\/td>\n<td>Parkes, Australia<\/td>\n<td>64-m steerable dish<\/td>\n<td>www.parkes.atnf.csiro.au<\/td>\n<\/tr>\n<tr>\n<td colspan=\"4\">Arrays of Radio Dishes<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Square Kilometre Array<\/span> (SKA)<\/td>\n<td>South Africa and Western Australia<\/td>\n<td>Thousands of dishes, km<sup>2<\/sup> collecting area, partial array in 2020<\/td>\n<td>www.skatelescope.org<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Atacama Large Millimeter\/submillimeter Array<\/span> (ALMA)<\/td>\n<td>Atacama desert, Northern Chile<\/td>\n<td>66 7-m and 12-m dishes<\/td>\n<td>www.almaobservatory.org<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Very Large Array<\/span> (VLA)<\/td>\n<td>Socorro, New Mexico<\/td>\n<td>27-element array of 25-m dishes (36-km baseline)<\/td>\n<td>www.science.nrao.edu\/facilities\/vla<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Westerbork Synthesis Radio Telescope<\/span> (WSRT)<\/td>\n<td>Westerbork, the Netherlands<\/td>\n<td>12-element array of 25-m dishes (1.6-km baseline)<\/td>\n<td>www.astron.nl\/radio-observatory\/public\/public-0<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Very Long Baseline Array<\/span> (VLBA)<\/td>\n<td>Ten US sites, HI to the Virgin Islands<\/td>\n<td>10-element array of 25-m dishes (9000 km baseline)<\/td>\n<td>www.science.nrao.edu\/facilities\/vlba<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Australia Telescope Compact Array<\/span> (ATCA)<\/td>\n<td>Several sites in Australia<\/td>\n<td>8-element array (seven 22-m dishes plus Parkes 64 m)<\/td>\n<td>www.narrabri.atnf.csiro.au<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Multi-Element Radio Linked Interferometer Network<\/span> (MERLIN)<\/td>\n<td>Cambridge, England, and other British sites<\/td>\n<td>Network of seven dishes (the largest is 32 m)<\/td>\n<td>www.e-merlin.ac.uk<\/td>\n<\/tr>\n<tr>\n<td colspan=\"4\">Millimeter-wave Telescopes<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">IRAM<\/span><\/td>\n<td>Granada, Spain<\/td>\n<td>30-m steerable mm-wave dish<\/td>\n<td>www.iram-institute.org<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">James Clerk Maxwell Telescope<\/span> (JCMT)<\/td>\n<td>Mauna Kea, HI<\/td>\n<td>15-m steerable mm-wave dish<\/td>\n<td>www.eaobservatory.org\/jcmt<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Nobeyama Radio Observatory<\/span> (NRO)<\/td>\n<td>Minamimaki, Japan<\/td>\n<td>6-element array of 10-m wave dishes<\/td>\n<td>www.nro.nao.ac.jp\/en<\/td>\n<\/tr>\n<tr>\n<td><span class=\"no-emphasis\">Hat Creek Radio Observatory<\/span> (HCRO)<\/td>\n<td>Cassel, CA<\/td>\n<td>6-element array of 5-m wave dishes<\/td>\n<td>www.sri.com\/research-development\/specialized-facilities\/hat-creek-radio-observatory<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/section>\n<section id=\"fs-id1167470853879\">\n<h1>Radio Interferometry<\/h1>\n<p id=\"fs-id1167470615520\">As we discussed earlier, a telescope\u2019s ability to show us fine detail (its resolution) depends upon its aperture, but it also depends upon the wavelength of the radiation that the telescope is gathering. The longer the waves, the harder it is to resolve fine detail in the images or maps we make. Because radio waves have such long wavelengths, they present tremendous challenges for astronomers who need good resolution. In fact, even the largest radio dishes on Earth, operating alone, cannot make out as much detail as the typical small visible-light telescope used in a college astronomy lab. To overcome this difficulty, radio astronomers have learned to sharpen their images by linking two or more radio telescopes together electronically. Two or more telescopes linked together in this way are called an interferometer.<\/p>\n<p id=\"fs-id1167470755666\">\u201cInterferometer\u201d may seem like a strange term because the telescopes in an interferometer work cooperatively; they don\u2019t \u201cinterfere\u201d with each other. Interference, however, is a technical term for the way that multiple waves interact with each other when they arrive in our instruments, and this interaction allows us to coax more detail out of our observations. The resolution of an interferometer depends upon the separation of the telescopes, not upon their individual apertures. Two telescopes separated by 1 kilometer provide the same resolution as would a single dish 1 kilometer across (although they are not, of course, able to collect as much radiation as a radio-wave bucket that is 1 kilometer across).<\/p>\n<p>To get even better resolution, astronomers combine a large number of radio dishes into an interferometer array. In effect, such an array works like a large number of two-dish interferometers, all observing the same part of the sky together. Computer processing of the results permits the reconstruction of a high-resolution radio image. The most extensive such instrument in the United States is the National Radio Astronomy Observatory\u2019s Very Large Array (VLA) near Socorro, New Mexico. It consists of 27 movable radio telescopes (on railroad tracks), each having an aperture of 25 meters, spread over a total span of about 36 kilometers. By electronically combining the signals from all of its individual telescopes, this array permits the radio astronomer to make pictures of the sky at radio wavelengths comparable to those obtained with a visible-light telescope, with a resolution of about 1 arcsecond.<\/p>\n<p id=\"fs-id1167471083541\">The Atacama Large Millimeter\/submillimeter array (ALMA) in the Atacama Desert of Northern Chile as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_ALMA\">Figure 5<\/a>, at an altitude of 16,400 feet, consists of 12 7-meter and 54 12-meter telescopes, and can achieve baselines up to 16 kilometers. Since it became operational in 2013, it has made observations at resolutions down to 6 milliarcseconds (0.006 arcseconds), a remarkable achievement for radio astronomy.<\/p>\n<figure id=\"OSC_Astro_06_04_ALMA\">\n<div class=\"title\" style=\"text-align: center\"><strong>Atacama Large Millimeter\/Submillimeter Array (ALMA).<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_ALMA-1.jpg\" alt=\"Photograph of the Atacama Large Millimeter Array in Chile, taken at night. Many of the telescopes are seen pointing in various directions, with the Moon and Milky Way prominent in the background sky.\" width=\"975\" height=\"273\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5.<\/strong> Located in the Atacama Desert of Northern Chile, ALMA currently provides the highest resolution for radio observations. (credit: ESO\/S. Guisard)<\/figcaption><\/figure>\n<\/figure>\n<div id=\"fs-id1167470625259\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">Watch this <a href=\"https:\/\/youtu.be\/_Ryctl1Gij4\">documentary<\/a>\u00a0from the European Space Agency that explains the work that went into designing and building ALMA, discusses some of its first images, and explores its future.\u00a0 The URL is:<a href=\"https:\/\/youtu.be\/_Ryctl1Gij4\">\u00a0https:\/\/youtu.be\/_Ryctl1Gij4\u00a0<\/a><\/div>\n<\/div>\n<p id=\"fs-id1167470963381\">Initially, the size of interferometer arrays was limited by the requirement that all of the dishes be physically wired together. The maximum dimensions of the array were thus only a few tens of kilometres. However, larger interferometer separations can be achieved if the telescopes do not require a physical connection. Astronomers, with the use of current technology and computing power, have learned to time the arrival of electromagnetic waves coming from space very precisely at each telescope and combine the data later. If the telescopes are as far apart as California and Australia, or as West Virginia and Crimea in Ukraine, the resulting resolution far surpasses that of visible-light telescopes.<\/p>\n<p id=\"fs-id1167471061686\">The United States operates the Very Long Baseline Array (VLBA), made up of 10 individual telescopes stretching from the Virgin Islands to Hawaii as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_Baseline\">Figure 6<\/a>. The VLBA, completed in 1993, can form astronomical images with a resolution of 0.0001 arcseconds, permitting features as small as 10 astronomical units (AU) to be distinguished at the center of our Galaxy.<\/p>\n<figure id=\"OSC_Astro_06_04_Baseline\">\n<div class=\"title\" style=\"text-align: center\"><strong>Very Long Baseline Array.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_Baseline-1.jpg\" alt=\"Diagram of the Very Long Baseline Array. The image shows the Northern Hemisphere of Earth centered on North America. Icons of radio antennas are shown distributed throughout the continental United States, as well as on Hawai\u2019i and Puerto Rico.\" width=\"975\" height=\"216\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6.<\/strong> This map shows the distribution of 10 antennas that constitute an array of radio telescopes stretching across the United States and its territories.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1167470602103\">Recent advances in technology have also made it possible to do interferometry at visible-light and infrared wavelengths. At the beginning of the twenty-first century, three observatories with multiple telescopes each began using their dishes as interferometers, combining their light to obtain a much greater resolution. In addition, a dedicated interferometric array was built on Mt. Wilson in California. Just as in radio arrays, these observations allow astronomers to make out more detail than a single telescope could provide.<\/p>\n<table id=\"fs-id1167470667239\" class=\"span-all\" summary=\"This table has 5 columns and 4 rows. The first row is a header with the values \u201cLongest Baseline (m)\u201d, \u201cTelescope Name\u201d, \u201cLocation\u201d, \u201cMirrors\u201d, and \u201cStatus\u201d. Under the \u201cLongest Baseline (m)\u201d column are the values, \u201c400\u201d, \u201c200\u201d, \u201c85\u201d, and \u201c22.8\u201d. Under \u201cTelescope Name\u201d are the values, \u201c\u201cCHARA Array(Center for High Angular Resolution Astronomy)\u201d, \u201cVery Large Telescope\u201d, \u201cKeck I and II telescopes\u201d, and \u201cLarge Binocular Telescope\u201d. Under \u201cLocation\u201d are the values, Mount Wilson, CA\u201d, \u201cCerro Paranal, Chile\u201d, \u201cMauna Kea, HI\u201d, and \u201cMount Graham, AZ\u201d. Under the \u201cMirrors\u201d column are the values, \u201cSix 1-m telescopes\u201d, \u201cFour 8.2-m telescopes\u201d, \u201cTwo 10-m telescopes\u201d, and \u201cTwo 8.4-m telescopes\u201d. Under \u201cStatus\u201d are the values, \u201cOperational since 2004\u201d, \u201cCompleted 2000\u201d, \u201cOperated from 2001 to 2012\u201d, and \u201cFirst light 2004\u201d.\">\n<thead>\n<tr>\n<th colspan=\"5\">Visible-Light Interferometers<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Longest Baseline (m)<\/th>\n<th>Telescope Name<\/th>\n<th>Location<\/th>\n<th>Mirrors<\/th>\n<th>Status<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>400<\/td>\n<td><span class=\"no-emphasis\">CHARA<\/span> Array (Center for High Angular Resolution Astronomy)<\/td>\n<td>Mount Wilson, CA<\/td>\n<td>Six 1-m telescopes<\/td>\n<td>Operational since 2004<\/td>\n<\/tr>\n<tr>\n<td>200<\/td>\n<td><span class=\"no-emphasis\">Very Large Telescope<\/span><\/td>\n<td>Cerro Paranal, Chile<\/td>\n<td>Four 8.2-m telescopes<\/td>\n<td>Completed 2000<\/td>\n<\/tr>\n<tr>\n<td>85<\/td>\n<td><span class=\"no-emphasis\">Keck I and II<\/span> telescopes<\/td>\n<td>Mauna Kea, HI<\/td>\n<td>Two 10-m telescopes<\/td>\n<td>Operated from 2001 to 2012<\/td>\n<\/tr>\n<tr>\n<td>22.8<\/td>\n<td><span class=\"no-emphasis\">Large Binocular Telescope<\/span><\/td>\n<td>Mount Graham, AZ<\/td>\n<td>Two 8.4-m telescopes<\/td>\n<td>First light 2004<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/section>\n<section id=\"fs-id1167470658378\">\n<h1>CHIME Radio Telescope in British Columbia, Canada<\/h1>\n<\/section>\n<p>The\u00a0<b>Canadian Hydrogen Intensity Mapping Experiment<\/b>\u00a0(<b>CHIME<\/b>) is an\u00a0<a class=\"mw-redirect\" title=\"Interferometer\" href=\"https:\/\/en.wikipedia.org\/wiki\/Interferometer\">interferometric<\/a>\u00a0<a title=\"Radio telescope\" href=\"https:\/\/en.wikipedia.org\/wiki\/Radio_telescope\">radio telescope<\/a>\u00a0at the\u00a0<a title=\"Dominion Radio Astrophysical Observatory\" href=\"https:\/\/en.wikipedia.org\/wiki\/Dominion_Radio_Astrophysical_Observatory\">Dominion Radio Astrophysical Observatory<\/a>\u00a0in\u00a0<a title=\"British Columbia\" href=\"https:\/\/en.wikipedia.org\/wiki\/British_Columbia\">British Columbia<\/a>,\u00a0<a title=\"Canada\" href=\"https:\/\/en.wikipedia.org\/wiki\/Canada\">Canada<\/a>\u00a0which consists of four\u00a0<a title=\"Antenna (radio)\" href=\"https:\/\/en.wikipedia.org\/wiki\/Antenna_(radio)\">antennas<\/a>\u00a0consisting of 100 x 20\u00a0<a title=\"Metre\" href=\"https:\/\/en.wikipedia.org\/wiki\/Metre\">metre<\/a>\u00a0cylindrical\u00a0<a title=\"Parabolic reflector\" href=\"https:\/\/en.wikipedia.org\/wiki\/Parabolic_reflector\">parabolic reflectors<\/a>\u00a0(roughly the size and shape of snowboarding\u00a0<a title=\"Half-pipe\" href=\"https:\/\/en.wikipedia.org\/wiki\/Half-pipe\">half-pipes<\/a>) with 1024 dual-polarization radio receivers suspended on a support above them. The antenna receives radio waves from\u00a0<a title=\"Hydrogen\" href=\"https:\/\/en.wikipedia.org\/wiki\/Hydrogen\">hydrogen<\/a>\u00a0in space at\u00a0<a title=\"Frequency\" href=\"https:\/\/en.wikipedia.org\/wiki\/Frequency\">frequencies<\/a>\u00a0in the 400\u2013800\u00a0<a title=\"Hertz\" href=\"https:\/\/en.wikipedia.org\/wiki\/Hertz\">MHz<\/a>\u00a0range. The telescope&#8217;s\u00a0<a title=\"Low-noise amplifier\" href=\"https:\/\/en.wikipedia.org\/wiki\/Low-noise_amplifier\">low-noise amplifiers<\/a>\u00a0are built with components adapted from the cellphone industry and its data are processed using a custom-built\u00a0<a class=\"mw-redirect\" title=\"FPGA\" href=\"https:\/\/en.wikipedia.org\/wiki\/FPGA\">FPGA<\/a>\u00a0electronic system and 1000-processor high-performance\u00a0<a title=\"General-purpose computing on graphics processing units\" href=\"https:\/\/en.wikipedia.org\/wiki\/General-purpose_computing_on_graphics_processing_units\">GPGPU<\/a>\u00a0cluster.<sup id=\"cite_ref-nature\/news_1-0\" class=\"reference\"><a href=\"https:\/\/en.wikipedia.org\/wiki\/Canadian_Hydrogen_Intensity_Mapping_Experiment#cite_note-nature\/news-1\">[1]<\/a><\/sup>The telescope has no moving parts and observes half of the sky each day as the Earth turns. It has also turned out to be a superior instrument for observing the recently discovered phenomenon of\u00a0<a title=\"Fast radio burst\" href=\"https:\/\/en.wikipedia.org\/wiki\/Fast_radio_burst\">fast radio bursts<\/a>\u00a0(FRBs).<\/p>\n<p>CHIME is a partnership between the\u00a0<a title=\"University of British Columbia\" href=\"https:\/\/en.wikipedia.org\/wiki\/University_of_British_Columbia\">University of British Columbia<\/a>,\u00a0<a title=\"McGill University\" href=\"https:\/\/en.wikipedia.org\/wiki\/McGill_University\">McGill University<\/a>, the\u00a0<a title=\"University of Toronto\" href=\"https:\/\/en.wikipedia.org\/wiki\/University_of_Toronto\">University of Toronto<\/a>\u00a0and the Canadian\u00a0<a title=\"National Research Council (Canada)\" href=\"https:\/\/en.wikipedia.org\/wiki\/National_Research_Council_(Canada)\">National Research Council<\/a>&#8216;s\u00a0<a title=\"Dominion Radio Astrophysical Observatory\" href=\"https:\/\/en.wikipedia.org\/wiki\/Dominion_Radio_Astrophysical_Observatory\">Dominion Radio Astrophysical Observatory<\/a>. A\u00a0<a title=\"First light (astronomy)\" href=\"https:\/\/en.wikipedia.org\/wiki\/First_light_(astronomy)\">first light<\/a>\u00a0ceremony was held on 7 September 2017 to inaugurate the commissioning phase.<\/p>\n<section id=\"fs-id1167470658378\">\n<figure id=\"attachment_1668\" aria-describedby=\"caption-attachment-1668\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-1668 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-1024x683.jpg\" alt=\"large radio telescope\" width=\"1024\" height=\"683\" srcset=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-1024x683.jpg 1024w, https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-300x200.jpg 300w, https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-768x512.jpg 768w, https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-65x43.jpg 65w, https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-225x150.jpg 225w, https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/Canadian_Hydrogen_Intensity_Mapping_Experiment_-_overall-350x233.jpg 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-1668\" class=\"wp-caption-text\">CHIME telescope in British Columbia, Canada. Photo credit: Image from Wikipedia CC BY-SA 4.0 Z22 This telescope has found a large number of FRBs = Fast Radio Bursts as of 2019.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h1>Radar Astronomy<\/h1>\n<p id=\"fs-id1167470691541\">Radar is the technique of transmitting radio waves to an object in our solar system and then detecting the radio radiation that the object reflects back. The time required for the round trip can be measured electronically with great precision. Because we know the speed at which radio waves travel (the speed of light), we can determine the distance to the object or a particular feature on its surface (such as a mountain).<\/p>\n<p id=\"fs-id1167470613198\">Radar observations have been used to determine the distances to planets and how fast things are moving in the solar system (using the Doppler effect, discussed in the <a class=\"target-chapter\" href=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/5-0-thinking-ahead\/\">Radiation and Spectra<\/a> chapter). Radar waves have played important roles in navigating spacecraft throughout the solar system. In addition, as will be discussed in later chapters, radar observations have determined the rotation periods of Venus and Mercury, probed tiny Earth-approaching asteroids, and allowed us to investigate the mountains and valleys on the surfaces of Mercury, Venus, Mars, and the large moons of Jupiter.<\/p>\n<p id=\"fs-id1167470586562\">Any radio dish can be used as a radar telescope if it is equipped with a powerful transmitter as well as a receiver. The most spectacular facility in the world for radar astronomy is the 1000-foot (305-meter) telescope at Arecibo in Puerto Rico (<a class=\"autogenerated-content\" href=\"#OSC_Astro_06_04_Largest\">[link]<\/a>). The Arecibo telescope is too large to be pointed directly at different parts of the sky. Instead, it is constructed in a huge natural \u201cbowl\u201d (more than a mere dish) formed by several hills, and it is lined with reflecting metal panels. A limited ability to track astronomical sources is achieved by moving the receiver system, which is suspended on cables 100 meters above the surface of the bowl. An even larger (500-meter) radar telescope is currently under construction. It is the <span class=\"no-emphasis\">Five-hundred-meter Aperture Spherical Telescope<\/span> (FAST) in China and is expected to be completed in 2016.<\/p>\n<figure id=\"OSC_Astro_06_04_Largest\">\n<div class=\"title\" style=\"text-align: center\"><strong>Largest Radio and Radar Dish.<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_06_04_Largest-1.jpg\" alt=\"Photograph of Arecibo Observatory in Puerto Rico, seen from above. The huge 1000-ft metal dish is built into a natural depression in the mountains.\" width=\"487\" height=\"364\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6.<\/strong> The Arecibo Observatory, with its 1000-foot radio dish-filling valley in Puerto Rico, is part of the National Astronomy and Ionosphere Center, operated by SRI International, USRA, and UMET under a cooperative agreement with the National Science Foundation. (credit: National Astronomy and Ionosphere Center, Cornell U., NSF)<\/figcaption><\/figure>\n<\/figure>\n<\/section>\n<section id=\"fs-id1167470881669\" class=\"summary\">\n<p id=\"fs-id1167470732320\">In the 1930s, radio astronomy was pioneered by Karl G. Jansky and Grote Reber. A radio telescope is basically a radio antenna (often a large, curved dish) connected to a receiver. Significantly enhanced resolution can be obtained with interferometers, including interferometer arrays like the 27-element VLA and the 66-element ALMA. Expanding to very long baseline interferometers, radio astronomers can achieve resolutions as precise as 0.0001 arcsecond. Radar astronomy involves transmitting as well as receiving. The largest radar telescope currently in operation is a 305-meter bowl at Arecibo.<\/p>\n<\/section>\n<div>\n<h2>Glossary<\/h2>\n<dl id=\"fs-id1167470606241\" class=\"definition\">\n<dt>interference<\/dt>\n<dd id=\"fs-id1167470759046\">process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another<\/dd>\n<\/dl>\n<dl id=\"fs-id1167470662948\" class=\"definition\">\n<dt>interferometer<\/dt>\n<dd id=\"fs-id1167470680988\">instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes<\/dd>\n<\/dl>\n<dl id=\"fs-id1167470585101\" class=\"definition\">\n<dt>interferometer array<\/dt>\n<dd id=\"fs-id1167470607777\">combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers<\/dd>\n<\/dl>\n<dl id=\"fs-id1167470746741\" class=\"definition\">\n<dt>radar<\/dt>\n<dd id=\"fs-id1167470709119\">technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it<\/dd>\n<\/dl>\n<\/div>\n","protected":false},"author":9,"menu_order":5,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-197","chapter","type-chapter","status-publish","hentry"],"part":169,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/197","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/users\/9"}],"version-history":[{"count":8,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/197\/revisions"}],"predecessor-version":[{"id":2747,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/197\/revisions\/2747"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/parts\/169"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/197\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/media?parent=197"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapter-type?post=197"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/contributor?post=197"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/license?post=197"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}