THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC Volume 1, Number 11 - June 1990 ########################### TABLE OF CONTENTS ########################### * ASA Membership/Article Submission Information * A Comparison of Optical and Radio Astronomy - David J. Babulski ########################### ASA MEMBERSHIP INFORMATION The Electronic Journal of the Astronomical Society of the Atlantic (EJASA) is published monthly by the Astronomical Society of the Atlantic, Inc. The ASA is a non-profit organization dedicated to the advancement of amateur and professional astronomy and space exploration, and to the social and educational needs of its members. Membership application is open to all with an interest in astronomy and space exploration. 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ASA Officers and Council - President - Don Barry Vice President - Bill Bagnuolo Secretary - Ken Poshedly Treasurer - Alan Fleming Board of Advisors - Edward Albin, Jim Bitsko, Bill Hartkopf Council - Jim Bitsko, Julian Crusselle, Toni Douglas, Eric Greene, Larry Klaes, Becky Long, Max Mirot, Paul Pirillo, Patti Provost, Michael Wiggs ARTICLE SUBMISSIONS - Article submissions on astronomy and space exploration to the EJASA are most welcome. Please send your on-line articles in ASCII format to Larry Klaes, EJASA Editor, at the following net addresses or the above Society addresses: klaes@wrksys.enet.dec.com or - ...!decwrl!wrksys.enet.dec.com!klaes or - klaes%wrksys.dec@decwrl.enet.dec.com or - klaes%wrksys.enet.dec.com@uunet.uu.net You may also use the above net addresses for EJASA backissue requests, letters to the editor, and ASA membership information. Please be certain to include either a network or regular mail address where you can be reached, a telephone number, and a brief biographical sketch. DISCLAIMER - Submissions are welcome for consideration. Articles submitted, unless otherwise stated, become the property of the Astronomical Society of the Atlantic. Though the articles will not be used for profit, they are subject to editing, abridgment, and other changes. Copying or reprinting of the EJASA, in part or in whole, is encouraged, provided clear attribution is made to the Astronomical Society of the Atlantic, the Electronic Journal, and the author(s). This Journal is Copyright (c) 1990 by the Astronomical Society of the Atlantic. A COMPARISON OF OPTICAL AND RADIO ASTRONOMY by David J. Babulski Electromagnetic radiation from space arrives at all wavelengths, from very short wavelength gamma rays to long wavelength radio waves. This wide spectrum of electromagnetic radiation is constantly bombarding the atmosphere of Earth. We humans live on the surface of the planet at the bottom of a giant ocean of atmosphere. This several-hundred-kilometer thick layer of mixed gases filters out most of the impinging electromagnetic radiation, leaving only two substantial "windows" into the Cosmos. The narrower of these two primary windows is called the Optical Window and includes three portions of the spectrum: The near-ultraviolet, that is, wavelengths longer than about three thousand Angstroms (an Angstrom is equal to one-hundred-millionth of a centimeter) or 0.3 micrometers (a micrometer is equal to ten to the negative six power or 0.000010 meter). The "visible" section, which includes the "colors" that we can see: Blue (about five thousand Angstroms or 0.5 micrometers) to red (about seven thousand Angstroms or 0.7 micrometers). Portions of the infrared (with wavelengths up to forty Angstroms or 0.04 micrometers). The other, larger portal into the Cosmos is called the Radio Window. It includes wavelengths from about one millimeter to about twenty meters. The long wavelength cutoff depends somewhat on conditions in Earth's ionosphere. Before we continue with a comparison of optical and radio astronomy, it may be useful to discuss the major component that these two disciplines have in common: Electromagnetic radiation. Electromagnetic Radiation - To fully understand electromagnetic radiation, we must first travel back in time. Our first stop is to early Greek history circa 585 B.C. It was around this period that Thales of Miletus (c.624-c.547 B.C.) noted that bits of amber rubbed with fur mysteriously attracted feathers and other lightweight objects. Also during this era, the curious property of the mineral lodestone to attract bits of iron was observed. These observations were long regarded as curiosities and were generally ignored until the early years of the Nineteenth Century, when such curiosities were described in terms of electricity and magnetism. In the early 1800s, Charles A. de Coulomb (1736-1806) in France measured the electrostatic force acting against a spring. About the same time, Count Alessandro Volta (1745-1827) in Italy invented a battery which was the first practical source of continuous electrical current. Stepping ahead twenty years to 1820, Oersted of Denmark demonstrated that a magnetic field existed around a wire which was carrying electrical current. In this same time period, a French physicist named Andre Marie Ampere (1775-1836) described the attraction and repulsion between two wires carrying electrical current in the same or opposite directions. It was shortly after these discoveries in the early 1820s that Georg S. Ohm (1787-1854) of Germany developed a thermocouple of iron and copper bars. When the junction of these bars was placed in a fire, Ohm noted that electrical current was generated. By using various lengths of copper wire, he formulated the famous laws that bear his name. These laws relate the flow of electric charges (current) in a conductor to the electrical potential (voltage) and the reaction of the circuit (resistance). By the year 1830, it was fairly well known in scientific circles that when current flowed through a conductor, a magnetic field was formed around the wire. The reverse case was not an easy experiment to verify. Several luckless investigators tried to wrap a wire around a magnet to generate an electrical current in the wire. In 1831 an English physicist named Michael Faraday (1791-1867) almost accidentally discovered that it was a moving magnet and not a stationary one that produced an electrical current in an adjacent conductor. Faraday proposed the daring concept that all space was filled with various force fields: Magnetic, electric, radiant, thermal, and gravitational. Based on his field theory, Faraday daringly postulated the principles of electromagnetic induction and the field theory, which proved to be the missing links between electricity and magnetism. In the United States one decade later, a scientist by the name of Joseph Henry (1797-1878) demonstrated the concept that electromagnetic energy could be propagated through space and detected at a distance by noting the inductive action of a spark circuit picked up by a parallel circuit some nine meters (thirty feet) away. By the 1850s, the various bits and pieces about electricity and magnetism were well known and documented: Coulomb's Law covering the relationship between the electric charge and the mechanical force it produced; Ampere's Law relating electrical current and magnetism; Gauss' Law covering the relationship between electric charge and the electric field; and Faraday's Law, which explained the relationship between the magnetic field and the induced voltage. It was not until the late 1860s that the brilliant Scottish theoretician James Clerk Maxwell (1831-1879) unified these various laws into a complete electromagnetic theory which is today ranked in importance with Sir Isaac Newton's (1642-1727) concept of gravitation and Albert Einstein's (1879-1955) theory of relativity. James Clerk Maxwell - This Scottish physicist and mathematician is generally considered to be the greatest theoretical physicist of the Nineteenth Century. Maxwell became a student of the senior scientist Faraday, studying all of Faraday's work in order to become thoroughly familiar with his concepts and line of reasoning. Maxwell was intrigued by the apparent lack of a unified theory of electric and magnetic behavior. From Faraday's work and his own experiments, Maxwell derived a breathtaking concept and a set of elegant equations that encompassed the various laws of electricity and magnetism and which form today's basis of electromagnetic theory. The unique picture presented by Maxwell of the interchange between electric and magnetic fields in free space was supported by a set of equations descriptive of any form of wave motion propagating itself freely from place to place. Equation 1: div E = 0 Equation 2: div H = 0 Equation 3: curl E = -1/c dH/dt Equation 4: curl H = 1/c dE/dt In these equations, the term "E" represents the electric field, commonly expressed in Volts/meter, and the term "H" represents magnetic induction and is expressed in "Teslas" (1 Tesla = 1 Weber/meter squared). The term "Div", referring to the mathematical operation "Divergence" is a measure of the total flow of E or H into or out of a point in space. Similarly, the term "Curl" is a mathematical operation measuring "vorticity", the amount of whirlpool-like structure about a point in space. A detailed analysis of Maxwell's Equations is beyond the scope of this article. However, a brief discussion of what each of the four equations describes follows: Equation 1 states that in the absence of electric charges, electric lines of force can neither be created nor destroyed. Equation 2 states that in the absence of electric charges, magnetic lines of force can neither be created nor destroyed. In addition, Equation 2 states that magnetic charges do not exist. Equation 3 is a generalized statement of Faraday's Law that a changing magnetic field produces an electric field and that the ratio of the electrostatic units to the electromagnetic units is a constant related to the speed of light (300,000 kilometers per second/186,000 miles per second). Equation 4 states that a changing electric field produces a magnetic field. These four equations also showed that a "disturbance" in the electromagnetic field did not die out, but rather could propagate outwards from its origin in the form of waves traveling at the speed of light. Maxwell termed such a disturbance in the electric and magnetic fields an "electromagnetic wave" and pictured it as an electric wave at right angles to a magnetic field with both waves propagating in phase. Maxwell was able to show that these waves have all the known properties of light. Light was therefore discovered to be an electromagnetic phenomenon. As the 1800s neared an end, Heinrich Hertz (1857-1894) of Germany succeeded in generating electromagnetic waves of long wavelength, which we now call radio waves. Further, Hertz found that the velocity of these long waves was exactly equal to the velocity of light. Thanks to Maxwell and Hertz, we now know that light and radio waves are both forms of electromagnetic radiation, with light being composed of very short wavelength and radio waves composed of very long wavelength. Equation 5 shows the mathematical relationship between wavelength and frequency: v = c/lambda In Equation 5, the symbol "c" equals the speed of light in meters per second; the Greek letter lambda is the wavelength in meters; and the symbol "v" equals the frequency in Hertz (or cycles per second). A more convenient arrangement is to convert Hertz to megahertz (abbreviated "MHz"). The revised mathematical expression is shown in Equation 6: v (in MHz) = 300/lambda (in meters) To see how this relationship works, let us convert the mid-point wavelength of the Optical and Radio windows to their corresponding frequencies. The mid-point wavelength of the Optical window is about two micrometers (or 0.000002 meter). Using the relationship in Equation 6, the corresponding frequency is 150 million MHz - a very high frequency indeed! At the other extreme, the mid-point wavelength of the Radio window is about 0.5 meters. Again, using the relationship in Equation 6, the corresponding frequency is about six thousand MHz, or six gigahertz (GHz). Most amateur radio telescopes operate in the meter wavelength range. For example, the radio telescopes described later in this series of articles will operate at a wavelength of about 2.9 meters. Using the relationship in Equation 6, the corresponding frequency is about 103 MHz. With this perspective in mind, we will now look at the instruments used to investigate these two regions of the electromagnetic spectrum. Short Wavelength Astronomy - For millennia, humans were able to observe the Cosmos with only their unaided eyes. History has shown that the construction of major observatories has been a continuous practice for a wide variety of cultures of varying levels of sophistication. However, for most of recorded time, these ancient observatories were concerned almost exclusively with astrology and not astronomy. Telescopes, as we know them today, were a relatively late addition to the complement of instruments in observatories. This is somewhat surprising, as the technology for making glass is ancient. The Mesopotamians were the first to fuse sand and ash together to form glass. By the year 1200 B.C., the Babylonians knew how to make objects of blown glass and the Chinese were manufacturing mirrors long before the birth of Jesus Christ. However, no one put this technology together to make a telescope until sometime in the early 1600s. Spectacles were in use in Europe by the late 1300s, and it was probably a happy accident by an obscure spectacle-maker that resulted in the first telescope. By chance, if one aligned two lenses, one concave and the other convex, objects far away appeared to be closer. Once the first telescope was made, word traveled quickly throughout Europe - so quickly, in fact, that it was impossible to ascertain the original inventor. Most scholars now believe that the actual inventor of the telescope was Dutch spectacle maker Jan Lippershey. On August 25, 1609, the Italian astronomer Galileo Galilei (1564-1642) demonstrated one of his first telescopes, which had a magnification factor of nine, to officials of the Venetian government. A magnification factor of nine, or 9x, means that the object being viewed appears nine times closer than it actually is. Although he had won over the Venetian officials, Galileo did not use this new instrument to observe the heavens right away. First he designed a more stable mount, then improved the optics to raise the magnification factor to thirty. Perhaps more importantly, Galileo had to gain confidence in this new instrument, the "telescope". In the early 1600s, the eyes were believed to be the final authority about the truth of sizes, shapes, and colors. Lenses and mirrors were known to distort images by enlarging them, reducing them, or even inverting them. Galileo spent several months and countless experiments to convince himself that what he observed through the telescope was indeed real. Once Galileo did turn the telescope toward the night sky, humanity's view of the Universe was changed forever. His observations of the four largest moons of the planet Jupiter and their motions demonstrated without doubt that Earth was not the center of orbital motion. This observation established empirically that one body (a moon in this case) could orbit a second (Jupiter) while the second body, in turn, orbited a third body (Earth's star, the Sun). The realization that the Milky Way Galaxy was composed of countless individual stars brought forth the idea that the Universe could be infinite in extent. The Refracting Telescope - The telescope that Galileo used to make these startling observations was a refractor telescope. A refractor consists of an objective lens at one end of a long tube and one or more lenses (which form the eyepiece) at the other end. Contrary to popular belief, it is the diameter of the objective and not the magnification of the eyepiece that is important in any telescope. The primary job of any telescope is to gather as much electromagnetic energy (in this case, visible light) as possible. An image of a far away object, say a star, is a point of light surrounded by a diffraction disk. The resolving power of a telescope (or how much "detail" can be revealed) is dependent on the size of the diffraction disk. The smaller the diffraction disk, the more detail can be resolved. The size of the diffraction disk is, in turn, directly related to the wavelength of the electromagnetic energy (light in this case) and the diameter of the telescope objective. In general terms, the larger the objective the smaller the diffraction disk. If the wavelength and objective diameter are measured in the same units (for example, both measured in centimeters), then the smallest angle alpha (Greek letter) in seconds of arc that can be resolved by a lens (or mirror) of diameter "d" is given by the relationship shown in Equation 7: alpha = 2.1 x 10 (to the fifth power) x lambda/d (arcseconds) The formula shown in Equation 7 is called the Dawes' Criterion. It represents the theoretical resolution for a telescope. An example for telescopes of ten-centimeter (four-inch) and twenty-centimeter (eight-inch) diameter objectives may be instructive: In the examples shown in Table 1, wavelength (lambda) will be ten to the negative five centimeters (or 5,500 Angstroms - visible light in the yellow-green portion of the spectrum). Table 1: Maximum Resolution of Optics (Dawes' Limit) Aperture Resolution 60 mm 1.93" 3" 1.52" 4.5" 1.02" 6" 0.75" 8" 0.58" 10" 0.46" 12.5" 0.37" 16" 0.29" From the table, a ten-centimeter (four-inch) diameter telescope has a Dawes' Limit of 1.16 seconds of arc. This means that two celestial objects separated by 1.16 seconds of arc or greater will be resolved as two separate objects. Celestial objects separated by less than 1.16 seconds of arc will not be resolved and will appear as one object. To give you some sense of scale, the diameter of a period appearing on this screen/page is about 180 seconds of arc when viewed from a distance of nine meters (three feet). That same period would subtend a distance of 0.023 seconds of arc when viewed from a distance of 0.8 kilometers (0.5 miles). By increasing the diameter of the telescope objective, the Dawes' Limit is correspondingly decreased. This same mathematical relationship also holds for radio telescopes, as we shall see. Telescopes larger than twenty-five centimeters (ten inches) cannot reach the Dawes limit except under exceptional seeing conditions because of turbulence in Earth's atmosphere. To achieve their true potential, methods must be employed such as speckle interferometry, which removes atmospheric blurring by analysis of many freeze-frame snapshots of an object. At the present time, the largest refracting telescope on Earth has an objective lens of one meter (forty inches) in diameter. This refractor is at the Yerkes Observatory in Williams Bay, Wisconsin. The instrument went into operation during the 1890s. Refractor telescopes tend to be somewhat expensive due to the high cost of the objective lens. Typically, modern refractor telescopes designed for the amateur astronomer tend to have objective diameters of ten centimeters (four inches) or less. The Reflecting Telescope - Most modern telescopes for short wavelength (optical) astronomy are of the reflector type. Unlike the refractor, the reflector telescope uses a parabolic mirror at the bottom of a long tube rather than a lens at the front of the tube. All of the large research telescopes are reflecting telescopes, with this type also being the most popular among amateur astronomers. The reflecting telescope was first conceived by James Gregory, a Scottish scientist, in 1663. Sir Isaac Newton, a British physicist, is generally credited with the first practical reflector telescope, which he built in 1668. There is a fundamental problem with using a mirror to collect and focus an image. To see the image you must place your eye at the point where the electromagnetic energy is focused (called the "prime focus"). Unfortunately, your head will get in the way and you will not see anything! Newton got around this problem by placing a small mirror at an angle in the middle of the telescope tube. This mirror deflects focused energy from the mirror to a point outside the tube. This type of reflector is called a Newtonian, in honor of its inventor. Another type of reflecting telescope was invented by the French mathematician Guillaume Cassegrain (c.1650-1700) in 1672. Cassegrain reflectors use a small convex mirror to intercept the energy from the primary mirror and reflect it back through a hole in the primary mirror. This type of telescope has the advantage of small physical size, as the energy path is folded inside the tube and is sometimes called a Catadioptric (meaning literally "many optical surfaces") telescope. A more recent reflecting telescope design which is very popular with amateur astronomers is the Schmidt-Cassegrain reflector. This is a catadioptric-type reflecting telescope using a spherical primary mirror at the bottom of the telescope tube and a lens-like corrector plate at the front of the tube. The corrector plate compensates for astigmatism from the spherical reflecting surface. The Cassegrain secondary mirror is mounted to the inside surface of the Schmidt corrector plate. This telescope type has the advantage of lower cost (It is cheaper to make a spherical mirror than a parabolic mirror) and small size with excellent field of view. The energy gathered by the telescopes described so far can be processed by our eyes. Because we can "see" the images produced by the telescope, astronomers use the term "magnitude" to describe the "brightness" (the amount of short wavelength energy flux, or light energy) received from a star or other luminous body. The system of magnitudes currently used by astronomers to denote the brightness of stars was invented by the ancient Greek astronomer Hipparchus (c.180-c.125 B.C.). The brightest stars Hipparchus saw in the sky he called first magnitude stars. Those about half as bright he called second magnitude and so on to the sixth magnitude, which were the dimmest stars he could see. Later, when telescopes came into use, astronomers extended the Hipparchus magnitude scale to describe the dimmer stars that they could see in their instruments. Over time astronomers have refined the magnitude scale so that each magnitude difference of one corresponds to a factor of 2.512 in light energy flux. The apparent magnitude scale is shown below: Sun (Sol) -27 Full Moon -12 Venus -4 Sirius -2 Unaided eye limit +5 to +6 Binocular limit +10 Pluto +14 Visual limit for large telescopes +20 Photographic limit for large telescopes +25 One should be careful not to confuse "apparent magnitude", which is how bright the star looks to us from Earth, with "absolute magnitude", which is how much light energy the star is actually producing. The absolute magnitude of a star is the apparent magnitude it would have if it were moved to a distance of exactly ten parsecs from Earth. Note: One parsec is a unit of astronomical distance equal to 3.26 light years. Therefore, ten parsecs is equal to a distance of 32.6 light years. For example: If the Sun was at a distance of ten parsecs from Earth, it would have an apparent magnitude of +4.8. The absolute magnitude range of other stars varies from -10 for the brightest to +15 for the dimmest. Now that you know about the most common instruments for short wavelength (optical) astronomy, let us next look at instruments for long wavelength (radio) astronomy. Long Wavelength Astronomy - Unlike optical astronomy, we humans cannot "see" energy flux at radio wavelengths. In order to process energy flux at radio wavelengths, we must use a collection device to capture the energy flux and a conversion device to convert the received energy to a form that we can readily process. Astronomers call the radio energy flux collection device an "antenna". Conversion of the received radio energy into a form that we can process is handled by a "receiver". In most cases, radio telescopes are categorized by the type and size of the antenna used. Most of the professional radio telescopes are categorized as Large Dish Antenna ("LDA") types. Because we can not "see" electromagnetic energy at radio wave-lengths, we must devise another way to describe the "brightness" of a radio source. Instead of the term "magnitude" to denote the brightness of an energy source, radio astronomers use the term "flux density". The basic unit of flux density is "Watts per Square meter per Hertz". However, as most radio sources are extremely small fractions of that value, the basic unit of flux density is defined as a flux unit. One flux unit is ten to the negative twenty-sixth power watt per square meter per hertz. To give you a sense of scale, the Sun has an average flux density of ten to the fourth power or ten thousand flux units at a frequency of about 150 MHz. Earth's Moon, in contrast, has a 150-MHz flux density of only 85 flux units. Unlike optical astronomy, the most powerful radio source in the sky is not the Sun, but an extragalactic radio source named Cassiopeia A at a 150-MHz flux density of seventeen thousand flux units! We can think of a radio telescope as analogous to an optical telescope in the following ways: The antenna in a radio telescope is analogous to the objective lens in an optical instrument. The Receiver is analogous to the eyepiece. Keep in mind that one does not "listen" to the output from a radio telescope in the same way that you listen to a conventional radio tuned to a typical radio station. The electromagnetic energy flux detected by the radio telescope covers a very wide band of frequencies and sounds very much like static. A conventional radio uses a radio frequency filtering system to select a very narrow range of frequencies and reject all others (as when you "tune in" a radio station). A radio telescope receiver, on the other hand, uses a similar radio frequency filtering system, not to select a very narrow frequency band, but rather to reject narrow band frequency interference from adjacent radio services. We want the radio telescope receiver to be as wide band as possible, in order to capture as much radio energy flux as we can. In most cases, the output from a radio telescope is converted to digital code for computer processing or, as is the case with most amateur instruments, is converted to strip chart recorder output. In this later case, the position of a line on a sheet of graph paper represents the level of radio energy flux received by the radio telescope. We can take our optical telescope analogy one step further in describing the types of radio telescopes. Remember, there are two fundamental types of optical telescopes, the refractor and the reflector. In like fashion, there are two basic types of radio telescopes: The Multi-Element Array (MEA)-type antenna, which is roughly analogous to the optical refractor, and the Large Dish Antenna (LDA), which is roughly analogous to the optical reflector. Radio Telescope Refractors - The typical MEA-type antenna is based on the Yagi-Uda multi-element antenna, usually called a "Yagi", in honor of its primary inventor. A conventional television antenna is an example of a Yagi antenna. This type of energy collector concentrates incoming radio energy flux toward a collecting element (usually called a "driven element") by using electrically isolated parasitic elements in front of and behind the collecting element. An electrically conductive feedline (usually coaxial cable) is connected to the collecting element to convey the received radio frequency energy to the receiver. Electric currents are induced in the parasitic elements by the passing electromagnetic wave. These induced currents in turn generate wave fronts of radio frequency energy that constructively or destructively interfere with the incoming radio energy. By adjusting the spacing between the parasitic elements and the collecting element and the length of the parasitic elements, the induced wavefronts can cause incoming radio energy flux to be concentrated at the collecting element. The parasitic elements in front of the collecting element are called "directors" and the parasitic element behind the collecting element is called a "reflector". A multi-element antenna, such as the Yagi type just described, is said to be "directive" and has "gain". That is, the antenna is most sensitive or responds most readily to incoming radio energy flux in the direction that the directors are pointing. The term "gain" refers to the power gain of the antenna, or put another way, how much of the radio energy is concentrated toward the collecting element and is usually expressed in decibels (dB). Another interesting antenna used in arrays is the Helical antenna invented by Dr. John Kraus of the United States in the early 1940s. This type of antenna uses a continuous antenna element in the shape of a coil or helix working against a ground plane. A helical antenna has two characteristics that make it especially interesting for radio astronomy applications: The helix is circularly polarized (Contrast this with the Yagi antenna, which is linearly polarized), and its combination of broad bandwidth and high gain. Unfortunately, the cross-sectional area, or aperture, of a single, multi-element antenna is rather small. The cross-sectional area, or the "effective aperture", of an antenna is not as straightforward as it might seem, but is a function of both antenna directivity and the antenna design center wavelength. The mathematical relationship called Dawes' Criterion also applies to radio astronomy as well as the optical branch. As we have learned earlier, an optical telescope with an aperture (objective lens diameter) of ten centimeters (four inches) can resolve an astronomical object to within about one arcsecond. The primary difficulty with radio telescopes is that the wave-length of radio energy is much greater than that for visible light energy. For example: Radio energy at a wavelength of twenty centimeters is roughly 400,000 times longer than the wavelength of visible light. As a result, to resolve the same angle of one arcsecond at a wavelength of twenty centimeters, a radio telescope would have to be nearly forty kilometers (24.8 miles) in diameter. Fortunately, there are ways around this problem which will be discussed later. How much detail a radio telescope can resolve is a function of its "beamwidth". The "beam" is a cone-shaped bundle of radio energy of such an angular size that within it details cannot be resolved. The beam width would correspond in optical astronomy to the angular size of the central diffraction disk of a stellar image. For a large optical telescope, that size would be roughly a few hundredths of a second of arc, but for a typical large radio telescope, the beam width is on the order of several minutes of arc. The beam width of a typical single multi-element Yagi antenna (similar to what may be used in a small amateur radio telescope), for example, is on the order of fifteen degrees! However, by connecting a large number of multi-element antennas together electrically in phase, the beam width can be reduced to about one to two degrees - still a far cry from one to two arc seconds! The size of the antenna beam depends to a large degree on the wavelength. In general, the shorter the wavelength (the higher the frequency) the smaller the antenna beam. One way around the problem of large beamwidth is to use a techniques called "interferometry". In this technique, two identical radio telescope antennas are placed some distance apart. Incoming radio energy flux will strike one antenna a brief instant before the other one so that the two antennas will receive the same energy at slightly different times and thus will be "out of phase" with each other. The difference in phase between the two antennas can be measured electronically. Because this phase difference depends on the angle of the radio source relative to the antennas, positional information on the radio source can be readily obtained with this technique. One of the projects to be discussed in this series is a two-station radio interferometer. Radio Telescope Reflectors - In an effort to reduce antenna beamwidth and improve the sensitivity of a radio telescope to weak sources of radio energy flux, radio astronomers shorten the radio wavelength at which they will observe and employ large reflecting "dish" antennas to collect and concentrate the radio energy. The form of these reflecting radio telescopes closely follows the form of their optical counterparts. It is interesting to note that the highest gain antenna on Earth (148 dB) is the five-meter (two hundred-inch) telescope on Mount Palomar in California. The very short wavelength of light energy causes such a high gain to be realized. One of the most common radio telescope reflectors is the "Prime Focus" arrangement In this radio telescope configuration, the receiver is located at the prime focus. Another common radio telescope reflector arrangement is the Cassegrain system. This arrangement is similar to its optical counterpart in that a primary parabolic reflector and a convex secondary reflector are used to "fold" the radio energy path. This arrangement has the advantage of slightly smaller size and placement of the receiver where it is easier to adjust and service. The Cassegrain system (and its cousin, the Gregorian system, which uses a concave rather than a convex subreflector) is usually only used in very short wavelength radio astronomy (centimeter wavelengths and shorter). As the radio wavelength gets larger, the size of the sub-reflector must also get larger. A point is soon reached where the amount of energy blockage caused by the sub-reflector negates any advantage of the system. Summary - Speaking in general terms, both optical and radio astronomy are observing the same thing: Electromagnetic energy. The primary difference is the wavelength of the electromagnetic energy: Optical wavelengths being very short and radio wavelengths being very long. Since we can "see" optical wavelengths as visible light, our brain can be used in conjunction with our eyes to process information gathered by the telescope. Because we can not see in radio wavelengths, we typically use electronic devices to process the received energy collected by a radio telescope and convert it to a form that we can then "see". Astronomically speaking, the primary advantage of radio astronomy is its ability to detect energy sources hidden behind dark obscuring matter. This dark matter blocks the shorter wavelengths of visible light, but the longer wavelengths of radio energy are able to pass through the dark matter. Another application of this characteristic of radio energy is Radar Astronomy. In this application, a high energy pulse of electromagnetic energy, at radio wavelengths, is directed toward an astronomical object. The reflected energy is then used to deduce details about that object. For example, the surface of the planet Venus is hidden from view at optical wavelengths by the planet's thick obscuring clouds. By using radar astronomy, we have been able to create computer-generated images of what the surface of cloud-covered Venus looks like. Radar astronomy has also recently been used to study the surface of the planet Saturn's largest moon, Titan, which is shrouded in thick orange clouds of nitrogen and methane. Although radio astronomy has been a relatively late addition to the discipline of astronomy, it serves as a valuable "second eye" into the Cosmos. Because radio astronomy is a relatively young discipline, it offers the amateur radio astronomer a unique opportunity to work alongside the professional astronomical community to make valuable contributions to science. Future articles in EJASA will describe how to construct small, relatively inexpensive radio telescopes for use by amateur radio astronomers. Editor's Note - ASA members Bill Black and Jeff Lichtman coordinate a Radio Astronomy Interest Group for those who wish to pursue this important branch of observational astronomy. Both men operate their own radio telescopes: Bill's instrument detects meteors by reflection of distant television stations from ionized trails in the upper atmosphere. Jeff's dish measures intensities of variable sources. Low-cost availability of computers and radio components has opened the doors for amateur participation in this field. If you wish to learn more about the RAIG, please contact the ASA at one of the addresses listed in this EJASA. Jeff is also President of the national Society of Amateur Radio Astronomers (SARA), an organization of nearly 250 radio hobbyists. For more information on SARA, please contact Jeff at the following address: 1425 Parkmont Drive Roswell, Georgia 30076 Telephone: (404) 992-4959 Jeff is the author of "Getting Started in Amateur Radio Astronomy", in the February 1990 issue of EJASA. About the Author - David J. Babulski is an amateur astronomer and professional science writer, with particular interest in aerospace and radio astronomy. David's science articles have been published in such magazines as RADIO HANDBOOK and others. David is the author of "Radio Astronomy: A Historical Perspective", in the February 1990 issue of EJASA. THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC June 1990 - Vol. 1, No. 11 Copyright (c) 1990 - ASA -- Donald J. Barry (404) 651-2932 | don%chara@gatech.edu Center for High Angular Resolution Astronomy | President, Astronomical Georgia State University, Atlanta, GA 30303 | Society of the Atlantic