9
A Review of
The ALMA Project and Radio Telescopes
Scott Rydbeck, Calvin College,
Abstract
This paper discusses the major concepts and ideas behind the use of radio telescopes, by giving a brief history, the basic design of a radio telescope, and finally discussing the ALMA project, which when completed in 2010, it will be the largest telescope every constructed. It will operate at the very highest radio frequencies, that is between 30 and 950 GHz, in 10 bands. The corresponding wavelength of the radiation detected is in the range 10 to 0.3 mm, so this part of the electromagnetic spectrum is often called the "millimeter / sub millimeter" band. It lies between the conventional radio bands and Far Infrared (FIR) bands. ALMA will use established radio astronomy techniques, but pushed to the very highest frequencies. This paper will discuss how this all is accomplished and what will come from these new technologies.
Indexed terms – ALMA, Interferometer, Radio Telescope, Sub-millimeter
I. Introduction
The human race has been infatuated with the sky and space for thousands of years. From the Greeks and the Romans, to Galileo and Copernicus, we have looked up to the sky for answers. It has been a mystery to many, but with new advances in technology, astronomers and scientists have developed many ways to improve our understanding of this Brave New World of Space.
Fig 1 – The Hubble Space Telescope was built in the mid-80s, and was launched in April, 1990. It has been a focal point for the astrological community for many years, and has plans to stay up in orbit for another decade.
Through the use of computers, new materials, and manufacturing techniques, the equipment used in technology has improved by leaps and bounds. One of the last great achievements of the astrological community was the commission of the Hubble space telescope. This project was worked on for over a decade before it actually made it into space with the Discovery Space Shuttle in April of 1990 [1].
The Hubble Space telescope revolutionized the astronomy community. It allowed for observers to view space from a whole new perspective. Normal telescopes on earth are distorted because the light must pass through the Earth’s atmosphere. Astronomers have dreamed of having a device in space which would be able to avoid the problems of the Earth’s atmosphere, which was found in the Hubble telescope. From this advancement some of the major jobs of the telescope have been to explore the age of the solar system, measure the age and size of the universe, chart the evolution of the universe, and learn more about the planets and stars around us [2]. Through the use of the telescope, many spectacular images have been produced.
Fig 2 – The Hubble Space Telescope has provided some of the most beautiful and amazing pictures of the Universe. Here is a picture of the Egg Nebula, which was taken by the telescope, uninhibited by the Earth’s Atmosphere.
II. Background
As human beings we are limited in our ability observe different frequencies of energy. The human eye can clearly see and interpret the visible frequency of energy, which is on the order of 1014 Hertz, with a wavelength of about 10-6 meters. These wave forms of energy in fact make up for a very small portion of the whole spectrum. Figure 3 shows the electromagnetic spectrum and it clearly demonstrates the small part of the entire spectrum that the visible spectrum is. It is with this fact in mind, that scientists and astronomy came up the idea of the radio telescope.
Fig 3 – The human eye is quite amazing as it only picks up a certain amount of the electromagnetic spectrum, which is interpreted as color by the brain. The electromagnetic spectrum is much larger than visible light, however, and includes ultra violet waves and radio waves, among other types.
A. History of Radio Telescopes
The history of the radio telescopes draws upon many people. The first key scientist was James Clerk Maxwell, who, in the 1870s, developed his four equations which have governed electromagnetism for the last two hundred years. He demonstrated the close connection between electricity and magnetism. Along with Maxwell’s equations, Heinrich Hertz was able to transmit electromagnetic waves and receive them over a distance of about 5 meters. With the results of these scientists, Thomas Edison first proposed the detecting of radio waves from the sun.
With the groundwork laid by these scientists Karl Guthe Jansky was to make one of the biggest discoveries for the science of radio telescopes. As an employee of Bell Telephone Laboratories in New Jersey, Janksy investigated wavelengths of about 10 to 20 meters. He built one of the first antennas to measure radio waves at a frequency of 20.5 MHz.[3]
Fig 4 – As an employee of Bell Telephone, Karl Janksy built his own rotating radio receiver, to help the phone company figure out about interference. He found out radio signals were coming from the Sun, but was unable to continue research.
The uniqueness of his design was in his receiver, which rotated in a circular fashion, which allowed for Janksy to know the direction from which the signal was coming. He noticed an arbitrary signal and first thought it was from the sun, but later realized it was coming from the Milky Way, coming from the constellation of Sagittarius. His discovery was widely publicized, but no one followed up on it for many years.
Grote Reber, a student of radio engineering, learned of Jansky’s discovery, and wanted to build upon it. He was so interested in the discovery that he even applied to work with Jansky at Bell Labs, although there were no positions available at the time. The ambitious twenty-six year old did not stop after not getting the job; instead he built his own telescope in his backyard. While working fulltime at a radio station in Chicago, he was able build his very own radio telescope in his spare time. The telescope consisted of a mirror of diameter 31.4 meters which he custom built.[3]
Fig 5 – Grote Reber built his own radio telescope in which he made many discoveries about radio signals, which helped fuel the love for radio astronomy.
He was able to magnify the cosmic signals by many millions of times. Reber tried multiple frequencies without success, until he settled on 160 MHz, which reaffirmed Jansky’s results, as well as put Reber on the map, and provided the basis for further studying of radio astronomy. Reber was able to publish many articles in both astronomy and engineering journals, but his major contribution was demonstrating the high number of radio signals coming from the Milky Way.
Reber’s experimentations had global repercussions. Despite the fact that the Second World War was going on, news of Reber’s experiments reached a Dutch astronomy by the name of Jan Oort. Oort, a scientist experienced only with optical astronomy, quickly fell in the love with the idea of being able to penetrate the dust clouds of space.
Due to the nature of light, one is only able to see a few thousand light years away before the light of distant objects is removed. Oort’s major discovery revolved around spectral lines. The velocity of a gas can be measured using spectral lines and the frequency shift caused by the Doppler Effect. Taking this into account, one can measure the distances to different gas cloud masses, and come up with a map of the galaxy. It was through these ideas that Oort and his student, H.C. Van de Hulst, started looking into the spectral lines for different substances. Hydrogen, the most abundant element in the universe, was the first element that was researched.
Through careful calculations, Van de Hulst was able to determine the frequency at which hydrogen produced radio signals as it moved from a ground state to neutral hydrogen. He found this frequency to be 1420 MHz, which correlates to a 21 cm wavelength. Through the fact that Hydrogen in its neutral stage could have a magnetic moment either parallel or anti-parallel to the proton’s, the scientists realized that the transition between these two states resulted in the release of 21 cm wavelength radiation.
Despite their efforts on paper, the two scientists were unable to test their results. Instead they left it up to Harold Ewen and Edward Purcell, two scientists from Lyman Laboratory, part of Harvard University at the time. Ewen decided to build a receiver to detect the 21 cm line of neutral hydrogen in order to get his doctorate in Physics. Ewen received a grant to work on his design, which included several new techniques. The first was the use of a horn antenna, which is shown in figure 6 attached to the fourth floor of the laboratory.
Fig 6 – Harold Ewen and Edward Purcell where able to build a Horn Antenna, which was used Lyman Laboratory in Massachusetts.
The second and far more crucial technique was the development of frequency switching, which allowed the removal of the background noise. While Ewen was working on this project, the Dutch were working on their own, with no success, until they found out about the frequency switching technique. The Dutch followed suit and used the novel technique and discovered the 21 cm line on May 11, 1951[3].
All of the scientists mentioned above contributed greatly to radio astronomy. After the detection of the 21 cm line radio astronomy really took off. All over the world antennas started popping up, bringing with them new technologies as well as new discoveries. The National Radio Astronomical Observatory (NRAO) was started during this time in West Virginia, in the United States.
The first major radio telescope built by the NRAO was the Howard E. Tatel Telescope of Green Bank, West Virginia. It was momentous for its time, having a diameter of 85 feet, it started operation in 1959 after a year of construction, having a pointing precision of about 30 arc seconds. Figure 7 shows a picture of it as it still stands today.
Fig 7 – Howard E. Tatel Radio Telescope was the first major telescope that was built by the National Radio Astronomical Observatory, which is still standing in 2004.
B. Functionality
Radio Astronomy has progressed much in the last few decades in part due to better equipment and also in part to scientists coming up with new ideas and ways of thinking about what we already know. Before we look at the technologies that we are on the horizon of using, we must take a look at the Radio telescope of recent decades. There are certain components that are similar to the original radio telescope of Harold Ewen, but there are also new designs that are used.
1) Basic Components
Each radio telescope has basic components which are common in all radio telescopes. Each component serves its own unique purpose and each component is crucial to make the radio telescope working correctly and efficiently. This generalized view of radio telescopes will serve as an introduction to the ALMA project, which is being built in the country of Chile in South America at this very moment. There are several components that make up the basic radio telescope, which will be discussed, and include receivers, amplifiers and recorders.
2) Dish and Antenna
The first component people think of when they hear about radio telescopes are the dish and the antenna. It is important to note that the use of the names dish and antenna can be interchangeable, as the dish is the antenna. Similar to the lens of an optical telescope, the antennas serve to magnify a distance source, so that it is able to be seen more clearly. The signals from space are in fact very weak; the antenna detects and amplifies the signal as show in figure 8. The dish acts as any satellite dish does, reflecting the radio waves to the antenna. The parabolic shape of the antenna, or sometimes called reflector, allows for the best collection of the radio waves. As shown in the history section, Reber built his own receiver, very similar to what is used today [4].
Fig 8 – Radio waves are flying through the air, and the dish provides a very efficient way to collect all of them, focusing them down to a signal point, called the focal point.
The most common form for the dish is the Cassegrain design, which uses both sub-reflectors and feedhorns. A sub-reflector is similar to the extra mirrors in optical telescopes. The feedhorn is the cylindrical device that receives the reflected signals of the dish at the focal point. These two parts form the Cassegrain design, which specifically uses a secondary mirrors placed close to the primary mirrors. The result is a more accessible focal point, as well as the reduction in signals arriving from wider angles, which are not from the primary direction [4].
Fig 9 – The Cassegrain Telescope design, uses two lenses to provide not only a clear a signal, but also a more directionally focused one [[5]].
The reflector serves two main functions, to collect power and to provide directionality. The equation for the power gathered is :
(1)Where A is the area of the dish, Sv is the flux density at the earth, and delta v is the frequency interval or bandwidth of the measured radiation. It is clear from the equation above that a larger antenna will gather more power, thus it is the target of many radio telescopes to have a very large telescope [6].
Resolving power is the ability to show smaller details with higher detail, which is very important when designing a radio telescope. The units of measure are arc seconds, as it is a measure of the absolute smallest angle that can be resolved in a given time frame.
(2)Where λ is the wavelength (mm), D, is the collector diameter (mm) [7]. There are a number of different types of antennas such as trough-like ones, cylindrical parabolics, yagi, horn antennas and Mills Crosses. Each different antenna type has its benefits and pitfalls. It is clear that some antennas are not as familiar, such as the Yagi antenna. Although they may look different their purpose remains the same, to receive electromagnetic signals.