Characteristics and techniques of Radio Telescopes

Transcription

1 Characteristics and techniques of Radio Telescopes Soon-Joon Yoon ( ), Hyun-Ju Rhee ( ), and Won-Seok Choi ( ) Department of Electrical and Electronic Engineering, Yonsei University TEL: Abstract This paper describes the principles of Radio Telescopes, Interferometer. And also the antennas which are use to observe Radio Astronomy; Cassegrain Antenna. Then we will focus on the method Phased Array Antennas which is frequently used in recent trends of radio telescope, for example VLA, VLBI. It's a very powerful technique for Radio Astronomy. And if overcome the interference of atmosphere, we can extend the ability of radio telescope much more. I. INTRODUCTION In Antennas for Radio Astronomy, during the past decades, important advances have been made in the resolving power of radio astronomy antennas by use of an extension of the synthetic-aperture principle. This principle has also made possible the radio mapping of astronomical objects and regions in much greater detail than was hitherto possible. The attainable resolution has exceeded that obtainable with the largest optical telescopes. The signals observed are radio signals emitted from objects or regions containing warm or hot matter which are radiated by energetic electrons spiralling around magnetic field lines and perhaps other radiative processes. However, the power which emitted from the heavenly bodies is too small, it s very difficult to gather them. Parabolic antenna can be one solution. However, with single parabolic antenna, the resolution which is obtained is very ambiguous. The resolution means the ability of analysis of radio wave source. It is proportional to the diameter of the antenna. However, to satisfy our specification, we need very large size of parabolic antenna. Therefore, it s very impractical. In addition, detecting shorter wavelengths of light allow greater precision, but are much more difficult to work with. To solve this problem, radio astronomy methods make use of the resolution properties of the interferometer. VLA and VLBI are the examples. In this paper, we present the principle of interferometer, the characteristic of Cassegrain antenna which is accepted for VLA and VLBI. II. CASSEGRAIN ANTENNA Cassegrain antennas have often been used as "radio telescopes" in the science of radio astronomy. The basic principle of Cassegrain antenna is composed of line or waveguide, feed, paraboloid reflector, hyperboloid subreflector.(fig 1) The Energy from the heavenly bodies illuminates the paraboloid reflector, which reflects it back to the subreflector. For this antenna, it is important to minimize the length of the transmission line between the primary radiator and the first stage of the receiver, or the transmitter output. Cassegrain antenna has many advantages than parabolic antenna of the same main reflector size. First, it has about 10% higher efficiencies than parabolic antennas. And it can be operated in dual frequency mode. One defect of this antenna is the aperture blocking which is produced by the Cassegrain subreflector. This blocking may be tolerated in many large-antenna applications in order to gain the advantages of an accessible primary radiator, but it is a defect of the Cassegrain scheme that may be serious. The performance of Cassegrain antenna is illustrated in Fig 2. A(θ) is a function of antenna efficiency respect to antenna s angular, and I(θ) is spectrum of received signal. It can be expressed by convolution systems, because the operation of antenna is based on convolution systems. Therefore, A(θ-θ ) I(θ ) dθ is response of Antenna. The signal which has same angle with antenna has the crest in spectrum. III. INTERFEROMETER The terms "interferometry'' and "interferometer'' are both derived from the word "interference". Interference is a phenomenon that occurs when one has waves of any kind i.e. sound waves, light waves, ocean waves, seismic waves

2 from earthquakes. However, a radio wave has longer wave length than light, its directivity and resolution isn t good. In this reason, radio astronomy use the interferometer of radio wave. When two waves come together at the same time and place, interference occurs. Resonance, beat frequencies, hetrodyning, dissonance are also all interference phenomena. Interference can be visualized as the adding together of two waves with each other. If both waves are in step or in phase, that is, the crest of both waves coincide, the two waves will add together to form a single wave. This combined wave will have a higher crest and deeper larger amplitude. In the case of light waves, two dimmer light beams will add together to form a brighter beam. This is called constructive interference. On the other hand, destructive interference occurs when the two waves are out of step with each other; that is, the crest of one coincides with the trough of the other. So, the amount of interference that occurs depends on both the amplitudes of the two waves and the degree to which their respective crests and troughs are in phase with each other. Interferometry is the use of interference phenomena for measurement purposes, either for very small angles or for tiny distance increments. Therefore, the application of interferometry to astronomy is very challenging. To receive the weak radio signal from universe, radio astronomy makes use of phased array antennas. The basic principle of phased array antennas is following. In Fig3, the baseline is D. and θ is angular between ground and antenna. In phased array antennas, the most important parameter is the "baseline," the distance between the antennas. At some point, the visibility drops to zero and the fringes disappear. This is called the "resolving point.' And if Dsin θ is nλ (multiple of wavelength), the waves effect constructive interference. However Dsin θ goes to (2k+1)λ/2, destructive interference will occur. The θ is variable angular, because earth s meridian is changed by time. In the real world, D can be changed by placing antennas on the rail and move antennas. This theory can be described in mathematical equation. The Power pattern of response signals are: P(θ)= F(θ) 2 = f(θ)g(θ) 2 Here, We assume the signals as unit step function. f(θ) is the pattern element and g(θ) is the element factor. f(θ) is relate with shape of signals. And g(θ) is characteristic of antennas. We concentrate on f(θ), because it describes the performance of phased array antenna. f(θ) for one antenna is : (Here, ψ is the function of θ) f(θ)=cos(ψ/2) And for phased array antenna, setting many antennas in array, formula is the form of integration. Therefore, sum of f(θ) is AF= Aejnψ=A(1+ejψ+ +ej(n-1)ψ)=a(1- ejnψ)/ (1- ejψ) =Asin(Nψ/2)/sin(ψ/2) And using variable ψ instead of θ, f(ψ), called Linear Array is Asin(Nψ/2)/Nsin(ψ/2). Fig 4-(a), (b), (c) are f(ψ) = Asin(Nψ/2)/Nsin(ψ/2). (a) and (c) is different respect to N, the number of antennas. This signal has maximum point at ψ=0. In (a), the angular for main lobe is much larger than (c). Wide angular for main lobe means increased ambiguity in observation. Therefore case (c) provides much better resolution than (a), (b). Therefore each one of the interferometer elements can be a full-fledged directional antenna. When one antenna apart from another, we can get the same resolution obtained by Cassegrain antenna which has diameter of distance between two antennas in former case. Using this technique, in these days, for example VLA, VLBI, we can get higher resolution than optical telescope. IV. VLA(Very Large Array) is a good application using Cassegrain antennas and interferometry. The VLA is a radio telescope located outside of Socorro, New Mexico. The array consists of 27 antennas 82 feet in diameter, spread out along a Y-shaped baseline.(fig 5) Each of the antennas can be moved via railroad tracks to different locations along the baseline. In the D configuration (this will be explained in next paragraph), the antennas are crowded together within a half mile from the center of the Y. The elements of this array are arranged in a "Y" pattern, and each leg of the Y is 20 kilometers in length, so that the greatest separation between any pair of elements is 35 kilometers. This array utilizes the interferometer principle, applying it to each. Since the Y has three (symmetrical) branches, there are 9 antennas in each branch. A railroad track runs along each leg of the Y, in which there are 72 three piered concrete stations at fixed points, to which the antennas can be moved and operated, in any one of four possible configurations. The period of antenna rearrangement is about 3 ~ 4 month. Four possible configurations are D, C, B, A. It goes from D to A, the distance of each telescope became more distant. These four conjurations have maximum separations of the outer lost antennas of 1.03, 3.5, 10, and 36.4 kilometers. Each of these configurations represents a comparable resolution VLA

3 capability at each of the four frequency bands in which the antennas can operate. There is separate horn feed for each of the four frequency bands, and the hyperbolic sub reflector is mechanically movable (remotely computer controlled) to focus the received signals on the horn in use. The received signals are amplified by electronics at each antenna and then transmitted via buried waveguides to the central station. In the A configuration, the antennas are stretched out over 20.9 kilometers from the center. When the dishes are in this configuration, their data can be combined to form a radio image equivalent to one produced by a single antenna 27.4 kilometers in diameter. Although the resolution of the VLA isn't as great as that of a VLBI, the greater total aperture area makes it the most sensitive and powerful radio astronomy instrument in existence. Thus it has different capabilities, which supplement and complement those of the VLBI. V. VLBI The VLBI(Very Long Baseline Interferometer) combines the resolution and gain of the individual antennas with the inherent high-resolution properties of an interferometer and makes use of the additional resolving capability of the synthetic aperture. A number of widely separated directional antennas are used as a combined multiple-interferometer / synthetic-aperture system. Like the VLA, VLBI utilizes the interferometer principle. In a typical experiment, several antennas distributed over the United States are used, together with an antenna in Europe. These are large steerable microwave paraboloidal reflectors - for example, the 140-foot-aperture reflector at the National Radio Astronomy Observatory in Green Bank, West Virginia, the "Hay-stack" 120-foot reflector in Boston, Massachusetts, and similar antennas at Ft. Davis, Texas, two in California, and a 100-meter antenna in Bonn, West Germany. Thus the interferometers baselines are thousands of miles long. These antennas make simultaneous highly synchronized observations over a period of hours, recording phase and amplitude data of the received signals. The most important thing of VLBI is time-synchronism. However, because of distance of between antennas, they can't be wired. Thus, very precise clock is needed like an atomic clock. The recordings are then sent to a central processing location, and by the use of appropriate mathematical algorithms the data are computer processed to yield high-resolution mapping of the observed region of the sky. This system represents the ultimate in angular resolution thus far attained in radio telescope. VI. CONCLUSION We have achieved lots of development in radio astronomy. And the advance is connected with the growth of radio telescope. The application of interferometer in radio telescope is the most important booster. However, the interference of atmosphere is still remained. The air is an obstacle in radio astronomy, because it will reduce the power of radio waves and non-linearly change the spectrum. Therefore, we think that by launching the VLBI system in universe, we can get much close and detail information. Comparing the ability of HUBBLE Telescope with other telescopes on earth, we can easily analogize the influencing power of radio telescope in universe. In addition, by launching VLBI on the orbit of earth, antennas can be located far away than on earth, and can change its distance by using the method of artificial satellite. This will result in huge innovational advance in radio telescope and radio astronomy. REFERENCES [1] A. Richard Thompson, Morgan, Swenson Interferometry and Synthesis in Radio Astronomy, 2 nd Edition " Wiley- Interscience.2001 [2] Warren L. Stutzman/ Gary A. Thiele, Antenna Theory And Design 2 nd Edition, Wiley-Interscience 1998 [3] Dordrecht-Holland. Radio Astronomy And Cosmology, D. Reidel Publishing Company. Symposium No 74. Aug 16-20, 1976 [4] Robert j. Mailloux. Phased Antenna Handbook, Artech House 1994 [5] D. Scott. Birney Observational Astronomy, Cambridge University Press [6] Morimoto misak, Seeing the universe with waves Wave-Science Publishing Company [7] Jung-Jin. Kang, Antenna Engineering, Jibmoondang 1996 [8] Kwang-Je Choi. EM waves and Antenna Engineering, Kwang Moon Kag. 1993

4 Fig 1. Cassegrain feed for a paraboloidal reflector. Fig 2. The power pattern of an antenna A(θ ) and the intensity profile of a source I 1 (θ ) used to illustrate the convolution relationship. The angle θ is measured with respect to the beam center OC and θ is measured with respect to the direction of the nominal position of the source OB. Fig 3. Geometry of an elementary interferometer. D is the interferometer baseline. Fig4-(a). 3 elements Fig 4-(b). 5 elements Fig 4.-(c). 10 elements Fig 5. VLA

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