Development of Radio Astronomy at the Bosscha Observatory

Transcription

1 Proceedings of the Conference of the Indonesia Astronomy and Astrophysics, October 2009 Premadi et al., Eds. c HAI 2010 Development of Radio Astronomy at the Bosscha Observatory T. Hidayat 1, M. Irfan 1, B. Dermawan 1, A. B. Suksmono 2, P. Mahasena 1, and D. Herdiwijaya 1 1 Bosscha Observatory and Astronomy Research Division, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Indonesia taufiq@as.itb.ac.id 2 School of Electrical Engineering and Informatics, Institut Teknologi Bandung, Indonesia Abstract. Radio telescopes for research and education are currently in phase of development at the Bosscha Observatory. A small parabolic radio telescope with diameter of 2.3 m working at 1420 MHz is already available for general purpose of radio astronomical observations. In addition, a Radio Jove telescope with dual dipole antenna working at 20 MHz is also available. It is suitable to monitor daily solar burst as well as Jupiter decametric emission. Moreover, ATLCP multielement radio interferometers are now under construction. It consists of non-tracking Radio Jove array as well as two-dimensional tracking interferometer. The latter is planned to utilize up to 5 antennas. Design and sensitivity of the interferometers will be presented. Multi frequency receivers are made available at 20, 406, 1420, and 1665 MHz. The latter will be used for VLBI in the near future. Keywords. radio astronomy, interferometer 1. Introduction Recently, Bosscha Observatory prepares to be a multiwavelength observatory as a long term project (Hidayat et al. (2009)). Considering present situation and location of the observatory, radio astronomy is naturally a first option to complement our classical optical programs. Besides its scientific importances, radio astronomy augments science education broadly and serves to promote interdisciplinary research and education (Pratap & Salah (2001)). This project comprises in one part to build prototype of low-cost radio telescopes with which one may work on radio astronomical techniques and instrument calibration. Subsequently, we develope further to aperture synthesis arrays for synthesis imaging which is a primary tool in modern radio astronomy. It is well known that synthesis imaging in radio astronomy has a rich history of mathematical, scientific, as well as technological advances and discoveries. In other part, this project can also be introduced to undergraduate program to discover the exciting world of radio universe and to encourage research by undergraduates. This paper thus presents the progress in our development of radio astronomy at the Bosscha Observatory. 2. Radio Telescopes As a prototype of our first radio instrument, we choose the MIT-Haystack Small Radio Telescope which provides all the basic operation of a radio telescope. It consists of a 2.3m parabolic dish with an alt-azimuthal mount controlled by a software written 143

2 144 Hidayat et al. Figure 1. The 2.3m radio telescope and its various components (left) and Radio JOVE dipole antenna and its receivers (right). Figure 2. Spectrum of NGC 7027 (left) and tentative detection of Pulsar J (right). in JAVA. The antenna is mounted in a meteorological tower at the Bosscha Observatory to get all the sky coverage above horizon. The receiver, low noise amplifier, feed horn, and a noise diode calibrator are shown in Figure 1. The receiver can cover frequency range of MHz and can be used for spectral observations with bandwidth between 100 khz and 1.2 MHz at a resolution of 1.9 khz and 7.8 khz. Typical system temperature is 200 K with aperture efficiency may reach 50%. The beam width is 6 and the pointing accuracy is 1. At this frequency range, this telescope is very suitable to conduct spectral line observations at the neutral hydrogen (HI) MHz frequency. Observations of extra

3 Radio Astronomy at the Bosscha Observatory 145 Figure 3. A 3m satellite dish (left) and a 6m antenna (right) for interferometer. galaxy and other objects are also possible. For examples, Figure 2 shows observations of a standard calibrator NGC 7027 and tentative detection of Pulsar J The study of Milky Way from HI emission using this telescope is currently undertaken by Utomo & Hidayat (2009) and Damanik et al. (2009). The second telescope to build is a 20.1 MHz using a dual-dipole array suitable to monitor wideband long-wavelength jovian emission and solar radiation from surface and coronal activity. This telescope follows the original design from NASA s Radio JOVE Project ( with a slight modification. Our antenna and receivers are shown in Figure 1. Next telescopes will employ receivers of MHz, Spectra Cyber 1420 MHz, and 1665 MHz from Radio Astronomy Supplies. The last two receivers will also be utilized in interferometric mode. Therefore, these multi-frequency receivers must be able to open various topics in radio observations. 3. Radio Interferometer As mentioned above, the second part in our radio astronomy development is to construct arrays of radio interferometer. Based on the existing radio telescopes, we decide to expand our radio JOVE to be non-tracking radio interferometer and two-dimensional tracking interferometer using paraboloidal reflector antennas. For the latter, obviously, the more the number of the antennas with sufficiently large aperture, the better the desirable interferometer. However, in this very early phase of development, an ATLCP interferometer is the only feasible one. We expect that its sensitivity is much better than K/Jy. As the budget is a strong constraint, we use low cost satellite dishes for the tracking interferometer. Considering the total area and topographic situation of the observatory, we estimate that the total number of the antenna cannot realibly be greater than 10 antennas. Those with 6m aperture are expected. So far, two 3m satellite dishes are already mounted and one 6m antenna is available (Figure 3). The 3m dishes will be used for experiment of fringe detection. The motorized mount for the 6m antenna is still under construction as well. Subsequently, we decide to construct 5-element interferometers for both type. As the terrain field is not flat at all, it is difficult to obtain a flexible configuration. Some fixed antennas with one movable antenna configuration, for example, could not be option in At the lowest cost possible

4 146 Hidayat et al. Table 1. Configuration of the Antennas Configuration Longitude Latitude Altitude Error Name Antenna (m) (m) Jove ± ± ± ± ±11 A ± ± ± ±8 B ± ± ± ±8 C ± ± ± ±8 D ± ± ± ±9 our case, since it is not allowed by the corresponding topographic situation. Hence, for ground preparation, we have to select some locations on which we can erect an antenna, and then combine with some other locations to obtain the best possible uv coverage. These possible configurations are shown in Table 1. The location of each aerial is measured using GPS without a very high accuracy (±5 m at best). It is not critical, however, since the uv-coverage does not depend on the absolute position of each aerial. Figure 4. Antenna configurations for radio JOVE interferometer (left) and configuration D for the tracking interferometer (right), described in Table 1.

5 Radio Astronomy at the Bosscha Observatory 147 Figure 5. Examples of uv-coverage of Jove interferometer from a source at δ = 10 and configuration A, C, and D for δ = 60 after 12 hours of observations. Later, for the sake of data analysis, once the aerials are mounted, we can update the position measurements using a more accurate geodetic GPS. As described in Table 1, we consider only one configuration for the radio JOVE Interferometer. Since the Bosscha Observatory is located near the equator, The Sun and Jupiter, the target objects for this interferometer are never far from the celestial equator, and consequently they are always at low declination. The advantage for our location is that their transits are always at high elevations which simplify the choice of antenna s beaming. Referring to Table 1, we found only one best possible configuration for this interferometer, called Jove. Figure 4 shows the antenna configuration obtained using the Google Earth Map. A grey circle in the top left of the figure is the dome of the observatory for reference. The longest distance for the East-West baseline is m at the working frequency of 20.1 MHz, which yields a synthesized beam, or angular resolution, of 154 arcmin only. The dipole array must be put in-phase to obtain a vertical beam. In any case, even the Sun cannot be resolved with this interferometer, but at least the position of the object can be determined. For the tracking interferometer, we consider 4 possible configurations, called A to D (Table 1 and Figure 4), and all of them consist of 5 fixed aerials. The main reasons

6 148 Hidayat et al. for studying these configurations are that the antenna can be erected at a relatively clear area and far from tall building or other obstacles. We also try to avoid to remove trees. Subsequently, we calculate the uv coverage for each configuration, given a source declination. We adopt a geoid model of WGS84 (Thompson et al. (1986), Walker (1994), Boone (2001)) to compensate altitude differences, where semimajor axis a = m, eccentricity e = 2f f 2, and f = 1/ Figure 5 shows examples of uv coverage for Jove configuration at a source declination of 10, and for configuration A, C, and D at high declination of δ = 60. Configuration D provides the longest East- West baseline of m, and at 1420 MHz, it yields a synthesized beam of 4, while the others give 4.4. After 12 hours of integration, configuration D also provides a better uv plane filling. 4. Further Work Simulations of expected fringe patterns have been done to be compared to actual observations. Two 3m parabolic dishes have been mounted and manually motorized to make experiments on fringe detection. Unfortunately, the receivers are not available yet. Constructions of mechanical parts and its associated controller for the 6m antenna are still undertaken. The antennas will be errected following the configuration D outlined above. Subsequently, this work includes to make a design of larger parabolic antenna of 12m aperture to be constructed by local manufacturer. The next step is to set up a VLBI with a baseline of 7.1 km between the Bosscha Observatory and ITB campus which provides an angular resolution of 3.7 arcsec at 18 cm wavelength. Acknowledgment This work is supported by Ministry of Research and Technology through Program Insentif Riset Dasar and HRIA-ITB 2009/2010, with which a multiwavelength astronomy in Indonesia could be initiated and to whom we are sincerely grateful. References Boone, F. 2001, A&A, 377, 368 Boone, F. 2002, A&A, 386, 1160 Damanik, S.R., Utomo, D., & Hidayat, T. 2009, This proceedings Utomo, D., & Hidayat, T. 2009, This proceedings Diamond, P.J. 1994, in: R.A. Perley, F.R. Schwab & A.H. Bridle (eds.), Synthesis Imaging in Radio Astronomy (ASP Conf. Series Vol. 6), p. 379 Hidayat, T., Mahasena, P., Dermawan, B., & Irfan, M. 2009, Menuju Observatorium Multiwavelength di Indonesia, Report of Insentif Ristek, KNRT Hjellming, R.M. 1994, in: R.A. Perley, F.R. Schwab & A.H. Bridle (eds.), Synthesis Imaging in Radio Astronomy (ASP Conf. Series Vol. 6), p. 477 Napier, P.J. 1994, in: R.A. Perley, F.R. Schwab & A.H. Bridle (eds.), Synthesis Imaging in Radio Astronomy (ASP Conf. Series Vol. 6), p. 39 Pratap, P. & Salah, J.E. 2001, J. Sci. Ed. Tech, 10, 127 Thompson, A.R., Moran, J. M., & Swenson, G.W. 1986, Interferometry and Synthesis in Radio Astronomy (New York: John Wiley & Sons) Thompson, A.R. 1994, in: R.A. Perley, F.R. Schwab & A.H. Bridle (eds.), Synthesis Imaging in Radio Astronomy (ASP Conf. Series Vol. 6), p. 11 Walker, R.C. 1994, in: R.A. Perley, F.R. Schwab & A.H. Bridle (eds.), Synthesis Imaging in Radio Astronomy (ASP Conf. Series Vol. 6), p. 355 Wilson, T.L., Rohlfs, K., & Hüttemeister, S. 2009, Tools of Radio Astronomy (Berlin and Heidelberg: Springer)