The Abu Simbel Radio Telescope Project

M.A. Mosalam Shaltout

National Research Institute of Astronomy and Geophysics Helwan, Cairo, Egypt
mamshaltout@frcu.eun.eg

Abstract. This paper emphasises the importance of building a radio telescope at Abu Simbel in the South of Egypt as part of the European VLBI Network (EVN) to cover the gap between the radio telescopes in Western Europe and the radio telescope at Hartebeesthoek in South Africa. The telescope can be used for solar and stellar observations at appropriate wavelengths and for geodesy studies. The suggested telescope has a diameter of 32 metres and works on a frequency range from 1.4 to 43 GHz. Abu Simbel is characterized by excellent atmospheric transparency, dry climate, low population density and little artificial interference. The possibilities for co-operation among international institutions is explored.

Sommaire. Cet article insiste sur l'importance de la construction d'un radio télescope à Abu Simbel dans le Sud de l'Egypte comme noeud du Réseau Européen du VLBI (EVN), pour combler le créneau entre les radio télescopes de l'Europe de l'Ouest et le radio télescope de Hartebeesthoek en Afrique du Sud. Le télescope pourra être utilisé pour des observations solaires et stellaires à des longueurs d'onde appropriées et pour des études en géodésie. Il est suggéré que le télescope ait un diamètre de 32 mètres et travaille sur une échelle en fréquence de 1,4 à 43 GHz. Abu Simbel est caractérisé par une excellente transparence atmosphérique, un climat sec, une faible densité de population et peu d'interférence artificielle. La possibilité de coopérations avec des institutions internationales est envisagée.

1. Introduction

In the earliest days of radio astronomy, compact cosmic radio sources were often referred to as ``radio stars'', although the vast majority were subsequently revealed to be extragalactic in origin. With the advent of sensitive interferometric arrays in the 70s and early 80s, however, bona fide stellar objects were detected, and by the mid-80s, the study of radio emissions from stars was a burgeoning new field of study. The study of radio emissions from stars overlaps, in several important respects, with the study of radio emission from the sun. While solar radio physics finds its roots in the very earliest days of radio astronomy, it continues to thrive, especially so in recent years. Practically important have been new instruments and techniques (e.g., the new 17 GHz radio heliograph at Nobeyama, the OVRO frequency agile solar array), the advent of multi-wavelength studies of solar phenomena, high resolution spectroscopy of the decimetric wavelength regime, and millimetre wavelength interferometry of active phenomena.^[1]

In recent years, there have been several developments in the techniques and instrumentation used for ground-based astronomical observations at millimetre and submillimetre wavelengths. The wavelength region of interest (5 mm to 300µm) is determined both by the nature of the instrumentation, and by the opacity of Earth's atmosphere.^[2] The frequency range above the oxygen absorption band at 69 GHz (wavelengths less than 5 mm) has been allocated to various services, but until recently only a few (military) applications used these high frequencies. On the other band, radio astronomers have used essentially all the spectrum in the various atmospheric windows up to frequencies of 1000 GHz in order to observe numerous lines from many molecules. This picture is changing rapidly as new technology becomes available.^[3] Most importantly, solar radio observations can now be used to study coronal magnetic fields directly, something optical and X-ray astronomers could not easily do. Microwave and millimetre observation are the only way to image electrons of energies greater than a few hundred KeV, up to 1 MeV in the solar atmosphere.^[4]

Table 1: Telescopes in the European VLBI Network.

Country	Location	Institute	Telescope Diameter (m)	Frequency Range (GHz)
Spain	Yebes	National Astronomical Observatory	14	2.3 - 4.3*
Italy	Bologna Noto	Instituto di Radioastronomia Instituto di Radioastronomia	32 32	1.4 - 4.3 1.4 - 4.3
The Netherlands	Westerbork	Netherlands Foundation for Radioastronomy	14 x 25	0.3 - 8.4
United Kingdom	Jodrell Bank Cambridge	Nuffield Radio Astronomy Laboratories Institute for Research in Astronomy	76 25 32	0.3 - 1.6 1.4 - 26 1.4 - 43
Sweden	Onsala	Onsala Space Observatory	25 20	2.3 - 8.4 2.3 - 100
Finland	Helsinki	Metsaehovi Radio Research	14	22 - 100
Germany	Effelsberg Wettzell	Max-Planck Institut für Radioastronomie Technical University of Munich	100 20	0.6 - 85 2.3 - 8.4*
Poland	Torun	Torun Radioastronomical Observatory	32	0.3 - 4.3*
Ukraine	Simeiz	Crimean Astrophysical Observatory	22	0.3 - 43*
China	Shanghai Urumqi	Shanghai Observatory Urumqi	25 25	0.3 - 22 0.3 - 22

Notes: * = Frequency range not yet full equipped.^[6]

World-wide Very Long Baseline Interferometry (VLBI) is undergoing a major expansion in capability at the present time. The new US Very Long Baseline Array (VLBA) is already producing eye-catching results even before its full capabilities have been realised. Its counterpart in Europe, the European VLBI Network (EVN), is carrying out a major upgrade of the radio-frequency performance and flexibility of its member telescopes and their VLBI equipment, as well as constructing a new state-of-the-art correlator. In the southern hemisphere the Australian VLBI array is also expanding its capabilities with a new correlator and recording terminals, and together with radio observatories in the Asia-Pacific region, began regular coordinated VLBI observations as the Asia-Pacific Telescope (APT). Millimetre-wave observatories across the globe have also banded together to form the Coordinated Millimetre-VLBI Array (CMVA) which has observed at 86 GHz three times a year starting from 1995 ^[5] (Fig.1). And perhaps the most spectacular of all, the space VLBI era began in February 1996 with the launch of the Japanese Muses-B satellite carrying an 8-m diameter radio telescope into earth orbit. The mission, called VSOP (VLBI Space Observatory Programme), combined the space-borne antenna with its ground-based counterparts around the world to form radio interferometers of dimension 32000 km and maximum angular resolution of 80 micro-arc-seconds. Russia plans to launch its 10-m diameter RADIOASTRON satellite into an even higher orbit than VSOP to provide a further increase of resolving power to 30 micro arcseconds.

In this paper, emphasis is placed on building a radio telescope at Abu Simbel in the south of Egypt, for solar and stellar observations on wavelengths ranging between microwave and millimetre. Abu Simbel is a dry desert with excellent atmospheric transparency and is one of the best sites in north Africa for optical and radio observations. Such a telescope could become a part of the European VLBI Network (EVN) for astronomical and geodetic observations.

2. Very Long Baseline Interferometry (VLBI)

The angular resolution of a single telescope is given by $\alpha$ = 1.22 $\lambda$ /D with D as the diameter of the telescope. The angular resolution is ~ 10 $^{\prime\prime}$ for observations at 3.5 mm, the shortest wavelength of the 100-m telescope at Effelsberg, Germany. This telescope has been successfully operated by the Max-Planck-Institute for Radio Astronomy in Bonn, Germany since 1972 [6]. With its diameter of 100 m, it is still the largest fully steerable radiotelescope in the world. The collecting area of about 7850 m² allows the detection of extremely weak radio signals of the order of a millijansky (the Jansky is the unit of radio flux: 1 Jy = 10^-26 W Hz^-1 m^-2). This angular resolution (10 $^{\prime\prime}$ ) for the largest fully steerable radio telescope in the world is much less than what the astronomers need to investigate the cores of radiogalaxies and quasars. However, Very Long Base Line Interferometry (VLBI) provides much higher angular resolution.

Figure 1: The European VLBI Network (courtesy by R. Schwartz).
The filled circle indicates the proposed site for the new radio telescope
at Abu Simbel; the open circle indicates Hartebeesthoek.

The idea behind VLBI is relatively simple: a network of several telescopes in various countries observes the same radio sources simultaneously with the individual telescopes. The technique relies on the interferometric properties of electromagnetic radiation as well as the earth's rotation. The angular resolution of the network is given from the baseline of the two most distant telescopes. An angular resolution of 10^-4 arc seconds is achieved by transatlantic VLBI.

3. The European VLBI Network (EVN)

In 1980, the European VLBI Network (EVN) was created with the Max Planck Institute for Radioastronomy (MPIFR) as one of the five founding members. The EVN consists of 12 institutes with 16 radio telescopes in Europe and China (see Table 1). The member institutes of the EVN agreed to devote observing time for 4 sessions of 3 weeks each per year. Most frequently, observing sessions are at 1.3 cm, 3.6 cm, 6 cm, and 18 cm. By combining with the American Very Long Baseline Array (VLBA) consisting 10 antennas spread over the United States, transatlantic VLBI Network observations are possible.

Recently, a VLBI network operating at 3mm was established with participating institutions in Chile, France, Spain, the United States and Sweden. Participation of the VLBA antennae is expected in the near future ^[6].

A major upgrade of EVN facilities is in progress, made possible by funding from multi-national sources in the European Union. The main aspects of the upgrade are: (1) the construction of a 16-station data processor at the newly established joint institute for VLBI in Europe (JIVE) in Dwingeloo, Holland. (2) Upgrade of the individual telescopes to allow recording at 1024 Mbit/sec (the MKIV standard), and (3) the employment of support scientists at JIVE and at some individual observatories to provide assistance for users of EVN and other arrays. In addition, a number of nationally funded upgrades are being carried out at EVN member stations, including receivers in new frequency bands, and replacement of older receivers with state-of-the-art HEMT-based systems.^[5]

Figure 2: Outline of the suggested Radio Telescope at Abu Simbel
in Upper Egypt. (Courtesy by Radio Telescopes Manufactory, Dusseldorf, Germany)

4. Gaps in the EVN

Considerable effort went into the optimum placing of the VLBA antennas for uv-coverage. No such license was available for the EVN: the telescopes are where they are! Figure 3 depicts the uv-coverage in 1999 for the EVN at 22 GHz. The coverage is good at northern declinations where radio sources pass overhead at the majority of telescopes, but gaps appear at lower declinations and the coverage becomes more one-dimensional the closer the sources are to equatorial. A telescope near the equator between western Europe and the South African telescope at Hartebeesthoek would help solve this problem.^[5]

5. A Radio telescope at Abu Simbel?

Abu Simbel is a small village on the western bank of Nasser Lake in Upper Egypt, with coordinates 22° 20.22´ N, 31° 36.97´ E, and altitude 200 metres above the sea level. It is not only the intermediate between Western Europe and South Africa, but it also completely satisfies two important conditions for radio observatories which are:

Radio observatories should be sited far from population centres, which affords them a degree of freedom from artificial interference.[7]
High frequency waves are harder to transmit and receive and they are hindered by moisture in the Earth's atmosphere.^[8]

Figure 3: Left: uv-coverage for the VLBA at declinations of 64°, 30°, 06°, and -18°. Units are 10³ km.
Right: uv-coverage of the EVN at 22 GHz. Telescopes in the EVN are Yebes, Jodrell Bank, Cambridge,
Effelsberg, Onsala, Bologna, Noto, Metsahovi, Torun, Urumqi, and Shanghai (see Table 1). Units are 10³ km
(courtesy by R.T. Schilizzi from ^[5]).

Figure 4: Radio Telescope at Cambridge (United Kingdom); antenna diameter 32 m;
frequency range from 1.4 to 43 GHz as the suggested Radio Telescope
at Abu Simbel in the Upper Egypt (courtesy by R. Schwartz).

The total population of Abu Simbel is not more than 5000 persons, mostly active in the field of tourism associated with the temple of Pharaoh Ramses II. It is 270 km south of Aswan, with a small local electric grid. Its climatic conditions are extremely favourable for astronomical observations in general. The atmospheric transparency is excellent, and the site almost cloudless all year round. The mean annual precipitation is 1 mm and evaporation per day is 20 mm. Humidity in summer is less than or equal to 13%, and in winter 37%. The mean yearly air temperature is +26° C; in January it is +16.7° C and in July +33.7° C. Minimum and maximum temperature reaches +21° C and +50° C once in every dozen years. The annual mean sky cover is one okta.

Abu Simbel is served by an international airport with low traffic density (three or four airplanes per day) handling mostly tourist traffic. This will be helpful for the fast transport of the magnetic tapes of observations to the headquarters of EVN. In Abu Simbel, there are two hotels of 3- and 5-star level, and a resthouse at our institute. These facilities can be used in carrying out further detailed studies for the site and climate before final consideration.

The suggested radio telescope in Abu Simbel (Fig.2) will be similar to three of the radio telescopes in EVN, two of them in Italy at Bologna and Noto and operated by Instituto di Radioastronomica, the third in the United Kingdom at Cambridge, and operated by Research Institutes in Astronomy. The diameter of the antenna of the telescope is 32 m, and it operates in the frequency range 1.4 to 43 GHz (Fig.4). Climatic conditions in Abu Simbel are ideal for microwave and millimetre radio observations. Also, we have taken into consideration the economic factor for the total price of the telescope, where Egypt is a developing country with lower labour costs. A rough estimation of the cost of the project is under way. The co-operation of international partners is sought.

The telescope will be a major addition to our research capacity in basic space science and geodesy in the National Research Institute in Astronomical & Geophysics, which was founded in 1903, and has since 1963 had the largest optical telescope in the Middle East, the 1.9-m telescope of Kottamia Observatory.

References

Report on the International Meeting on `Radio emission from the stars and the sun', Barcelona, Spain, 3-7 July 1995, hosted by the University of Barcelona (1995).
J.E. Carlstrom and J. Zmuidzinas: `Millimetre and submilimetre techniques,' Review of Radio Science 1993-1996, pp. 839-882, edited by W. Ross Stone, Oxford Science Publications (1996).
W.A. Baan, `Annual Report from IUCAF,' Radio Science Bulletin No.281, pp. 23-26 (1997).
M.R. Kundu, `Recent Advances in Solar Radio Astronomy,' Review of Radio Science 1993-1996, pp. 913 - 951, edited by W. Ross Stone, Oxford Science Publications (1996).
R.T. Schilizzi, `Current Developments in VLBI Astronomy on the Ground and in Space,' Radio Science Bulletin No.273, pp. 14-28 (1995).
R. Schwartz, `The Activities of the Max-Planck-Institute for Radio Astronomy (MPIFR) Under the Aspect of Scientific Cooperation', Very Long Baseline Interferometry, Invited paper No.2, Proceedings of the Fourteenth National Radio Science Conference (NRSC 97), Cairo-Egypt, March 23-25 1997. Published by the Egyptian National Radio Science Committee, Academy of Scientific Research and Technology, Cairo, Egypt (1997).
M. Ishiguro and Radio protection group in NRO, `Radio Interference at Nobeyama Radio Observatory,' 23th General Assembly of the International Astronomical Union, August 17-30, 1997, Kyoto, Japan.
J. Roth, `Will the Sun Set on Radio Astronomy,' Sky and Telescope, April 1997, pp. 40-43 (1997).

WGSSA
2000-02-28