The Onsala Twin Telescope Project
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The Onsala Twin Telescope Project R. Haas Abstract This paper described the Onsala Twin Tele- 2 The Onsala Space Observatory scope project. The project aims at the construction of two new radio telescopes at the Onsala Space Obser- The Onsala Space Observatory (OSO) is the National vatory, following the VLBI2010 concept. The project Facility for Radio Astronomy in Sweden and has the starts in 2013 and is expected to be finalized in 2017. official mission to support research within radio as- tronomy and geosciences. The observatory was estab- lished in 1949 and is located at Råo¨ on the Onsala- Keywords Onsala Space Observatory, VLBI2010, pensinsula at the Swedish West coast, about 40 km Twin Telescope south of Gothenburg. Onsala belongs to the municipal- ity of Kungsbacka. An aerial photo of the observatory is presented in Figure 1. 1 Introduction Since 1949 the observatory has been equipped with several radio telescopes of various sizes. The three ex- isting ones today are the 25 m diameter radio telescope In September 2011 a project team consisting of Hans built in 1963, the 20 m radio telescope built in 1976, Olofsson, the director of the Onsala Space Observa- and the LOFAR station built in 2011. However, remain- tory, Gunnar Elgered, the head of the Department for ing parts of older telescopes, e.g. concrete foundations, Earth and Space Sciences at Chalmers University of are still there. Technology, Rudiger¨ Haas, the research group leader The observatory has a long and very successful of the Space Geodesy and Geodynamics research record in Very Long Baseline Interferometry (VLBI) group at Chalmers, Mikael Lilje, the head of the going back to 1968 (Scherneck et al., 1998). Onsala Geodesy Division of Lantmateriet,¨ the Swedish Map- was the first European observatory to contribute to ping, Cadastral and Land Registration Authorty, and Jan Johansson, the deputy head of the Department for Measurement Technology at SP Technical Research Institute of Sweden, submitted a proposal to the National Infrastructure programme of the Knut and Alice Wallenberg (KAW) Foundation. This proposal concerned a twin-telescope system for geodetic Very Long Baseline Interferometry (VLBI) at the Onsala Space Observatory, following the VLBI2010 recom- mendations (Petrachenko et al., 2009). The proposal was accepted in April 2012 by KAW, and a total amount of 29.7 MSEK was granted for the project. Rudiger¨ Haas Fig. 1 An aerial photo of Råo¨ with the Onsala Space Observa- Chalmers University of Technology, Onsala Space Observatory, tory (Credit: Onsala Space Observtory/Vastkustflyg,¨ 2011). The SE-439 92 Onsala, Sweden white spot approximately in the center of the photo is the 30 m diameter radome that is enclosing the 20 m radio telescope. 61 62 Haas VLBI observations (Whitney, 1974). Today OSO is contributing to observations in the European VLBI Network for Astronomy (EVN) and the International VLBI Service for Geodesy and Astrometry (IVS). The geoscientific observations are performed us- ing the 20 m radio telescope for geodetic VLBI, sev- eral receiving stations for Global Navigation Satel- lite Systems (GNSS), a superconducting gravimeter, a seismometer, a GNSS-based tide gauge, and several ground-based microwave radiometers for observations of the atmosphere (Haas et al., 2012). Fig. 2 Two of the parabolic antennas at Onsala in the 1960’ies. Right: The 12 m diameter telescope (”nr. 1”) that was destroyed 3 The planned location of the Onsala in a storm in 1969. Left: A 9 m diameter Wurzburg¨ antenna Twin Telescope (”nr. 2”). The antennas formed a west-east baseline (”nr. 2” to ”nr. 1”). The photo is taken from Rydbeck (1991). The geological situation at Råo¨ is very suitable for the Table 1 Distances (m) between the existing and the planned construction radio telescopes since the area is domi- telescopes at OSO. nated by bed rock of type Gneiss. A first geotechnical 25 m OTT1 OTT2 inspection indicated that new radio telescopes could be 20 m 601.1 397.3 465.3 constructed anyywhere on the observatory premises. 25 m 209.0 136.1 − However, there are additional constraints. The new OTT1 75.7 − − telescopes should be located not too far away from each other, so that they share the same atmospheric condi- tions, but not too close to each other either in order the 25 m telescope and the planned twins, OTT1 and to avoid sky blockage. Their elevation axes should be OTT2. The local topography is indicated by contour approximately at the same height in order to guaran- lines with 2 m resolution. The three telescopes are on tee equally good visibility. Furthermore, the majority a small peninsula that is surrounded on three sides by of the horizon shall be free down to an elevation an- the sea and wetland, respectively. In about 200 to the gle of 5 , and the existing equipment at the observa- ◦ north, there is a rocky hill with a height of more than tory should not be disturbed by the new telescopes. Other considerations concern the closeness to the sea, wind influence and closeness to a natural reserve in the 6 6 16 600 10 20 8 12 2224 8 14 northern part of the observatory. 24 550 4 4 4 12 24 8 6 26 22 Based on these considerations we located two suit- 28 6 500 8 88 10 6 10 16 18 30 28 10 able sites for the telescopes. They are in about 140 m 450 6 20 32 26 2018 12 22 1614 14 24 and 210 m distance to the existing 25 m telescope and 16 2 400 8 12 4 6 10 2 Wetland 8 6 8 4 form a short east-west oriented baseline of approxi- 6 35012 4 2 OTT2 OTT1 mately 76 m distance. Actually, the chosen places were 300 10 6 6 4 4 16 8 6 2 4 6 occupied in the 1950’ies and 1960’ies by two so-called north (m) 4 250 8 2 2 8 10 2 6 4 Wurzburg¨ antennas with 9 m diameter and the con- 200 22 25 m 4 6 6 6 4 2 6 2 crete foundations for these antennas are still there. The 150 The sea 4 The sea 22 4 2 2 Wurzburg¨ antenna at the Eastern location was rebuild 100 2 The sea into a 12 m telescope in the 1960’ies. A photograph of 50 these two antennas, taken in the 1960’ies, is presented 0 0 50 100 150 200 250 300 350 400 450 500 550 600 in Figure 2. In late 1969 the 12 m telescope was unfor- east (m) tunately destroyed in a storm (Rydbeck, 1991). Fig. 3 Digital elevation model of a selected area of OSO, show- Simulations were performed to investigate the hori- ing the location of the 25 m telescope, and the planned Onsala zon masks for the Onsala Twin Telescope. Also the im- Twin Telescope antennas, OTT1 and OTT2. These three tele- pact on the horizon mask of the existing 25 m and 20 m scopes are on a small peninsula that is surrounded by the sea radio telescopes was inspected. Figure 3 depicts a digi- from south-west to south-east and wetland in the east. In the north, there is a rocky hill of more than 32 m height. tal elevation model of the area, showing the location of The Onsala Twin Telescope Project 63 32 m. The foundations of the OTT twins are planned Table 2 Information on pressure (P), temperature (T), relative to be at a height of 5.5 m. Table 1 lists the distances humidity (RH), mean wind (MW) and gust wind (GW) recorded between the existing and planned telescopes. at the OSO during 2012–2012. The planned OTT telescopes do not significantly P T RH MW GW impact the horizon masks of the existing 20 m and 25 m (hPa) (◦C) (%) (m/s) (m/s) telescopes. Since the twin telescopes are not located in maximum 1047.1 +27.1 98.6 31.5 38.0 the same direction towards the 25 m telescope, they median 1010.8 +8.2 79.3 6.4 7.6 mean 1010.6 +7.7 77.8 5.7 8.4 will not see the 25 m telescope in the same azimuth minimum 962.5 15.6 22.0 0.0 0.0 direction. This reduces the area of the horizon that is − blocked for both telescopes together. Figure 4 depicts the horizon masks of the twin telescopes individually (dashed and dashed-dotted lines), and the combined constructions that are located in this environment need horizon mask of both telescopes (solid line). The cal- a very good corrosion protection to survive this harsh culations were performed as seen from the lower edge marine climate. of the prime reflectors, i.e. this is a kind of worst case Figure 5 depicts the meteorological records of scenario. The combined horizon of OTT is completely air pressure, air temperature, and relative humidity free above 7◦ elevation and blocked by less than 11 % recorded at OSO during 2010–2012. The pressure above 5◦ elevation. The largest obstacle is the rocky variation show the frequently passing weather fronts. hill towards the north of the twin telescope. The annual temperature variation extends over about 40 ◦C, and the median relative humidity is 75 %. The extreme values that were recorded during this period are listed in Table 2. 4 The environmental conditions at Onsala Table 3 Statistics on mean wind (MW) and gust wind (GW) for OSO during 2010–2012. The OSO site is located directly at the Swedish west coast, see Figure 1, and surrounded by the salty sea percentage of time MW GW % (m/s) (m/s) waters of the Kattegatt.