Giuseppe Maccaferri

Sardinia Radio Telescope: the new Italian project

This contribution gives a description of the Sardinia Radio Telescope (SRT), a new general purpose, fully steerable antenna proposed by the Institute of Radio Astronomy (IRA) of the National Institute for Astrophysics. The radio telescope is under construction near Cagliari (Sardinia) and it will join the two existing antennas of Medicina (Bologna) and Noto (Siracusa) both operated by the IRA. With its large antenna size (64m diameter) and its active surface, SRT, capable of operations up to about 100GHz, will contribute significantly to VLBI networks and will represent a powerful single-dish radio telescope for many science fields. The radio telescope has a Gregorian optical configuration with a supplementary beam-waveguide (BWG), which provides additional focal points. The Gregorian surfaces are shaped to minimize the spill-over and the standing wave between secondary mirror and feed. After the start of the contract for the radio telescope structural and mechanical fabrication in 2003, in the present year the foundation construction will be completed. The schedule foresees the radio telescope inauguration in late 2006.

Status of the Sardinia Radio Telescope project

We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The instrument is the result of a scientific and technical collaboration among three Structures of the Italian National Institute for Astrophysics (INAF): the Institute of Radio Astronomy of Bologna, the Cagliari Astronomy Observatory (in Sardinia,) and the Arcetri Astrophysical Observatory in Florence. Funding agencies are the Italian Ministry of Education and Scientific Research, the Sardinia Regional Government, and the Italian Space Agency (ASI,) that has recently rejoined the project. The telescope site is about 35 km North of Cagliari. The radio telescope has a shaped Gregorian optical configuration with a 7.9 m diameter secondary mirror and supplementary Beam-WaveGuide (BWG) mirrors. With four possible focal positions (primary, Gregorian, and two BWGs), SRT will be able to allocate up to 20 remotely controllable receivers. One of the most advanced technical features of the SRT is the active surface: the primary mirror will be composed by 1008 panels supported by electromechanical actuators digitally controlled to compensate for gravitational deformations. With the completion of the foundation on spring 2006 the SRT project entered its final construction phase. This paper reports on the latest advances on the SRT project.

Water masers in the Kronian system

The presence of water has been considered for a long time as a key condition for life in planetary environments. The Cassini mission discovered water vapour in the Kronian system by detecting absorption of UV emission from a background star (Hansen et al. 2006). Prompted by this discovery, we started an observational campaign for search of another manifestation of the water vapour in the Kronian system, its maser emission at the frequency of 22 GHz (1.35 cm wavelength). Observations with the 32 m Medicina radio telescope (INAF-IRA, Italy) started in 2006 using Mk5A data recording and the JIVE-Huygens software correlator. Later on, an on-line spectrometer was used at Medicina. The 14 m Metsahovi radio telescope (TKK-MRO, Finland) joined the observational campaign in 2008 using a locally developed data capture unit and software spectrometer. More than 300 hours of observations were collected in 2006-2008 campaign with the two radio telescopes. The data were analysed at JIVE using the Doppler tracking technique to compensate the observed spectra for the radial Doppler shift for various bodies in the Kronian system (Pogrebenko et al. 2009). Here we report the observational results for Hyperion, Titan, Enceladus and Atlas, and their physical interpretation. Encouraged by these results we started a campaign of follow up observations including other radio telescopes.

A VLBI experiment using a remote atomic clock via a coherent fibre link

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

An active surface for large reflector antennas

We present a solution adopted for the Noto antenna (Sicily) in order to overcome the degradation of antenna efficiency due to the gravitational deformation of the structure of large antennas. This new setup allows a substantial increase in the operating frequency, and eliminates the dependence of the antenna efficiency on elevation.

Diving into the Sardinia Radio Telescope minor servo system

The Sardinia Radio Telescope (SRT) is a new 64-metre, Gregorian-shaped antenna built in Sardinia (Italy). It is designed to carry out observations up to 100 GHz. The telescope is provided with six focal positions: primary, Gregorian and four beam-waveguide foci. This paper describes the project of the servo system which allows the focus and receiver selection during the instrument setup. This system also operates, at the observation stage, the compensation of some of the stucture deformations due to gravity, temperature variations and other environmental effects. We illustrate the system features following a bottom-up approach, analysing all the project layers ranging from low-level systems, as the hardware controls, to the design and implementation of high-level software, which is based on the distributed objects ACS (ALMA Common Software) framework. Particular focus will be put on the links among the hierarchical levels of the system, and on the solutions adopted in order to guarantee that the control of the servo system is abstracted from the underlying hardware.

Observations and analysis of phase scintillation of spacecraft signal on the interplanetary plasma

Aims. The phase scintillation of the European Space Agencys Venus Express (VEX) spacecraft telemetry signal was observed at X-band (lambda = 3.6 cm) with a number of radio telescopes of the European Very Long Baseline Interferometry (VLBI) Network in the period 2009-2013. Methods. We found a phase fluctuation spectrum along the Venus orbit with a nearly constant spectral index of -2.42 +/- 0.25 over the full range of solar elongation angles from 0 degrees to 45 degrees, which is consistent with Kolmogorov turbulence. Radio astronomical observations of spacecraft signals within the solar system give a unique opportunity to study the temporal behaviour of the signals phase fluctuations caused by its propagation through the interplanetary plasma and the Earths ionosphere. This gives complementary data to the classical interplanetary scintillation (IPS) study based on observations of the flux variability of distant natural radio sources. Results. We present here our technique and the results on IPS. We compare these with the total electron content for the line of sight through the solar wind. Finally, we evaluate the applicability of the presented technique to phase-referencing VLBI and Doppler observations of currently operational and prospective space missions.

Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode

In September 2016, the microquasar Cygnus X-3 underwent a giant radio flare, which was monitored for 6 days with the Medicina Radio Astronomical Station and the Sardinia Radio Telescope. Long observations were performed in order to follow the evolution of the flare on a hourly scale, covering six frequency ranges from 1.5 GHz to 25.6 GHz. The radio emission reached a maximum of 13.2 ± 0.7 Jy at 7.2 GHz and 10 ± 1 Jy at 18.6 GHz. Rapid flux variations were observed at high radio frequencies at the peak of the flare, together with rapid evolution of the spectral index: α steepened from 0.3 to 0.6 (with S

Water masers in the Saturnian system

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