Claudio Bortolotti

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.

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.

A coherent fiber link for very long baseline interferometry

We realize a coherent fiber link for application in very long baseline interferometry (VLBI) for radio astronomy and geodesy. A 550-km optical fiber connects the Italian National Metrological Institute (INRIM) to a radio telescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. We use an ultrastable laser frequency- referenced to the primary standard as a transfer oscillator; at the radio telescope, an RF signal is generated from the laser by using an optical frequency comb. This scheme now provides the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7 × 10-16 accuracy. The fiber link never limits the experiment and is robust enough to sustain radio astronomical campaigns. This experiment opens the possibility of replacing the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common- clock VLBI based on a network of telescopes connected by fiber links.

An RFI mitigation project at the Italian radio telescopes

In this paper we present the first results of a project aiming at the mitigation of one of the most pressing problems for observational radio astronomy in Italy and the cm-wavelength telescopes world-wide: the ever-deteriorating situation of Radio Frequency Interference (RFI). We illustrate the campaigns conducted at the Noto 32m radio telescope and the Sardinia Radio Telescope (SRT) observing sites to monitor the evolution of RFI at these locations in the frequency range 0.05–40 GHz. A new FPGA-based spectrometer and an offline software tool for RFI detection and excision are presented and their performances are summarized.

The optical fiber link LIFT for radioastronomy

Optical fiber links are beneficial not only for frequency metrology, but also for other fields of physical research, such as radioastronomy and geodesy. We realized the first optical fiber link from a National Institute of Metrology to a Radiotelescope used for Very Long Baseline Interferometry and for geodetic measurements, over a distance of 544 km. We performed a remote calibration of the hydrogen-maser used as a frequency reference at Medicina Radiotelescopes, in central Italy, against the Italian Cs fountain primary frequency standard. The comparison was limited by the statistical uncertainty of the hydrogen-masers. This experiment demonstrates that optical links can provide radioastronomical facilities with very accurate and stable frequency references, in perspective better than the currently used hydrogen-masers. This opens new perspectives in the ultimate limits of VLBI and a more precise space geodesy.

The RFI monitoring systems for the Medicina and the Sardinia Radio Telescopes

We present here two similar RFI monitoring systems now operational at the radio astronomy observatories at Medicina (30 Km East of Bologna) and the Sardinia Radio Telescope (SRT, a 115 GHz fully steerable 64 m dish, expected to see the first light by the end of 2010, at 35 Km North of Cagliari). Both systems were specifically designed for monitoring the radio spectrum with an extremely wide frequency range, in combination with very high sensitivity and dynamic range. Their frequency coverage spans both over the narrow bands (internationally) allocated to the Radio Astronomy Service as well as across the much wider frequency bands receivable by the advanced radio astronomical receivers operating at both observatories. While these systems are expected to monitor the interferences generated mainly outside our own premises, we are well aware that all modern electronics are susceptible to produce RFI. Then we are also planning to search for any self-interference case originated within our radio observatories. In order to identify the exact location of the RFI source, we make use of a fixed monitoring station, with antennas on top of a tower placed nearby the radio telescope, as well as of a mobile laboratory. This last is composed of a van fully equipped with high performance instrumentation (spectrum analyzers, receivers, extensible antennas, filtered preamplifiers, etc) and their associated ancillary devices (GPS navigators, antenna pointing readouts, autonomous power supplies, living aids, etc). We will describe in details all the RF components and the careful calibration of their electrical characteristics.