Alessandro Orfei

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.

A Multi-Feed Receiver in the 18 to 26.5 GHz Band for Radio Astronomy

A large-bandwidth, state-of-the-art multi-feed receiver has been constructed to be used on the new 64 m Sardinia Radio Telescope (SRT) (http://www.srt.inaf.itl), an antenna aiming to work from 300 MHz to 100 GHz with an almost continuous frequency coverage. The goal of this new receiver is to speed up the survey of the sky with high sensitivity in a frequency band that is very interesting to radio astronomers. In the meantime, the antenna erection has been finalized, and the receiver has been mounted on the Medicina 32 m antenna to be tested (http://www.med.ira.inaf.itl). We present a complete description of the system, including a dedicated backend, and the results of the tests.

The dual-band LP feed system for the Sardinia Radio Telescope prime focus

We present the design of the passive feed system of the dual-band receiver for the prime focus of the Sardinia Radio Telescope (SRT), a new 64 m diameter radio telescope which is being built in Sardinia, Italy. The feed system operates simultaneously in P-band (305-410 MHz) and L-band (1300-1800 MHz). The room temperature illuminators are arranged in coaxial configuration with an inner circular waveguide for L-band (diameter of 19 cm) and an outer coaxial waveguide for P-band (diameter of 65 cm). Choke flanges are used outside the coaxial section to improve the crosspolarization performance and the back scattering of the P-band feed. The geometry was optimized for compactness and high antenna efficiency in both bands using commercial electromagnetic simulators. Four probes arranged in symmetrical configuration are used in both the P and the L-band feeds to extract dual-linearly polarized signals and to combine them, through phased-matched coaxial cables, into 180 deg hybrid couplers. A vacuum vessel encloses the two P-band hybrids and the two L-band hybrids which are cooled, respectively at 15 K and 77 K. For the P-Band, four low loss coaxial feedthroughs are used to cross the vacuum vessel, while for the L-Band a very low loss large window is employed. The P-band hybrids are based on a microstrip rat-race design with fractal geometry. The L-band hybrids are based on an innovative double-ridged waveguide design that also integrates a band-pass filter for Radio Frequency Interference (RFI) mitigation.

A Novel Broadband Q-Band Polarizer with Very Flat Phase Response

Abstract We present a 33–50 GHz circular waveguide polarizer based on a broadband combination of retarders. The device was designed within the FP7 RadioNet APRICOT (‘All Purpose Radio Imaging Cameras on Telescopes’) consortium for the development of focal plane arrays for radiotelescopes. The device is based on a Pancharatnam recipe used in optics and behaves as a polarization transformer, from circular to linear and vice versa. This novel design of polarizer exhibits an unprecedented performance combining extremely flat differential phaseshift across the whole band, very low losses and very low Amplitude Unbalance (AU). It can be used in astronomical instruments or telecommunication antennas where very high purity circular polarization is required. The design was manufactured and then tested using a Vector Network Analyzer (VNA). Averaged across a 40% bandwidth the measured RL, IL, amplitude unbalance and differential phase-shift were respectively:−31.4/−35.3 dB, −0.09/−0.06 dB, 0.04 dB and 90.0 ± 0.4°.

InP MMIC amplifiers for cryogenic radioastronomical applications

Radioastronomical observation development pushes technology research to develop equipment working up to 100-150 GHz. This trend makes the noise contribution of the low noise amplifiers (LNA) more serious. A significant improvement comes from the use of indium phosphide (InP) technology, as it gives the best noise performance in this range. Moreover, as radioastronomy is moving towards the array receiver configuration, this implies high integration and repeatability. In the frame of the FARADAY Project, we have been involved in designing several kinds of MMIC amplifiers in the 18-26 GHz range in order to approach the InP technology and the MMIC design methodology.

Commissioning of the Sardinia Radio Telescope in Italy: Results and perspectives

On 30 September 2013, the opening ceremony of SRT has signed the contractual ending of the instrumental commissioning of the Sardinia Radio Telescope. In February 2014 it has been completed also the “fine tuning process” aimed at defining the first optimizations parameters needed to make calibrated radio astronomical observations. Since then, the final Astronomical Validation, that was just started in parallel with the previous activities, has taken the lead of the users allocated time. At the time of the real presentation in August we expect to be able to present the experimental quantitative results of the commissioning that at the time of this writing are still under analysis.

Electromagnetic analysis of the Petzval surface for Medicina Radio Telescope

Multi-feed represent one of the new frontiers in the field of radio astronomy. The technology is now quite mature to allow the fabrication of receivers with high performance at relatively low cost. Hence, one can get numerous benefits by simultaneously using many feeds, placed in the focal plane of the reflector antenna. However the array dimension is set by the maximum loss acceptable in the antenna gain due to off-axis feed aberrations. These effects can be limited by adjusting the position along the reflector axis according to the Petzval surface. Nowadays, the very powerful electromagnetic software makes the optimization of such a best-focus position easier. In this contribution we show several numerical results of the electromagnetic performance exploiting the Petzval surface. The numerical analysis has been conducted for the Italian radio telescope of Medicina (Bologna) at 43 GHz.

An agile receiver-switching solution for use with parabolic antennas

We present a solution adopted for the Medicina antenna to carry out the fast, remote, and automatic change of many microwave receivers used for observation in radio astronomy. This new setup allows the capability to switch receivers located at the antennas primary and secondary foci in a couple of minutes, without the need of manpower or equipment, and regardless of weather constraints.

Metamaterial-based Toraldo pupils for super-resolution at millimetre wavelengths

Using the long-established Cardiff metal-mesh filter technology, we have exploited our ability to artificially manipulate the phase of a wavefront across a device in order to produce a dielectric-based Toraldo pupil working at millimeter wavelengths. The use of a Toraldo pupil to push the angular resolution of an optical imaging system beyond the classical diffraction limit is yet to be realized in the millimeter regime, but is an exciting prospect. Here we present the design and measured performance of a prototype Toraldo pupil, based on a 5 annuli design.