Giampaolo Zacchiroli

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.

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.

The microwave holography system for the Sardinia Radio Telescope

Microwave holography is a well-established technique for mapping surface errors of large reflector antennas, particularly those designed to operate at high frequencies. We present here a holography system based on the interferometric method for mapping the primary reflector surface of the Sardinia Radio Telescope (SRT). SRT is a new 64-m-diameter antenna located in Sardinia, Italy, equipped with an active surface and designed to operate up to 115 GHz. The system consists mainly of two radio frequency low-noise coherent channels, designed to receive Ku-band digital TV signals from geostationary satellites. Two commercial prime focus low-noise block converters are installed on the radio telescope under test and on a small reference antenna, respectively. Then the signals are amplified, filtered and downconverted to baseband. An innovative digital back-end based on FPGA technology has been implemented to digitize two 5 MHz-band signals and calculate their cross-correlation in real-time. This is carried out by using a 16-bit resolution ADCs and a FPGA reaching very large amplitude dynamic range and reducing post-processing time. The final holography data analysis is performed by CLIC data reduction software developed within the Institut de Radioastronomie Millimétrique (IRAM, Grenoble, France). The system was successfully tested during several holography measurement campaigns, recently performed at the Medicina 32-m radio telescope. Two 65-by-65 maps, using an on-the-fly raster scan with on-source phase calibration, were performed pointing the radio telescope at 38 degrees elevation towards EUTELSAT 7A satellite. The high SNR (greater than 60 dB) and the good phase stability led to get an accuracy on the surface error maps better than 150 μm RMS.

A Novel Application of the Active Surface of the Shaped Sardinia Radio Telescope for Primary-Focus Operations

The 64-m Sardinia Radio Telescope (SRT) is a shaped dual-reflector antenna designed to optimize secondary-focus operations. A novel application of the SRT active primary reflector aimed at achieving efficient operations also at the primary focus is presented. To this purpose, we show how the panels of the shaped primary mirror can be moved to best fit a parabolic surface. The modified primary reflector permits increasing the maximum frequency for primary-focus operation and also improves low-frequency multifeed capabilities. In fact, our numerical simulations show that at 80 GHz, the aperture efficiency from the primary focus drops only 10% with respect to 300 MHz.

Design of super-resolving Toraldo Pupils for radio astronomical applications

More than half a century ago, in 1952, Giuliano Toraldo di Francia suggested that the resolving power of an optical instrument could be improved using a filter consisting of finite-width concentric coronae of different amplitude and phase transmittance, now known as Toraldo Pupils (TPs). The concept of “super-resolution” was born, and in the current literature it is generally associated with various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. In the microwave range, the first successful laboratory test of TPs was performed in 2003. These first results suggested that TPs could represent a viable approach to achieve superresolution in Radio Astronomy. We have therefore started a project devoted to an exhaustive study of TPs and how they could be implemented on a radio telescope. In this work we present a summary of the status of this project, and then we describe a preliminary design to implement a TP system on a 32-m radio telescope.

A control loop closure system for the Sardinia Radio Telescope active surface

The Sardinia Radio Telescope (SRT) is a 64 meters (diameter) single dish radioantenna which is in the building phase in Italy. One of the most challenging characteristics of SRT is its capability to observe up to a frequency of 100 GHz thanks to its main reflector active surface. The active surface is composed by 1008 panels and 1116 mechanical actuators which may modify the segmented shape of the main reflector making possible the correction for wavefront distortions induced by the gravitational and thermal deformations. In order to observe at a frequency of 100 GHz the surface shape must be accurate below of a value of 150 μm r.m.s.. This value may be reached during the initial alignement phase using the microwave holography but it cannot be maintained during the scientific operations because of the (dynamical) deformations. In order to permit the observations at any time, a system able to measure the surface deformations with the necessary accuracy and a time-response of few minutes (the time-scale of the deformations) must be operative. We propose here three simple and robust methods to measure the relative deformations of the segmented panels with respect to an initial aligned surface (reference surface). The ultimate choice on which one of the three systems will operate on SRT will be taken after final testing on all of them. Prototypes of each system have been realized and two of them have been also successfully tested on the active optics radiotelescope of Noto (Italy). The test on the third system will be done in the next few months.

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.

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.