Ray-tracing and physical-optics analysis of the aperture efficiency in a radio telescope

The performance of telescope systems working at microwave or visible/IR wavelengths is typically described in terms of different parameters according to the wavelength range. Most commercial ray tracing packages have been specifically designed for use with visible/IR systems and thus, though very flexible and sophisticated, do not provide the appropriate parameters to fully describe microwave antennas, and thus to compare with specifications. In this work we demonstrate that the Strehl ratio is equal to the phase efficiency when the apodization factor is taken into account. The phase efficiency is the most critical contribution to the aperture efficiency of an antenna, and the most difficult parameter to optimize during the telescope design. The equivalence between the Strehl ratio and the phase efficiency gives the designer/user of the telescope the opportunity to use the faster commercial ray-tracing software to optimize the design. We also discuss the results of several tests performed to check the validity of this relationship that we carried out using a ray-tracing software, ZEMAX and a full Physical Optics software, GRASP9.3, applied to three different telescope designs that span a factor of $\simeq 10 in terms of D/lambda. The maximum measured discrepancy between phase efficiency and Strehl ratio varies between$\simeq 0.4 and 1.9 up to an offset angle of >40 beams, depending on the optical configuration, but it is always less than 0.5 where the Strehl ratio is >0.95.Comment: 34 pages, 7 figure

Electromagnetic Gap Leakage Analysis for the SKA Mid-Frequency Dish

This paper presents electromagnetic analysis as performed on the Square Kilometre Array (SKA) mid-frequency dish in order to estimate the ground leakage caused by gaps in the main reflector. The effects of the ground leakage are shown in terms of conventional performance parameters such as gain and aperture efficiency, as well as the percentage of feed power hitting the ground behind the main reflector. Analysis were performed for SKA Band 1 and 2 with a combined frequency range of 0.4 - 1.8 GHz. It is shown that the gaps in the main reflector reduce the gain between 0.23 and 0.05 dB and could increase the noise temperature by approximately 14 K when pointed at zenith. The transmission percentage of the incident power hitting the ground is below 0.2% across the combined bands

Electromagnetic analysis and preliminary commissioning results of the shaped dual-reflector 32-m Ghana radio telescope

This paper presents results from the electromagnetic analysis of the African VLBI Network shaped Ghana radio telescope at the operating frequencies of 5 and 6.7 GHz. The geometry implemented in commercial electromagnetic software provides insight into the effects of the slanted beam-waveguide, shaped reflector illumination and mechanical tolerances, which are known to be more stringent compared to a perfect paraboloid. It is shown that the theoretical maximum gain and aperture efficiency at 5 GHz are 63.80 dBi and 85.45%, respectively. The corresponding values at 6.7 GHz are 66.47 dBi and 88.00%, respectively. Comparisons to sidelobe maps produced from astronomical observations are also discussed, showing possible misalignment in the structure when utilised outside its originally intended purpose

Feasibility Study of Angular Super-Resolution with the Active Surface of a Radio Telescope

The concept of super-resolution refers to various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. A feasible method to design antennas and telescopes with angular resolution better than the diffraction limit consists of using variable transmittance pupils. The simplest transmittance pupils are binary phase shifts masks, also known as Toraldo Pupils, consisting of finite-width concentric coronae which modify the phase of the incident wavefront. In this work we present a preliminary feasibility study to determine if and how the active surface of the 32m Noto radio telescope can be used to modify the wavefront in the same way a Toraldo Pupil would do. Our preliminary analysis suggests that an ideal reflector with fully independent active panels would be able to achieve the super-resolution effect, but the real Noto active surface, where each actuator is connected to four distinct panels, adversely affects the operation of the simulated Toraldo Pupil. We are planning to apply the same analysis to the shaped active surface of the Sardinia Radio Telescope

Inaugurata la più grande antenna radioastronomica italiana

Con una cerimonia estremamente suggestiva è stato di recente inaugurato il Sardinia Radio Telescope. Oltre a numerose autorità nazionali e locali ed un’ampia rappresentanza di scienziati provenienti dai principali centri internazionali di radioastronomia, alla cerimonia hanno partecipato circa 2000 cittadini giunti dai comuni limitrofi interessati a conoscere questo nuovo “vicino di casa”. Risalta infatti in un passaggio naturalisticamente incontaminato, quale è il sito di Pranu Sanguni, questo nuovo impianto dalle dimensioni imponenti e costruito con tecnologia avanzatissima tale da renderlo uno dei più evoluti e potenti strumenti del mondo per lo studio delle emissioni radio provenienti dai corpi celesti e per applicazioni di geodinamica e di scienze spaziali.The Sardinia Radio Telescope was recently inaugurated in the course of an impressive ceremony. In addition to the representatives of local and national authorities, and scientists from leading centres of radio astronomy all over the world, the ceremony was also attended by around 2,000 local people from the nearby areas who were curious to meet this ‘new neighbour’ of theirs. This new installation of spectacular dimensions does indeed stand out in the unspoilt natural landscape of the site of Pranu Sanguni. Built with groundbreaking technology, it is in fact one of the most evolved and powerful instruments in the world for the study of radio emissions originating from celestial bodies and for geodynamics and space science applications

Electromagnetic modelling of the 32-m Ghana radio telescope

This paper presents the electromagnetic modelling and supporting results of the African VLBI Network (AVN) Ghana radio telescope at the operating frequencies of 5 and 6.7 GHz. Working from limited technical data, we establish suitable geometrical parameters for the unshaped version of the Cassegrain system, including modelling and rotation of the four mirrors within the slanted beam-waveguide (BWG). The geometry implemented in GRASP (General Reflector Antenna Software Package) produces several performance values including the effects of the BWG, struts and mechanical tolerances. It is shown that the theoretical maximum gain and aperture efficiency at 5 GHz are 63.09 dBi and 72.56%, respectively. The corresponding values at 6.7 GHz are 65.80 dBi and 75.42%, respectively

Preface to the special section on “Radio astronomy: A continuous demand for breakthrough technology”

Radio astronomy is a relatively young science: about an average human lifetime has passed since Karl Jansky’s measurement campaign took place at Holmdel, New Jersey, in the early 1930s, now celebrated as the birth date of radio astronomy. Most people working in the field today can claim to have personally known the pioneers in their countries. In the case of Italy, one such pioneer was Gianni Tofani, who passed away in February 2015. He fully devoted his professional life to scientific and technological research in astrophysics, mainly from the wonderful Arcetri hill of his beloved city, Florence. Furthermore, his management style was highly respected, bringing him to hold leadership positions such as the Director of the Institute of Radio Astronomy

Suppression of spurious harmonic responses in superconducting microstrip spiral filters using gold overlay and stagger tuning

The higher order resonances of microwave superconducting filters with spiral resonators were suppressed, giving a very wide stop-band. In one filter spirals were modified to have equal fundamental resonances, but different higher order resonances, by adding stubs and using non-uniform line width. The higher resonances were reduced to −39 dB, in a stop band up to 4.3 times the pass band centre. In another filter, parasitic resistive resonators were added, suppressing the responses to −57 dB. The resistance is provided by patterning the 200 nm gold overlay already required for the external contacts. The increase in pass band loss is not measurable. Measurements of a single spiral resonator with a gold-YBCO bi-layer (200 and 600 nm respectively) suggest that the resistivity of the gold layer at 15 K is the order of 22 times that of bulk gold, a slight advantage for the present application. At 100 K, it is about double

Compatibility studies between Radio Astronomy and three upcoming technologies

In recent years, the increased use of high-frequency technology in the millimetre and microwave range, including mobile phones, automobiles, and industrial equipment, has further reduced and threatened the spectrum assigned to the radio astronomy service (RAS). As a scientific and passive service, RAS requires protection from commercial services to observe the extremely faint celestial signals. As spectrum use for land, air, and space communications grows, protecting RAS operations from radio frequency interference is becoming more challenging. This report examines the impact of advanced technologies on radio astronomy, specifically car radar at 77 GHz, 5G and Wi-Fi device deployments at 6.6 GHz. These technologies are evaluated for their potential impact on Italian radio telescopes: the Radio Observatories of Medicina and Noto and the Sardinia Radio Telescope (SRT). Of particular concern is the potential threat posed by car radars to future high-density prospects, as well as the historical importance of the 6.6 GHz frequency for radio astronomical observation of methanol emissions from stars