2nd URSI AT-RASC, Gran Canaria, 28 May – 1 June 2018 A Review of Front-End Receivers for the INAF Radio Telescopes P. Bolli(1), M. Beltrán(1), M. Burgay(2), C. Contavalle(3), P. Marongiu(2), A. Orfei(3), T. Pisanu(2), C. Stanghellini(3), S.J. Tingay(4), G. Zacchiroli(3), and A. Zanichelli(3) (1) INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy (2) INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy (3) INAF – Istituto di Radioastronomia, via Piero Gobetti 101, 40129 Bologna, Italy (4) Curtin University – Curtin Institute of Radio Astronomy, 1 Turner Ave, Technology Park, Bentley 6102, Western Australia Abstract facilities was charged to conduct this review. The group provides the authorship for this paper. This paper describes a rigorous process started within the Italian National Institute for Astrophysics (INAF) to The working group interacted with many Italian and review existing and future radio astronomical front-end foreign astronomers and technologists. The two most receivers for the INAF radio telescopes. The main goal of significant examples of this approach have been: i) the the review was to increase the level of coordination in survey of the characteristics of some of the major receiver development, improving at the same time the International radio telescopes to benchmark the scientific impact from the national facilities. The working performance of Italian telescopes against recognized top- group in charge of the review collected information on the class worldwide radio astronomy facilities; ii) the call for front-ends in operation and under development at the INAF ideas addressed to the whole Italian astronomical radio telescopes and new ideas for future receivers from the community to survey scientific interest in the development Italian astronomical community. The outcome of the of new receivers. During the review process, a national working group consists of recommendations on receivers workshop was held to present to the INAF community the under development and on strategies for future receivers at preliminary recommendations of the working group and at the suite of Italian radio telescopes. the same time to stimulate discussion and collect feedback. Finally, in May 2017 the working group concluded its 1. Introduction mandate by delivering the final report, available in [1], to the INAF management. In the northern summer of 2016, the newly established Section II (Radio Astronomy) of the INAF Scientific This paper describes and summarizes the most significant Directorate started a process aimed at harmonizing and aspects of the review and is organized as follows: Section coordinating efforts and resources in radio astronomy. The 2 presents the radio telescopes covered in the review; first step of this process was to review the radio Section 3 illustrates the receivers currently in operation and astronomical front-end receivers for the INAF radio under development at the INAF radio telescopes. The telescope facilities. Three main goals were identified: i) scientific perspectives for future receivers that emerged develop a list of work to undertake on existing receiver from the Italian community are reported in Section 4. systems that require major maintenance/repair; ii) Finally, Sections 5 and 6 contain the final undertake a review of receiver developments currently recommendations and the conclusions, respectively. underway; iii) develop a roadmap for future receiver developments. 2. INAF Radio Telescopes The process was designed to be science-driven, but also The INAF radio telescopes considered in the review relatively challenging and ambitious regarding engineering process were the Sardinia Radio Telescope (SRT, [2]), the developments. Moreover, it was requested to consider Medicina Radio Telescope (MED, [3]) and the Noto Radio boundary conditions, like the telescope operations and Telescope (NOTO, [4]). The main technical characteristics dependencies, as well as the International radio of each antenna are reported in Table 1. Even if the original astronomical contexts where INAF was involved in mandate of the review included the Medicina Northern technological/astronomical research and development. Cross, then it was realized that a significant infrastructure refurbishment in view of its use for radio astronomical A working group composed by ten people from INAF with studies is required, and this was outside the scope of the different professional backgrounds as well as a wide review. Also the space science applications of the SRT geographic representation from different groups and have been excluded from the review. In order to provide accurate recommendations, the working is composed by 10 FTE covering almost all the necessary group acquired technical information from other areas technical areas from active and passive microwave fundamental for the operational of the radio telescopes. components to mechanics and cooling, integration and These are briefly summarized in the remaining part of this testing. However, in specific areas the activity is Section. commissioned to external industries or to Universities and Research institutes. In the last 15 years, the receiver groups The three antennas run the same telescope control software: have designed and fabricated 13 receivers for the three the worldwide used Field System for Very Long Baseline Italian radio telescopes. Interferometry (VLBI) and a modern control system for single dish operations, developed within the DISCOS An important step of the review process was to collect project and integrating receiver and back-end controls. The technical, scientific and management data for the 14 data acquisition equipment available at the Italian radio receivers in operation at the various radio telescopes (five telescopes consists of a total-power detector, two different at MED, five at NOTO and four at SRT). The more relevant digital spectrometers, a digital correlator and a digital base technical information of these receivers (frequency range, band converter adopted by the European VLBI Network feed system typology, maximum antenna gain and system (EVN). A software-based correlator can also be used to noise temperature at zenith) are listed in Table 2, where perform VLBI observations. ranges are given whenever the performance is not constant over the whole frequency band. Opacity measurements at the SRT site show that in the winter period there is 50% probability to have an opacity An analysis of the scientific impact of the receivers in below 0.2 Np at 100 GHz; while the other Italian sites, operation has been carried out for MED and NOTO, while despite their very low altitudes, show opacity values at 90 it was not possible for the SRT due to its recent start of GHz below 0.25 Np in winter months. operations. The percentage of observing time (including VLBI, radio astronomical and geodetic, and single-dish) As far as Radio Frequency Interference (RFI) is concerned, allocated to each receiver shows that since 2010 at MED below 6 GHz RFI is seriously affecting the operations of the S/X-, Clow- and K-band receivers have been the most the receivers at MED where no geographic shields protect used with approximately 25-30% each. The situation at the radio telescope, especially in the not purely passive NOTO is slightly different: the total allocated time for frequency bands. The situation improves at SRT and S/X-, Clow- and Q-band receivers is 20-25% each, while NOTO, where above 3 GHz the RFI level is acceptable. At for the K-band it decreases to 15% due to its lower higher frequency (from 13 GHz at MED and from 6 GHz performance than the one in MED. In terms of publication at SRT and NOTO) the RFI environment is considered rate, for MED and NOTO the S/X- and Clow-band clean. receivers are the more productive due to their participation in VLBI observations. Scientific publications in single-dish Table 1. Main technical characteristics of the INAF radio observing mode typically use the higher-frequency telescopes receivers like the Clow-, X-, K- and Q-bands. Parameter MED NOTO SRT Inauguration date 1983 1988 2013 Table 2. Frequency coverage and measured performance Optical Cassegrain Cassegrain Shaped for the receivers in operation at the INAF radio telescopes configuration antenna antenna Gregorian Receiver Freq. range Feed Gain T_sys antenna [GHz] system* [K/Jy] [K] Primary mirror 32 32 64 1.35-1.45 0.12 55 MED L S diameter [m] 1.595-1.715 0.11 65 Active surface No Available Available 2.2-2.36 0.12 55 MED S/X DF Frequency range 1.35 – 26.5 1.4 – 86 0.3 – 116 8.18-8.98 0.14 38 [GHz] MED Clow 4.3-5.8 S 0.16 28 Frequency agility Available Partially Available MED Chigh 5.9-7.1 S 0.15 60-90 available MED K 18-26.5 D 0.11 50-80 Surface accuracy 350÷400 700÷900 300 2.2-2.36 0.15 120 μ NOTO S/X DF [ m] 8.18-8.58 0.15 110 Pointing accuracy 0.002 0.002 0.002 NOTO Clow 4.62-5.02 S 0.16 30 [deg] NOTO Chigh 5.10-7.25 S 0.13 120 NOTO K 21.5-23.0 S 0.08 110 3. Front-end receivers in operation and under NOTO Q 39-43.5 S 0.08 120 0.305-0.410 0.52 50-80 development SRT P/L DF 1.3-1.8 0.55 25-35 The design and development of front-end receivers for the SRT Chigh 5.7-7.7 S 0.6 24-28 8.2-8.6 0.64 180 INAF radio telescopes is in charge to the receiver groups SRT X/Ka DF distributed across the three INAF structures: Istituto di 31.85-32.25 0.57 190 SRT K 18-26.5 M 0.46-0.66 40-70 Radioastronomia, Osservatorio Astronomico di Cagliari * Legend for the feed system typology: mono-feed (S), dual- and Osservatorio Astrofisico di Arcetri.