Broadband receiving systems for 4.58-8.8 GHz radio astronomical observations at Irbene radio telescopes RT-32 and RT-16

Since 2011 Ventspils International Radio Astronomy Centre has been involved in the large scale infrastructure project which allowed significant speeding-up of the upgrading activities related to radio telescopes RT-32 and RT-16 as to its fitting with appropriate VLBI receiving and recording equipment. Radio telescopes were instrumented with new state-of-art broadband cryogenic receivers for frequency range of 4.5 - 8.8 GHz developed and installed by company Tecnologias de Telecomunicaciones e Informacion. In this paper architecture of receiving system as well as significance and working principles of key subsystems are described. The receiver is formed by a cooled RF subsystem and a room temperature IF subsystem. The RF and IF subsystems are designed to process two C and X band signals (LCP and RCP) in parallel. Normally, during observations, the measured vacuum level in the receivers dewar is from 10e-6 to 10e-8 mbar and the temperature inside dewar is at level of 14 K at second stage, 20 K at polarizer and 46 K at the first stage. Since October 2015 radio telescope RT-32 with new receiver system took part in several successful international VLBI sessions. During preparation for VLBI observations preliminary aperture efficiency, system temperature and beam pattern measurements were carried out to evaluate RT-32 performance after the stations renovation that besides the receiver also included repairing of the main reflector. Performance parameters were derived with the help of switching noise diode and on-off observations of calibration sources with known flux density at various elevations. First results measured at 4836 MHz are summarized in this manuscript.