We give a brief overview of the technical development related to geodetic VLBI done during 2017 and 2018 at the Onsala Space Observatory. 1 General Information The technical development work for geodetic VLBI at the Onsala Space Observatory (OSO) was mainly dedicated to commissioning the Onsala twin telescopes (OTT). Additional technical development concerned a new broadband feed horn and activities related to water vapor radiometry and the tide gauge station. The main activities are summarized as follows and discussed in more detail in the subsequent sections: • Installation and testing of the OTT DBBC3s. • Testing the OTT CDMS systems. • Temperature monitoring system in the OTT towers. • Broadband feed horn. • Water vapor radiometry. • Tide gauge station. Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory OSO Technology Development Center IVS 2017+2018 Biennial Report 2 Installation, Testing, and Fine-tuning the DBBC3s The two DBBC3s for OTT were delivered in March 2017 and successively installed and tested. In the following weeks and months, extensive tests were performed and improvements made in close cooperation with Gino Tuccari and Sven Dornbusch. Several upgrades of the DBBC3 hardware and software were done, and in 2018 Ed Himwich included full support of the DBBC3 in the VLBI Field System. VGOS observations were started in September 2017, and by the end of 2018 the OTT VGOS systems worked quasi-operationally. As an example, Figure 1 depicts spectra that are produced immediately after each scan in VGOS sessions and displayed on the OTT VLBI FS computers. There are spectra for all 64 channels, each one with 32-MHz bandwidth, covering the current VGOS frequency and polarization setup, as well as sampler statistics. The graphs are displayed for each scan during a VGOS session, as well as stored, and thus allow an online quality control of the ongoing session as well as a post-session identification of potential problems due to, for instance, radio frequency interference (RFI) or other problems.

A new receiver for the Onsala 20 m antenna with the possibility of being equipped with 3 mm and 4 mm bands has been built and the 3 mm channel has been commissioned during the Spring 2014. For single-dish operation, the receiver uses an innovative on-source/off-source optical switch. In combination with additional optical components and within the same optical layout, the switch provides two calibration loads (for the 3 mm and 4 mm channels), sideband rejection measurement, and tuning possibilities. The optical layout of the receiver employs all cold (4 K) offset elliptical mirrors for both channels, whereas the on-off switch employs flat mirrors only. The 3 mm channel employs a sideband separation (2SB) dual polarization receiver with orthomode transducer (OMT), 4-8 GHz intermediate frequency (IF), x? 2pol x? upper and lower sidebands (USB? +? LSB). The cryostat has four optical windows made of high density polyethylene (HDPE) with anti-reflection corrugations, two for the signal and two for each frequency band cold load. The cryostat uses a two-stage cryocooler produced by Sumitomo HI? RDK? 408D2 with anti-vibration suspension of the cold-head to minimize impact of the vibrations on the receiver stability. The local oscillator (LO) system is based on a Gunn oscillator with aphase lock loop (PLL) and four mechanical tuners for broadband operation, providing independently tunable LO power for each polarization. This paper provides a technical description of the receiver and its technology and could be useful for instrumentation engineers and observers using the Onsala 20 m telescope.

A new decade bandwidth circular Eleven feed for future radio telescope projects has been developed, with a significantly improved BOR 1 efficiency (therefore aperture efficiency). This Eleven feed is constructed of “circularly” curved folded dipoles on a flat printed circuit board (PCB), with the aim to make this antenna structure more rotationally symmetrical at a very low manufacture cost. The simulated results show that the BOR 1 efficiency of the circular Eleven feed achieves better than -1 dB over a decade bandwidth of 1.3-14 GHz, which is considered as a significant improvement for decade-bandwidth feed technology.

The design of the cryogenic 2-14 GHz Eleven feed for reflector antennas in future wideband radio telescope involves electrical design of the Eleven antenna, design of center puck, different alternative solutions for integrating with low noise amplifiers (LNAs), mechanical and cryogenic design and tests, system noise temperature estimation and so on. All these design aspects will be described in the paper. A large quantity of simulated and measured data has been obtained in order to verify the electrical, mechanical and cryogenic performance, and the system noise temperature. The objective of this work is to provide a good feed candidate for reflector antennas in VILB 2010 and US SKA projects. The system configuration of the Eleven feed can be represented by the block diagram shown in Fig. 1, which consists of three parts: the Eleven antenna, center puck and the LNA-integration circuit board. Fig. 2 shows a photo of the Eleven feed. The reflection coefficient measured at Caltech (California Institute of Technology) is shown in Fig. 3, and the aperture efficiency and other subefficiencies measured at DTU (Denmark University of Technology) are shown in Fig. 4. Fig. 5 shows Steady state analysis of the thermal distribution along the feed with head load of 20W/m2. More simulated and measured data will be shown in the paper. In conclusion, the Eleven feed has good performance in the frequency range of 2 – 14 GHz, and has gone through several cryogenic tests.

APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world's outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.