4 Channel Receiver Beam Alignment with Quasi-optics System
Jiman Kang, Seog-Tae Han, Jung-Won Lee, Do-Heung Je, Seog-Oh Wi
Korean VLBI Network(KVN) is the world's first VLBI network that could measure 4-channel signals(22, 43, 86, 129GHz frequency band) simultaneously. In order to receive the signals at the same time, the signal should be divided into each frequency band using Low Pass Filters(LPFs) and they are transmitted through the Quasi-Optics System.
It is very difficult to align all elements because we use three LPFs, eight ellipsoidal mirrors and two flat mirrors. Therefore we made a three-axis beam measuring device, and we performed the beam alignment using a laser device and a beam measuring device.
The goal of the beam alignment is to align within 10% of the size of each frequency band beam. So we should adjust the beam within 2 arcseconds in 129GHz, 4 arcseconds in 43GHz, and 7 arcseconds in 22GHz of the each beam relative to 86GHz. Through repeated measurement experimentations and analysis of Quasi-Optical System, we aligned 22&43GHz band beams within 4 arcseconds and 129GHz band within 2 arcseconds based on 86GHz beam, and then we installed on the actual telescope.