Single-Dish Performance of KVN 21 m Radio Telescopes: Simultaneous Observations at 22 and 43 GHz

We report simultaneous multifrequency observing performance at 22 and 43 GHz of the 21 m shaped-Cassegrain radio telescopes of the Korean VLBI Network (KVN). KVN is the first millimeter-dedicated VLBI network in Korea having a maximum baseline length of 480 km. It currently operates at 22 and 43 GHz and is planned to operate in four frequency bands: 22, 43, 86, and 129 GHz. The unique quasi optics of KVN enable simultaneous multifrequency observations based on efficient beam filtering and accurate antenna-beam alignment at 22 and 43 GHz. We found that the offset of the beams is within less than 5′′ over all pointing directions of the antenna. The dual-polarization, cooled, high electron mobility transistor (HEMT) receivers at 22 and 43 GHz result in receiver noise temperatures less than 40 K at 21.25–23.25 GHz and 80 K at 42.11–44.11 GHz. The pointing accuracies have been measured to be 3′′ in azimuth and elevation for all antennas. The measured aperture efficiencies are65%(K)/67%(Q) 65 % ( K ) / 67 % ( Q ) ,62%(K)/59%(Q) 62 % ( K ) / 59 % ( Q ) , and66%(K)/60%(Q) 66 % ( K ) / 60 % ( Q ) for the three KVN antennas, KVNYS, KVNUS, and KVNTN, respectively. The main-beam efficiencies are measured to be50%(K)/52%(Q) 50 % ( K ) / 52 % ( Q ) ,48%(K)/50%(Q) 48 % ( K ) / 50 % ( Q ) , and50%(K)/47%(Q) 50 % ( K ) / 47 % ( Q ) for KVNYS, KVNUS, and KVNTN, respectively. The estimated Moon efficiencies are77%(K)/90%(Q) 77 % ( K ) / 90 % ( Q ) ,74%(K)/79%(Q) 74 % ( K ) / 79 % ( Q ) , and80%(K)/86%(Q) 80 % ( K ) / 86 % ( Q ) for KVNYS, KVNUS, and KVNTN, respectively. The elevation dependence of the aperture efficiencies is quite flat for elevations greater than 20°.