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Over the past several years the millimeter wave VLBI (Very Long Baseline Interferometry) observations have been intensively performed. However phase fluctuation due to troposphere is one of the key issue in terms of degradation of sensitivity and limits imaging capability in millimeter wave VLBI observations. We describe the details of designed receiver optics for the Korean VLBI Network to calibrate tropospheric phase fluctuation for the millimeter wave VLBI observation. These optics guide beams from one position on sky to 22, 43, 86, and 129 GHz-band receivers simultaneously. Several topics, such as the design principles of imaging and power loss due to phase errors on common ellipsoidal mirrors are discussed.

The Korean VLBI Network (KVN) is a new mm-VLBI dedicated array with capability for simultaneous observations at multiple frequencies, up to 129 GHz.The innovative multi-channel receivers present significant benefits for astrometric measurements in the frequency domain. The aim of this work is to verify the astrometric performance of the KVN using a comparative study with the VLBA, a well established instrument. For that purpose, we carried out nearly contemporaneous observations with the KVN and the VLBA, at 14/7 mm, in April 2013. The KVN observations consisted of simultaneous dual frequency observations, while the VLBA used fast frequency switching observations. We used the Source Frequency Phase Referencing technique for the observational and analysis strategy. We find that having simultaneous observations results in a superior performance for compensation of all atmospheric terms in the observables, in addition to offering other significant benefits for astrometric analysis.We have compared the KVN astrometry measurements to those from the VLBA.We find that the structure blending effects introduce dominant systematic astrometric shifts and these need to be taken into account. We have tested multiple analytical routes to characterize the impact of the low resolution effects for extended sources in the astrometric measurements. The results from the analysis of KVN and full VLBA datasets agree within 2-σ of the thermal error estimate.We interpret the discrepancy as arising from the different resolutions. We find that the KVN provides astrometric results with excellent agreement, within 1-σ, when compared to a VLBA configuration which has a similar resolution. Therefore this comparative study verifies the astrometric performance of KVN using SFPR at 14/7 mm, and validates the KVN as an astrometric instrument. Subject headings: Astrometry -techniques: interferometric -quasars: individual (OJ287, 0854+213)GHz, along with KVN observations using a similar configuration, both carried out in April 2013. The VLBA observations were carried out using fast frequency switching between both bands, while the KVN used simultaneous dual frequency observations. This study will serve to deepen the understanding on the limitations imposed by the fast frequency switching observing mode (i.e. with the VLBA), and the benefits derived from simultaneous dual frequency simultaneous observations with the multi-channel receivers in the KVN. The driver for this is to investigate whether to equip global baselines with KVN-like systems or more conventional fast frequency-switching systems.The interest and basis of the SFPR method have been described in detail in other publications ; most recently, the application to spectral line observations of H 2 O and SiO masers in evolved stars, with a non integer frequency ratio, is presented in Dodson et al. (2014). In summary, the SFPR technique provides at mm-wavelengths the benefits that conventional PR has in the cm-wavelength regime, where the moderate tropospheric phase fluctuations can be match...

Oxygen-rich Asymptotic Giant Branch (AGB) stars can be intense emitters of SiO (v=1 and 2, J=1→0) and H 2 O maser lines at 43 and 22 GHz, respectively. VLBI observations of the maser emission provide a unique tool to probe the innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned H 2 O and v=1 and v=2 SiO maser maps have traditionally limited the physical constraints that can be placed on the SiO maser pumping mechanism. We present phase referenced simultaneous spectral-line VLBI images for the SiO v=1 and v=2, J=1→0, and H 2 O maser emission around the AGB star R LMi, obtained from the Korean VLBI Network (KVN). The simultaneous multi-channel receivers of the KVN offer great possibilities for astrometry in the frequency domain. With this facility we have produced images with bona-fide absolute astrometric registration between high frequency maser transitions of different species to provide the positions of the H 2 O maser emission, and the centre of the SiO maser emission, and hence reducing the uncertainty in the proper motion for R LMi by an order of magnitude over that from Hipparcos. This is the first successful demonstration of source frequency phase referencing for mm-VLBI spectral-line observations and also where the ratio between the frequencies is not an integer.

We have developed a new millimeter wave receiver system with input optics that support simultaneous observations in four bands of 22, 43, 86, and 129 GHz to facilitate calibrating tropospheric phase fluctuations for millimeter-wave VLBI observations. In order to make simultaneous observations in four bands pointing at the same position in sky, it is crucial that errors among the beams from any misalignments should be kept small. After doing the beam alignment in the laboratory, on-site test observations were carried out so as to evaluate the performance. The result is that the beam centers of the four bands with reference to the 86 GHz beam center were aligned within 2″ over most of the elevation range of the Korean VLBI Network (KVN) 21 m telescope. Measured telescope aperture efficiencies including the multiband receiver optics are 65% at 22 GHz, 62% at 43 GHz, 57% at 86 GHz, and 38% at 129 GHz. Through this novel optics covering wide RF bandwidth effectively, we can simultaneously observe the SiO maser transitions at 43, 86, and 129 GHz and in addition the water maser line at 22 GHz.