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تسجيل مستخدم جديدKorean VLBI Network: the First Dedicated Mm-Wavelength VLBI Network in East Asia
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Korean VLBI Network (KVN) is the first dedicated mm-wavelength VLBI Network in East Asia and will be available from the middle of 2008. KVN consists of three stations and has the maximum observation frequency of 129 GHz with the maximum baseline length of 480 km. KVN has unique characteristics in the multifrequency, simultaneous observing system. By taking advantage of this we are considering various science topics, including not only maser emitting regions and young stellar objects in our galaxy, but also extragalactic objects. Construction of the first site is in progress. We are concurrently developing components, including receivers, data acquisition systems, and a correlator, and also arranging the international collaboration.
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Koreas new VLBI project to construct the Korean VLBI Network (KVN) started in 2001, as a 7-year project that is fully funded by our government. We plan to build 3 new high-precision radio telescopes of 21-m diameter in 3 places in Korea, which will b
e exclusively used for VLBI observations. We will install the 2/8, 22 and 43 GHz HEMT receivers within 2007 as a first target, and later we will expand the receiving frequency up to 86 and 129 GHz for astronomical, geodetic, and earth science VLBI research. The millimeter-wave VLBI will be the ultimate goal of KVN. For the front-ends, we are going to install a multi-channel receiver system that employs low-pass filters within a quasi-optical beam transportation system. This receiver system will give reliable phase calibrations for millimeter-wave VLBI as well as enable simultaneous multi-frequency band observations. The hard-disk type new Mark 5 will be used as the main recorder of KVN. We have completed the design of the KVN DAS system of 2 Gsps sampling rate, which will use 4 data streams to meet the multi-channel requirement. A VERA type DAS modified for Mark 5 recorder is also under consideration. A new correlator project for KVN was recently approved from Korean government, and will start in the second half of 2004.
We present the results of comparative study of amplitude calibrations for East-Asia VLBI Network (EAVN) at 22 and 43 GHz using two different methods of an a-priori and a template spectrum, particularly on lower declination sources. Using observationa
l data sets of early EAVN observations, we investigated the elevation-dependence of the gain values at seven stations of the KaVA (KVN and VERA Array) and three additional telescopes in Japan (Takahagi 32m, Yamaguchi 32m and Nobeyama 45m). By comparing the independently obtained gain values based on these two methods, we found that the gain values from each method were consistent within 10% at elevations higher than 10 degree. We also found that the total flux densities of two images produced from the different amplitude calibrations were in agreement within 10% at both 22 and 43 GHz. By using the template spectrum method, furthermore, the additional radio telescopes can participate in the KaVA (i.e. EAVN) so that it can give a notable sensitivity increase. Therefore, our results will constrain the detailed conditions to reliably measure the VLBI amplitude using EAVN and give a potential to extend possible telescopes comprising EAVN.
This paper presents the catalog of correlated flux densities in three ranges of baseline projection lengths of 637 sources from a 43 GHz (Q-band) survey observed with the Korean VLBI Network. Of them, 623 sources have not been observed before at Q-ba
nd with VLBI. The goal of this work in the early science phase of the new VLBI array is twofold: to evaluate the performance of the new instrument that operates in a frequency range of 22-129 GHz and to build a list of objects that can be used as targets and as calibrators. We have observed the list of 799 target sources with declinations down to -40 degrees. Among them, 724 were observed before with VLBI at 22 GHz and had correlated flux densities greater than 200 mJy. The overall detection rate is 78%. The detection limit, defined as the minimum flux density for a source to be detected with 90% probability in a single observation, was in a range of 115-180 mJy depending on declination. However, some sources as weak as 70 mJy have been detected. Of 623 detected sources, 33 objects are detected for the first time in VLBI mode. We determined their coordinates with the median formal uncertainty 20 mas. The results of this work set the basis for future efforts to build the complete flux-limited sample of extragalactic sources at frequencies 22 GHz and higher at 3/4 of the celestial sphere.
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 t
he 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-$sigma$ 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-$sigma$, 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.
Guided by the recently published science case for the future of European VLBI, EVN2015, a roadmap for the future of the EVN is sketched out in this paper. The various desired technical improvements are being discussed with an emphasis on the role of
e-VLBI. With this innovation new scientific capabilities are introduced. In this way the EVN is also positioned as an interesting platform for exercising new techniques and operational models, complementary to other SKA pathfinders. In return, the technology development for the SKA can have a positive impact on the scientific capabilities of VLBI, for example on the development of a next generation correlator, capable to process much larger data-rates. The development of cheap, frequency agile antennas can also be of great importance for VLBI. This adds to the potential for maintaining a Northern hemisphere, global VLBI array in the SKA era.
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