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1 Introduction

The 25 m radio telescope of the Urumqi Astronomical Station (UAS), Chinese Academy of Sciences, is located at Nan Mountain, which is south 40 km from Urumqi city, Xingjiang province, in the most northwestern part of China (81$^{\circ }10' 41'' $E, 43$^{\circ }28{{'}}16{{''}}$N). It is situated 2080 m above sea level, a very dry site for the short centimeter observations. The telescope was built in 1992 as an element of Chinese VLBI project and has been in operation since 1993. It has been outfitted with receivers for six wavelength bands centered near $\lambda=92$, 20, 13, 6, 3.6 and 1.3 cm. Table 1 summarizes the relevant information. The table shows the center frequency, bandwidth, antenna efficiency, polarization and system noise temperature.

Table 1: Receiver information

Band & 92 & 18 & 13 & 6 & 3.6 & 1.3 \\  \hline
 ...K) \\  
System temp. & 100 & 150 & 150 & 120 & 130 & 115 \\  \hline\end{tabular}

The antenna as an element of the EVN is important because of its unique location in the middle of the Asia-European land mass and in an area with a very stable geological structure. It has served the VLBI schedule more than 50% of the time per year. As a single dish, the Urumqi telescope has a relatively small aperture for observing cosmic sources. We can take advantage, however, of the good observing conditions of the site such as dry weather and low radio interference and work on special projects in deep integration and wide survey. There are several very important lines in the existing bands, water vapor (6$_{16}\rightarrow 5_{23}$) and ammonia (J, K=1, 1) lines in the K band, methanol (51-60A-) in the C band and hydroxyl lines in the L band. The K band receiver shown in Table 1 covers two lines, water vapor and ammonia without any modification in the receiver system.

The 22 GHz H2O maser lines are most spectacular, displaying many unusual characteristics of astrophysical masers, with their high intensity, point-like structure, great variability and wide velocity range. The masers can be used to trace the youngest star forming regions of the Milky Way, the dynamical properties of outflow in star forming regions, envelope expansion in late-type stars (Moran 1996), as well as gas motions of accretion disks in active galactic nuclei (Miyoshi et al. 1995). The largest surveys to date for H2O maser emission in the 6$_{16}\rightarrow 5_{23}$ transition in the Galactic and extra-galactic sources were carried out by the Arcetri group (Comoretto et al. 1990) and Braatz et al. (1996). About one thousand H2O maser sources, including perhaps one hundred megamasers, have been found in the north sky. Despite a bounty of information for the Galactic H2O maser emission, many aspect still remain unknown. For instance, the large-scale distribution of maser sources in the Milky Way is far from the completely defined (Wang et al. 1992). The variability has been studied in a systematic fashion only for a very limited sample of objects. For H2O megamaser from distant galaxies, increasing the size of the samples will be quite important for further defining the properties of accretion disks in active galactic nuclei (Greenhill et al. 1997).

At the (J, K) = (1,1) NH3 inversion line at 13 mm wavelength (23.694495 GHz), the HPBW of the Urumqi telescope is about 2' which is about 4 times the resolution of the 115 GHz CO survey of the Galaxy by Dame & Thaddeus (1985). Furthermore, The electric quadruple hyperfine structure of NH3 allows the optical depth and the density of molecular clouds to be derived from the observed relative brightness temperature of the different hyperfine components (Ho & Townes 1983). The structure of the Milky Way would be further elucidated by comparing the maps of NH3 and CO lines.

Investigating the large-scale structure and properties of the 22 GHz H2O maser and 23 GHz NH3 emission in the Milky Way are the main motivations for building a spectral line receiving system for the Urumqi telescope. The receiver is a cryogenically cooled 22 GHz system with the front end being a low noise HEMT amplifier. The back end is a surface acoustic wave (SAW) Chirp-Z-Transform (CZT) spectrum analyzing system.

The aim of this paper is to describe the instrument for the new spectral receiving system. All the calibrations and the first observational data are discussed elsewhere (Zheng et al., in preparation).

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