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    {\small\rm\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm  suppl.\hfill J. Esimbek et al.: Spectral System of the Urumqi
    Telescope and Water Maser Studies %%（作者：题目名称）
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\zihao{5}\songti

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    Vol.44~ Suppl.
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    \small ACTA ASTRONOMICA SINICA
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf Spectral System of the Urumqi
    Telescope and\\[2mm] Water Maser Studies    %%（文题）
%\\[1ex]
$\!\!^*$}

\footnote[0]{$^{\ast}$ supported by the NSFC Grant 10203003,
10128306, 10133020 and the G1999075405 NKBRSF}

\vs\vs{\normalsize Jarken Esimbek\up{1,2},\hs\hs Yuefang Wu\up{1}
\hs\&\hs Hongbo Zhang\up{2}}  %%（本文作者）

\vs ({\footnotesize 1~{\it Astronomy Dept, CAS-PKU Joint Beijing Astrophysics Center}~ 100871}) \\%%（本文作者单位）
({\footnotesize 2~{\it Urumqi Astronomical Observatory ,NAO,CAS,Urumqi}
~ 830011}) \vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
For spectral line observations in short centimeter wavelength,
 a new system was installed in the 25\,m telescope of Urumqi Station of NAOC.
The receiver is a cryogenically
 cooled HEMT receiving system with a 2\,GHz wide band, a 20\,K working
  temperature and an average noise temperature of 50\,K. The back end
  is SAW chirp-transform spectrum analyzing system with a total
  bandwidth of 40\,MHz and a frequency resolution of 40\,kHz. Many known
  $H_{2}$O maser sources were observed. A new digital
correlation spectrometer will be installed at the end of this
year. The site and new facilities at Urumqi Astronomical Station
provide excellent conditions for the spectral line observations in
centimeter wavelength region. A frontier subject in astrophysics,
studying for water masers and star formation was designed.

 \vn
 \no{\bf Key words}~~  astronomical technique-spectrometer,
 ISM: masers, stars: formation
\end{quotation}

%%%% 正文
 \para{\large\bf{1}~~ The Spectral Systerm}

\vs\vs The 25-meter radio telescope of Urumqi Astronomical Station
of NAOC, is located at Nanshan Mountain near the sourth suburb of
Urumqi city, Xinjiang province. It situates 2080\,m above the sea
level, a very dry site for the short centimeter observations. The
telescope is mainly used as a VLBI station. It has receiving
systerms for even bands: 1.3\,cm, 3.6/13\,cm, 6\,cm, 18\,cm,
49\,cm and 92\,cm.

To investigate large-scale structure of the Milky Way and
dynamical properties of the 22\,GHz water maser emission, the
1.3\,cm spectral line receiving system was built. The receiver is
a cryogenically cooled HEMT amplifier. Its major parts are
designed and developed for the VLBI projects (Ma et al. 1999). A
slight modification was made and a second mixer with a reference
frequency synthesizer was added in order to compensate the Doppler
shift due to the motions of the sources. This generates a fixed
output at the 529\,MHz central frequency. The SAW CZT-based
spectrometer consists of three parts (Chen et al. 1990).
Pre-amplifiers and mixers constitutes the first stage with two
highly stable L.O. ($F_{1}$=710\,MHz,$F_{2}$=690\,MHz). Part II is
the SAW CZT processor containing four parallel CZT subsystems.
Each subsystem operates in a bandwidth 20\,MHz with 50\% working
duty cycle and a frequency resolution 39\,kHz. The SAW CZT
processor, therefore, has a bandwidth of 40\,MHz and 100\% duty
cycle. The equivalent transformation rate is greater than 1024
points/66.6\,$\mu$s. Part III is a digital processor to carry out
the A/D conversion and digital integration. The longest
integration is about 300\,ms(Zheng et al. 1999).

The spectrometer, synthesizer, antenna controlling computer and
noise source are operated by observing computer. The diagram of
the system is shown in Fig.1.

\begin{figure}[h]
\center {\includegraphics [bb=0 221 408 443,scale=0.6]{fig1.eps}}

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   %{\hspace{3.5cm}
   {\center
   \footnotesize

      Fig.\,1~~ The diagram of the 13mm water maser
spectral system}

    \end{figure}
    \begin{figure}[tbhp]
\center {\includegraphics [bb=0 186 408 453,scale=0.5]{fig2(1).eps}}\\
\center {\includegraphics [bb=0 184 408 453,scale=0.5]{fig2(2).eps}}\\
\center {\includegraphics [bb=0 178 408 456,scale=0.5]{fig2(3).eps}}\\
\center {\includegraphics [bb=0 175 408
456,scale=0.5]{fig2(4).eps}}

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   {\center
   \footnotesize

      Fig.\,2~~ Observed results of water maser
emission}

    \end{figure}

\para{\large\bf{2}~~Observational Tests}

\vs\vs With the SAW CZT-based spectrometer the early experimental
observations were carried out at 13.7\,m radio telescope at
Qinghai Station of the Purple Mountain Astronomical Observatory.
Since then the digital processor in the spectrometer has been
improved(Zheng et al. 1996; Chen et al. 1991). The 13\,mm spectral
receiving system on the 25\,m radio telescope at Urumqi was
experimentally used to observe water masers to test the properties
of the system and site of the Urumqi Station for short centimeter
astronomical observations. Water vapor maser emission from a
number of Galactic sources were observed. Observations were  made
in position-switching mode and a 31\,K state noise was used as a
second calibrator. The system temperature at zenith was 180\,K.
The absolute calibration of flux density was about 20\%. The
measured lines from some known sources, such as W3(OH), ON1 are
shown in Fig.2. The results present the feasibility of the
receiving system and the excellent meteorological conditions of
the Urumqi Station for short centimeter wavelength band.



\para{\large\bf{3}~~Digital Correlation Spectrometer}

\vs\vs Though the SAW CZT spectrometer can be used in astronomical
observations, it was designed in 1983. Its band width and
resolution are difficult to meet the further requests of
astronomical measurements.

 At present, digital auto-correlation is used as spectrum analyzing
 systems for radio astronomy. The digital auto-correlation spectrometer
 provides a flexible selection of bandwidth and high resolutions and
 convenient digital components.

 For improving the resolution and
 stability of the equipment, a new digital correlation spectrometer, as a
 back end will be
installed to the spectral system of the telescope at the end of
this year. The S80S4096 digital correlation spectrometer has 4096
frequency channels with the maximum analog bandwith ranging from
0.5MHz to 80MHz. At the maximum signal bandwith of 80MHz the frequency
resolution is 19.53KHz. The bandwidth and the
frequency resolution can be changed by selecting the appropriate
clock frequency.



\para{\large\bf{4}~~Project}

\vs\vs (1). Search new water masers. Select IRAS sources with high
color index without 6\,cm radiation as samples. Such objects may
be at early stages of young stellar objects and are important for
study initial conditions of star formation.

(2). Investigate water maser emissions of different stages of star
formation, to obtain the correlation of the water maser emissions
and the star forming and early evolution activities.

(3). Select masers of different star formation stages to monitor
their time variation and to examine the affect of the maser environment.

(4). We will cooperate with NAOC and select masers of different
star formation stages to observe the infrared emission with
1.26\,m telescope at the same time to investigate pumping
mechanism.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~
This project is supported by the NSFC Grant 10203003, 10128306 and
10133020 and the G1999075405 NKBRSF.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \nref Chen D., Gong J., Zhou X., et al.  A New Type of Radio
Astronomical Spectrometer Acta Astrophys.Sin.13, 1990, 85
 \nref
Zheng X., Zhou Y., Xu Z., et al., A\&AS, 1999, 137, 537 \nref Ma
L., Inatani J., Miyazawa K.,  et al. Report of a 22\,GHz
ReceiverSystem development,Wurumqi Station,Academia Science of
China,1995 La Canada, California, Model S80S4096 Digital
Correlation Spectrometer, July, 2001

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