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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill C. Jin et al.: An Introduction to the Miyun 50\,m Radio Telescope  %%（作者：题目名称）
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    \zihao{6}\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
An Introduction to the Miyun 50m Radio Telescope    %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize C. Jin,\hs\hs H. Wang\hs\&\hs X. Zhang}  %%（本文作者）

\vs
({\footnotesize \nao}) \\%%（本文作者单位）

\vs\vs \ec

\normalsize
\begin{quotation}

\no{\bf A{\scriptsize\bf BSTRACT }}~~~\small The National
Astronomical Observatories of China
 is planning to build a 50m radio telescope at Miyun Station.
In this paper, we briefly introduce the scientific motives and the
design specifications of this radio telescope. The primary
scientific motive is the gravitational wave detection by long-time
monitoring of a group of millisecond pulsars. A digital backend
like COBRA\footnote{COBRA-Coherent On-line Baseband Recorder for
Astronomy.} (Kramer et al. 2001)  is under consideration. Other
applications of the telescope are also introduced.

\end{quotation}

\para{\large\bf{1}~~ Introduction}

\vs\vs Pulsars are rotating neutron stars. Since first discoveries
more than 30 years ago, it has become a very attractive object to
astronomer because of the extreme density and activity. We can
consider a pulsar, especially millisecond pulsar  (Backer et al.
1982), as a very precise natrual clock on the sky. Early in 1979,
Detweiler considered the influence of gravitational wave on this
natrual pulsar clock, and suggested the method to detect
gravitational wave by mornitoring pulsar periods (Detweiler 1979)
. A group in Princeton has made long term monitoring of
millisecond pulsar using the Aricebo telescope. They had reached
an accuracy which is comparable with that of atomic clock (Paspi,
Taylor, \& Ryba 1994) . However, the sensitivity requirement made
such experiments only could be performed with few large radio
telescopes. But these large telescopes are always occupied with
various observational tasks, thus could not be fully devoted into
pulsar observations.

In 2000, Prof. Wang Shouguan has proposed to build a low-cost L-band 50m
pulsar radio telescope at the Miyun Station. The main scientific motive
is the gravitational wave detection, by doing long-time mornitoring of
millisecond pulsars. With the relatively large collecting area,
this telescope could carry out other astronomical observations 
(e.g IPS\footnote{IPS - Interplanetary Scintillation.}, SNR) .
And it also could act as an element, to join the current VLBI array.

We introduce the 50m telescope in this paper. In the following section,
we describe the design specifications and expected
performation of the telescope. In the third section, the status of the
project is introduced. In the fourth section, other possible astronomical
applications are introduced. In the last section, a summary is given.

\para{\large\bf{2}~~ Design Specifications and Expected Performance}

\vs\vn{\bf{2.1~~Design Specifications}}

Considering the sensitivity requirement for pulsar observations and the
limited budget available, we have decided to build a L-band 50m radio
telescope. The main design specifications are presented in Table 1.

\newpage
\bc{\small\bf Table 1~~ The main design specification of the
50m radio telescope
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{ccc}
\hline\\[-3.6mm]\hline
 Dish diameter & : & 50\,m \\
Mounting      & : & Altitute-Azimuth \\
Surface       & : & Aluminium plate  (inner 30\,m)  \\
              &   & Welded metal mesh  (outer part)  \\
RMS surface accuracy & : & 1 mm  (inner 30\,m)  \\
                     &   & 3 mm  (outer part)  \\
Short wavelength limit & : & 2\,cm  (inner 30\,m)  \\
Pointing accuracy & : & 19 arcsec \\
Slew rates (elevation)  & : & 0.5 degree per second \\
Slew rates (azimuth)    & : & 1.0 degree per second \\
Optics                & : & Prime focus, f/D= 0.35 \\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

Large bandwidth is needed to meet the sensitivity requirement for
pulsar observations. The ionized ISM, through with the radio
signal propagates before reaching us, would cause a dispersion to
the received signal. This despersion effect would then lead to a
smearing of the pulses when observing on earth with a finite
bandwidth. One method to reduce this dispersion is to divide the
observing bandwidth into small channels, and then record the
signals from the small channels separately. After that, the
signals from different channels are combined together, taking the
time-delay among the channels caused by the dispersion effect into
account. This method could reduce the dispersion to a large
extent, and is widely use for many years. But small residual
smearing still resides within each channel, and the response time
of the filters prevent dividing the observing bandwidth into more
narrower channels. The coherent de-dispersion method  (Hankins \&
Rickett 1975)  can, in principle, remove the dispersion completely
when the dispersion measure is known in advance. This method
require the phase information of the signal to be kept during the
data processing procedure. However, the required large
computational power made this method routinely implemented
possible only in recent years. The de-dispersion calculation could
be done in hardware  (Backer et al. 1997)  or software  (Stairs et
al. 2000), eighter on-line or off-line. Jodrell Bank now are
developing a Coherent On-line Baseband Recorder for Astronomy
(COBRA), which will perform the pulsar data processing on-line in
software (Kramer et al. 2001) .

We plan to equit the new 50m radio telescope with a COBRA-like digital
backend system, but with a wider bandwidth of 300 MHz. The NAOC and JBO have
signed a Memoremdum of Understanding concerning cooparation on developing
the receiver system for this 50m telescope. The pre-detection votage data will
be sampled and processed on-line, these offers large flexibility to the
data processing.

\vs\vn{\bf{2.2~~Expected performance}}

\bc{\small\bf

 Table 2~~ Expected performance at L-band
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{ccc}
\hline\\[-3.6mm]\hline
 Antenna Efficiency & : & 50\% \\
System Temperature  & : & 30K \\
Center Frequency   & : & 1665\,MHz  (18cm)  \\
Bandwidth          & : & 300\,MHz \\
Sensitivity        & : & 0.3\,mJy  (30minutes, 300MHz) \\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

The expected performance of the 50\,m radio telescope at L-band is
presented in Table 2. At L-band, the whole aperture will be used,
the total effective collecting area is about 1000 squre meters.
The lower limit of observable flux density is 0.3mJy, with 30 minutes 
integration time and 300 MHz observing bandwidth. If we extend the 
integration time to 10 hours, then the sensitivity would be 0.07mJy. With
sensitivity like this, more than 30 millisecond pulsars could be
monitored. This is esential to our prime scientific objective --
the gravitational wave detection.

\newpage
\vspace*{1mm}
\para{\large\bf{3}~~ Status of the Project}

\vs\vs In the second half of 2001, we invited four domestic
institutes to make conceptual design for this 50\,m radio
telescope. By comparing their designs and considering their
background in telescope-related domain, we have chosen the 54th
institute to build the telescope for us. Their conceptual design
is shown in Fig 1. This is a traditional wheel-on-track system.
The r.m.s. error of the surface within central 30 meter is 1\,mm,
thus the telescope can work up to 15\,GHz.

\begin{figure}[h]
\center {\includegraphics [bb=46 0 393 504,scale=0.5]{50m.eps}}

   %\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize Fig.\,1~~ The conceptual design of the 50m radio telescope}

    \end{figure}

The NAOC and the 54th institute have signed the contract of
building the 50m telescope. The construction of the telescope is
schedueled to be completed at the beginning of 2005.

\para{\large\bf{4}~~Other Astronomical Applications}

\vs\vn{\bf{4.1~~IPS Observations}}

The first IPS observation was carried out at Miyun Station with
the MSRT radio telescope (Wu, Zhang, \& Zheng 2001) . With the
relatively large collecting area, the 50m telescope could also do
IPS observations. And larger collecting area could be obtained by
combining the signals from the MSRT and the 50\,m telescope
together.

\vn{\bf{4.2~~Extending the MSRT and a VLBI unit}}

The 50\,m will be located near the current MSRT. The MSRT is an
east-west earth rotation synthesis array, consisting of 28 9\,m
radio telescopes. The sensitivity of the array will be increased
by a factor of 2 after the signal from the 50\,m telescope is
combined.

Due to its location and relative large collecting area, the 50\,m
telescope could join the current VLBI array. The 50\,m telescope
covers most of the EVN working frequencies. The antenna locations
of the current EVN and the 50\,m telescope is shown in Figure 2.
The 50\,m telescope will contribute mostly on the longer
baselines.

\begin{figure}[h]
\center {\includegraphics [bb=45 234 393
440,scale=0.7]{evnmiyun.eps}}

   %\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize

      Fig.\,2~~ The EVN and the Miyun station. The circled position is the Miyun
Station.}

    \end{figure}

\no{\bf{4.3~~Space missions}}

The telescope could work up to 15\,GHz, and it is the largest
singal dish in China. If equipped with proper receiver and
transmitter, it then could act as a ground station to
telecommunicate \& receiving data for space missions (e.g. the
coming WSO project) .

\para{\large\bf{5}~~ Summary}

\vs\vs We introduced the new 50m L-band pulsar radio telescope.
The main scientific motive is gravitational wave detection, by
long-time mornitoring of a group of millisecond pulsars. With the
relatively large collecting area, this 50m radio telescope could
carry out other astronomical observations, e.g. IPS, SNR, etc. The
telescope could also join the current VLBI array. Being the
largest singal dish in China, it could also work as a ground
station for space missions.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~We thank the four domestic institutes
that participated in this project. The figures about the
conceptual design of the telescope were provided by the 54th
institute.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

\nref Backer D.C., Kulkarni S.R., Heiles C., et al., Nature, 1982,
300, 615 \nref Baker D.C., Dexter M.R., Zepka A., et al., PASP,
1997, 109, 61 \nref Detweiler S., ApJ, 1979, 234, 1100 \nref
Hankins T.H., Rickett B.J.,  In: Methods of Computational
           Physics.  Radio Astronomy.  New York: Academic Press, 1975, 14,
           55
\nref Kramer M., Lyne A.G., Joshi B.C., et al., Contribution
           to SKA/RFI mitigation workshop, Boon, 2001
\nref Paspi V.M., Taylor J.H., Ryba  M.F.  ApJ, 1994, 428, 713
\nref Stairs I.H., Splaver E.M., Thorsett S.E., et al.,  MNRAS,
2000, 314, 459 \nref Wu J., Zhang X., Zheng Y.,  ApSS, 2001, 278,
189

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