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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 D. Yang \& J. Cheng: Current Projects and New Initiatives
    for Radio Telescope  %%��ڕ؞��ނ��ȓ����
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\zihao{5}\songti

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    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \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
Current Projects and New Initiatives \\[2mm]for Radio Telescopes   %%����ނ��
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}

\vs\vs{\normalsize Dehua Yang\up{1} \hs\&\hs Jingquan Cheng\up{2}}  %%������ڕ؞��

\vs ({\footnotesize 1~{\it Institute of Astronomical Optics \&
Technology (NIAOT)/ National Astronomical Observatories, CAS,
China (NAOC),
Nanjing }~ 210042}) \\%%������ڕ؞�����
({\footnotesize 2~{\it National Radio Astronomy Observatory, 949
North Cherry Avenue Tucson, AZ 85721, USA, The NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.}}) \\
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
With the development of radio, X-ray and gamma ray astronomy,
observation now is covering the whole band of electromagnetic
radiation. In the radio band, a lot of research interest lies in
the millimeter and sub-millimeter wavelength range, requiring new
and more powerful observing facilities. This paper
reviews current projects and new initiatives for radio telescopes,
including telescopes working at the millimeter and
sub-millimeter wavelength bands. The paper also describes key
technologies and special considerations in new telescope design,
such as surface accuracy, thermal control and other issues. Finally the
paper lists all current radio telescope programs in China.

 %\vn
 %\no{\bf Key words}~~  pulsar-mode  change-spectrum
\end{quotation}

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

\vs\vs Radio wave bands are not only complementary but also unique
compared with optical band in some fields of astrophysics. Due to
the expansion of the universe, relic radiation emitted from the
Big Bang has been shifted to the radio regime, and now is detected as the
cosmic microwave background, whose intensity distribution
allows astronomers to study the large-scale structure of the
universe (NRC 2001). The Earth's atmosphere which is opaque to many wavelengths is almost transparent
to radio waves,
which enables astronomers to have a
clear view to those optically ``unseen'' astronomical objects.
Hence, almost all radio telescopes are ground-based, and it is
possible to build very large dishes and complex instruments.

The last century has seen a variety of radio telescopes built, from the
Arecibo 305 m reflector to 100 m fully steerable dishes at Effelsberg and Green
Bank, from separated single dishes to big interferometry arrays, which cover a
wide radio band of electromagnetic radiation. However, there is still a ``gap''
from the far infrared to sub-millimeter and to millimeter sections of the
electromagnetic spectrum. The astronomy
at these wavelengths is particularly informative to study the early stage of star
formation (K\"archer \& Baars 2000). In recent years, many
radio telescopes and arrays have been designed for these sub-millimeter and millimeter
wave bands, such as the SMA, LMT, ALMA and EVLA (Expanded VLA). Because of shorter
wavelengths, the design of such telescopes is difficult, involving a number of
new technologies to achieve an all-weather, stable surface, and good pointing accuracy. To
attain these goals, either new thermally stable carbon fiber reinforced plastic material
is used or an active surface technology is employed. At the same time,
corresponding receiver instruments and facilities are in development. In this
short waveband, absorption of the atmosphere is serious. Therefore site
selection is also important. It is expected that many new discoveries will be made with
all these new telescopes.

\vspace*{1mm}\para{\large\bf{2}~~Current Projects and New
Initiatives}

\vs\vs In 2001, the US National Research Council reviewed all the
existing radio facilities and recommended a number of projects and
new initiatives in the centimeter, millimeter, and sub-millimeter
wavebands. Table 1 lists the major projects and new
initiatives in the centimeter waveband, and Table 2 lists those in
millimeter and sub-millimeter wavebands (NRC 2001). Some of them
are currently under construction or in upgrading.

\vn{\bf{2.1~~Projects in Centimeter Waveband}}

\begin{figure}[h]
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   \small

      \bf Table\,1~~ Major centimeter waveband telescopes
      \ec}

\center {\includegraphics [bb=0 0 983 649,scale=0.3]{tt1.eps}}
\end{figure}

For a radio telescope facility, there are two major concerns in its design:
the sensitivity and the angular resolution. The former is the telescope's
ability to detect very faint signals, while the latter is its ability
to see the fine details in the celestial objects. Higher sensitivity
requires a larger collecting area. Higher resolution requires longer baselines
if the wavelength is fixed, shorter wavelenth if the baseline is fixed. All
the proposed telescopes listed in both Tables 1 and 2 are fit with these
requirements. They have either a larger collecting area or a longer baseline.
Other new projects are in the shorter millimeter and sub-millimeter wavebands.

The largest fully steerable antennas are the 100\,m Effelsberg and
Green Bank telescopes. The Effelberg 100\,m dish, is a symmetrical
design and has used the so-called homology technique which enables
it to observe at the shortest wavelength of $\sim$4\,millimeter. The Green
Bank Telescope is an offset design and it uses an active control
method to improve its surface accuracy. The two giant antennas are
useful tools for studying complex or weak radio sources requiring
various frequencies and a large collecting area. For compact,
smaller-scale radio sources, interferometry is necessary. The
MERLIN, VLBA and VLA can provide spatial resolutions of 0.1
arcseconds or better.

For studying the star formation and supermassive black holes in powering
luminous active galactic nuclei, extremely fine angular resolution is needed. The
Expanded VLA (EVLA) will be the answer. The EVLA will have one order higher
sensitivity and angular resolution of the VLA. It will be possible to combine the
EVLA and VLBA systems to enhance VLBA's field of view and sensitivity.

The Advanced Radio Interferometry between Space and Earth (ARISE)
project includes a 25-m-class space antenna. The space antenna
will be linked with the VLBA. ARISE will achieve the
highest spatial resolution which is about six times better than
that of the VLBA. It will be used for studying compact objects and
will be operated at about the 3\,mm waveband.

SKA (Square Kilometer Array), at present, is under coherent technology
development. It is extremely important for studying how the first galaxies
condensed out of vast clouds of atomic hydrogen. It is also capable of studying the
distribution of dark matter on the largest scales by means of gravitational lensing.

Many of the current telescopes and arrays, such as EVLA, VLBA, LOFAR and the
privately funded 1HT, could be linked together to form the foundation
of ground-based interferometric centimeter-wave astronomy for the new decade.


\vn{\bf{2.2~~Projects in Millimeter and Sub-millimeter Wavebands}}

Table 2 shows all major projects and initiatives in the millimeter
and sub-millimeter wavebands. Among the projects, ALMA (Atacama
Large Millimeter Array) is the first-ranked ``new initiative'' of
radio projects started a decade ago. It is an international
collaboration among the United States, Canada, and Europe. Japan
has also expressed interest in participating in the project.
And Chile, as a host nation for ALMA, participates in the project
by making available the superb high dry astronomical site under
the southern sky in the Atacama Altiplano. ALMA is an array of 64
dishes with 12\,m aperture making a total area of
7,200\,m$^{2}$. It will image at 1\,mm wavelength with an angular
resolution as high as 0.01 arcseconds that will be achieved by the
optical Next Generation Space Telescope (NGST). At present, the
celestial sky at sub-millimeter wavelengths can only be studied
from space with small orbiting telescopes.
Telescopes like the Far Infrared and Sub-millimeter Telescope (FIRST)
can only provide a coarse angular resolution and a limited
sensitivity.
ALMA will bring to millimeter and sub-millimeter astronomy the
aperture synthesis techniques. This enables precision imaging
on sub-arcsecond angular scales. ALMA will also observe
at short centimeter and millimeter wavelengths, with the
same image detail and clarity as its optical and radio counterparts
do. ALMA is to be
the most powerful complete imaging instrument accessible to
astronomers.

\begin{figure}[h]
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   \small\bf

      Table\,2~~ Major ground based millimeter and sub-millimeter telescopes
      \ec}

\center {\includegraphics [bb=0 0 897 671,scale=0.4]{tt2.eps}}
\end{figure}

To be complementary to ALMA, a new initiative CARMA
(Combined Array for Research in Millimeter-wave Astronomy) is recommended on the northern hemisphere. It is
actually a hybrid array comprising nine of the current 6-m BIMA (Berkeley
Illinois Maryland Association) antennas, six 10.4-m OVRO (Owens Valley radio
Observatory) dishes, and ten new 2.5-m small antennas in California.

To cover all the sky, the South Pole Sub-millimeter-wave Telescope (SPST), a 7-
to 10-m aperture sub-millimeter-wave telescope, is recommended. Thanks
to its low opacity and stable seeing, the South Pole is the best site in the
world for ground-based observations at sub-millimeter wavelengths. SPST would
be equipped to survey the dusty universe and to study the distortion of the cosmic microwave
background caused by clusters of galaxies. Its
survey capability will make the SPST an important complement to ALMA.

The Large Millimeter Telescope (LMT), is a joint project between
Mexico and the US. It is a 50\,m telescope operating at millimeter
wavelengths (K\"archer \& Baars 2000; Eisentr\"ager \& S\"u$\beta$
2000). It is to be equipped with multi-beam receiving systems
to make full use of its great sensitivity. The LMT, as the biggest
single dish working at millimeter band with high resolution,
involves special technical considerations and material selections
in achieving its design goals.


\para{\large\bf{3}~~Key Technology Related to Millimeter and Sub-millimeter
Wavelength Telescopes}

\vs\vs The difficulties in the design of millimeter and
sub-millimeter telescopes are intrinsically associated with the
comparatively short wavelengths. Gravity, wind, and especially
thermal effect tend to be very serious sources of surface, phase,
pointing, and tracking distortions. In order to overcome such
problems to achieve several tenths RMS surface accuracy and
$\sim$1 arcsecond pointing accuracy under all operating
conditions, various measures are taken into consideration. These include:
special structure design, material selection, active feature,
thermal control, combination of open-loop and close-loop
compensation control, site investigation, etc.

Traditionally, the surface accuracy of radio telescopes is
achieved passively by homology and the pointing accuracy by a
two-axes control system with encoders for measurement. The
Effelsberg 100\,m dish has been notably successful for its
elegant homologous design making it capable of 4\,mm
observation. However, the homology concept may be limited due to
the laws of mechanics. So, a new design concept
or additional active features, including active surface control
and active sub-reflector, have to be introduced to some new
sub-millimeter telescopes. For very large reflectors, it is
impossible or impractical to achieve and to maintain a higher
accuracy by constructing sufficiently stiff truss for their dish structures.
Therefore, some antennas will have dishes involve not only truss
members. On some antenna design, it becomes beneficial to
deliberately make pliable homologous backup structure. With this
concept, taking advantage of the inherent pliability of the
trusswork, control efforts may contribute to couple drivers
electronically by encoders (K\"archer \& Baars 2000; Yang 2002).

In the concept of active surface control, actuators are installed at each corner
of surface panels and focusing-centering device is to be implemented for
actively correcting
sub-reflector position. For compensating the gravitational deflection, an open-loop
control and
a look-up table are possibly a solution. Gravity loads are repeatable as well as
predictable. The deflection pattern
for a series of elevation angles can be calculated or be measured using
some kind of laser range system. Correspondingly, the control of each surface actuators can be
derived or pre-stored in a control computer.  As to the deflection of sub-reflector, the same idea
is applicable. The deflection can also be calculated by finite element method or
measured by 5D or 6D laser metrology system (Chen et al. 2000).

Radomes had been used for comparatively small astronomical antennas in the past
to shield wind disturbance and solar radiation. However it is not practical
for very large antennas. Radome also causes serious signal loss resulting
lower sensitivity of the telescope in the shorter wavelength.
For this reason, new antennas all adopt exposed configuration. These antennas have to
undertake severe open environment, such as wind, solar and infrared radiation.
Nonuniform temperature field will cause deformations of telescope surface.
It also produces pointing and phase errors. Like gravitational
deflections, the thermal deformations may produce problems in large bull gear as well.
A special technique like ``buoyant'' pinion carriage for elevation drivers is
necessary for large antennas (Yang 2002). Unlike gravitational loads, thermal effect is randomly distributed.
Thanks to its low frequency behavior, it is possible to employ a close-loop correction,
namely to perform a real-time finite element model (FEM) calculation with actually measured
temperature field by thermal sensors. These sensors are installed over the telescope structure (Eisentr\"ager
\& S\"u$\beta$ 2000). The calculation will provide axes' misalignments,
pointing error and surface error. Therefore, the surface error can also be
corrected for by surface actuators while axes' misalignments and pointing error
may be compensated with control effort. Thermal treatments for alleviating
radiation induced surface error are advisable. Temperature changes between the
surface and backup structure due to solar radiation during day and infrared
radiation at night can be tempered by cladding the panel units. Ventilation
system, for example, the forced air cooling, can be used to get a better thermal
performance of the telescope structure. For overcoming wind effect, it is
recommanded that the telescope should be strong enough to work for maximum
operational wind velocity and to keep the wind-introduced errors within budget. Also,
for large reflectors, the site wind behavior needs to be investigated carefully
as it may cause inadmissible deflections of axes, especially that of elevation
axis. Some of the structural deflections can be detected by inclinometers and be evaluated by FEM model. From these measurement and calculation
pointing error can be compensated for by the same
idea as discussed above.

To reduce deflections and various errors, carbon fiber reinforced
composites (CFRP) are widely selected for some existing and newly
designed telescopes at these short wavelengths (Cheng 2000; Cheng 2002).
CFRP has a high stiffness over weight ratio and a low thermal
expansion coefficient. It has a density of 2268\,kg/m$^3$ and a
thermal expansion less than 2$\times$10$^-$$^6$. As an aniostropic
material, its dominant Young's modulus can be higher than that of
steel. These features of CFRP enable it to be used for fabricating
new telescope structure, especially, reflector back-up structure.
The disadvantage of using CFRP material consists in
complexity of CFRP joints and high material and labour cost.

Because the sub-millimeter wave band is the shortest radio wave window opened up
to the ground, in order to minimize the high absorption of the earth's
atmosphere of millimeter and sub-millimeter radiation, sites for locating
telescopes at these wavelengths need to be high and dry with low level man-made
radio frequency interference (RFI). The harmful RFI has been increasing dramatically in
recent years with the rapid growth of wireless communications
(K\"archer \& Baars 2000). Moreover, locally generated
RFI at site can be also troublesome, so remote control and operation may preferable to keep
the RFI sources at a minimum level (Jewell 2000) .

\para{\large\bf{4}~~Radio telescope programs in China}

\vs\vs In the past century, several radio telescopes have been
built at observatories of China. The largest dishes are two 25-m
centimeter wavelength telescopes, one is situated near Urumqi and
another in Shanghai. Both are part of international VLBI
cooperation. A 13.7\,m enclosed millimeter wavelength dish was
built in Delinha in 1986. And a meter-wave aperture synthesis
array is operated at Miyun station of NAOC.

At present, a 50-m pulsar radio telescope is underway at NAOC and hopefully to
be erected in 2 years. The Five hundred meter Aperture Spherical Telescope
(FAST) is, under feasibility study, as an effort for the international
cooperation in the Kilometer Square Array-SKA.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~The authors are grateful to the
support of NIAOT and NAOC on the preparation of this paper.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \nref NRC, USA 2001Astronomy \& astro-physics in the New
Millennium, \nref K\"archer J., Baars M., in Proc. of SPIE, 2000,
4015, 155 \nref Eisentr\"a ger P., S\"u$\beta$ M.,  in Proc. of
SPIE, 2000, 4015, 488 \nref Raffin P., et al.,  in Proc. of SPIE,
2000, 4015, 169 \nref Cheng J., in Proc. of SPIE, 2000, 4015, 48
\nref Cheng J., Thermal shape change of some CFRP-aluminum
honeycomb sandwiched structures, to be in Proc. of SPIE, 2002
\nref Cheng J., in Proc. of SPIE, 2000, 4015, 48 \nref Jewell
P.R., in Proc. of SPIE, 2000, 4015, 136 \nref Yang D., Design of A
50-m Pulsar Radio Telescope, to be in Proc.of SPIE", 2002, \nref
Chen F., Brown G.M., Song M., Opt. Eng., 2000,  39(1), 10
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