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Vol.25 No.3 2011
Interview
Large Astronomical Telescope Development in
China: Achievements and Prospects
SONG Jianlan (Staff Reporter)
“LAMOST represents a brand-new concept for telescope design and development.” —CAS Member SU
Dingqiang
“We are taking the lead in some aspects, including large-scale spectral sky survey, active optics and
positioning of optical fibers.” —CAS Member CUI Xiangqun
From the point of view of an opticist, the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope (LAMOST)
marks a miracle-like milestone. It integrates a lot of innovative
ideas once thought to be impossible: both the primary mirror
and the corrector segmented and adjustable using active
optics; an incredible 5-degree field of view achieved by an 8m
telescope; the highest spectra acquiring rate in the world; an
amazing variable optical system capable of forming a series of
Schmidt telescopes through real-time adjustment, and many
others (refer to the description of LAMOST on page 231) .
The successful completion of LAMOST, a project
once thought “too ambitious to be possible,” has put CAS
Member SU Dingqiang, co-creator of the concept, in
spotlight together with CAS Member CUI Xiangqun, the
General Engineer supervising the whole project who has
turned Su’s ambitious ideas true.
Immediately before the news release of the 28th General
Assembly of the International Astronomical Union (IAU) in
Beijing, BCAS got an invaluable chance to talk with Su and
Cui on the development of large astronomical instruments
CAS Member Prof. SU Dingqiang,
co-designer of LAMOST and the
main inventor of the active optics
technology critical to the optical
system.
The Large Area Multi-Object Fiber Spectroscopic Telescope
(LAMOST), the biggest Schmidt reflecting telescope in the
world, is located in the Xinglong Observational Station under the
National Astronomical Observatories, CAS.
in China and strategy for its future advancement, starting
from LAMOST.
CAS Member Prof. CUI Xiangqun,
the General Engineer supervising
the whole LAMOST project.
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LAMOST—challenges, implications and creativity
Combining large field of view and large aperture
first time
Astronomy has long been cursed by a dilemma in
telescope development: the bigger the aperture is, the
smaller a field-of-view can a telescope cover. In other
words, it is impossible to build large-aperture telescopes
with very large fields of view. Thanks to the invention of the
Schimdt optical system, scientists can expand the field of
view through adding a corrector to the optical system, and
scientists built the first Schimdt telescope with an aperture
as big as 1.0 meter in the 1940s, around the same time
when people obtained the first 5m general telescope. Things
remained frustrating until the 1990s for large-aperture
Schimdt telescopes however, with the biggest aperture
barely beyond 1.0 meter, in a sharp contrast with the 10m
diameter for general-type telescopes achieved by scientists
in the same decade.
What adds to the difficulty is the limitation of optical
glass manufacture: it is widely agreed in the industry that
the aperture of a single piece of high-quality optical glass
suitable for a telescope can not be made bigger than 2
meters in diameter. This limit directly smashed any hope
to expand the field of view of a very large telescope, say
with an aperture of 8/10 meters, by just adding a refracting
corrector to the telescope system: a refracting corrector big
enough for this sake is simply not available at all. An 8m
telescope of general type has a field of view of merely 0.5
degree. To obtain a big field of view in such a big-aperture
telescope, scientists have to turn to another solution, just
as commented by R. Wilson, the founder of active optics:
“Extension of the Schmidt telescope to bigger diameters
could then only be achieved by replacing refracting
correctors to reflecting correctors. But no one had dared to
attempt such a difficult technical project until the Chinese
optical experts of Nanjing Observatory 1 proposed the
LAMOST project, published by Wang et al. in 1996.”
This predicament had fettered the growth of some
emerging disciplines, including large sample astronomy,
cosmology and galaxy evolution and structure, which rely
on large-scale sky surveys to obtain massive data on large
celestial objects like galaxies and galaxy clusters. Small
telescopes cover bigger areas of the sky and are convenient
for the search of valuable objects, but because of their
small apertures they can only collect limited amount of
light, making it very difficult to observe dim objects, let
alone to capture their spectra; a big telescope can focus on
a very far-away and dim object, but it can only cover a very
small area of the universe and hence can only close up to
the detail of an object. Neither of them is suitable for sky
1
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Light beams glitter inside the huge tube of the LAMOST.
This mammoth astronomical telescope represents the most
challenging application of active optics so far, having achieved
both an extremely large field of view (5 degree) and a big aperture
(7.8m).
surveys.
Now LAMOST has solved the problem through
applying active optics to a reflecting Schimdt system.
However, it used to be seen as a project “too ambitious to
be possible.”
“We did not realize its great implications to telescope
development until 1994 when I attended a symposium at
Cambridge,” recalled Cui when asked about challenges
posed by LAMOST. “After I presented the design many
attendees managed to have a brief talk with me. People
were excited by a possible renaissance of the Schimdt
telescope resulted from this well-anticipated breakthrough
in extending such optical system to much larger apertures.
However this excitement was soon replaced by a cautious
expectation. Some peers doubted that this great idea could
come true, as at that time many necessary technologies
were still missing. On the other hand, it was just too bold
to adopt two segmented mirrors in the same optical system:
their control itself could be a nightmare, let alone applying
active optics, which had never been tested in any telescope
till that time, to both of them.”
The suspicion held by peers was understandable, as
admitted by Su and Cui. “Even I was very nervous at the
early stages of the development,” Su disclosed: “Yes as
the inventor, I am confident of the theory itself, but I was
very clear that it was a dream very difficult to realize. And
very importantly, it was the first in the world—we had
nothing similar to lean on. Sometimes Cui’s team met
with temporary obstacles in experiments, and I got really
worried. You know, if it failed, I would have become the
culprit.” In a joking tone he continued: “I put forward
It is a mistaken name for then CAS Nanjing Astronomical Instruments Research Centre, now the CAS Nanjing Institute of Astronomical Optics & Technology.
Bulletin of the Chinese Academy of Sciences
Vol.25 No.3 2011
the main idea of LAMOST together with CAS Member
WANG Shouguan, an astronomer. As an opticist I must be
responsible for the technological issues occurring in the
construction process. What would happen if I was wrong
and it failed?”
As a sharp contrast, Cui, a mid-age female opticist
who used to be a backbone developer of the four 8.2m
telescopes installed at the European Southern Observatory
(ESO), showed surprising calm. “Of course we could make
it. I trusted this from the very beginning,” she responded
peacefully with a smile.
Time flew and when Cui arrived at Rio de Janeiro,
Brazil in 2008 to attend an international conference
on astronomical telescopes and instruments to give
a presentation on the progress, the team had finished
assembling all the sub-mirrors for both the primary mirror
and the corrector. “I got the photos of the completed mirrors
and presented them to the audiences. Long, loud applauses
showed their congratulations to us. It was a happy surprise
for the whole astronomical community. Attendees from
different countries kept coming to shake hands with
me, Japanese, French, German, Russian, American and
British…”
Cui's team completed assembling both the mirrors in 2008.
Shown is the complete primary mirror (MB). For a look at the
complete corrector (MA), please refer to page 232 of this issue.
Brand-new concept
Now as the biggest Schimdt telescope to date, LAMOST
possesses a set of properties dwarfing any other existing
astronomical instrument for sky surveys. For example, it
expands the field of view by 10 times compared to a general
telescope of equivalent aperture, and it can obtain 4,000
spectra at just one shot, capable of performing sky surveys
producing tens of million spectra. However, it over-shadows
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other astronomical telescopes mainly in a very special way.
“LAMOST represents a brand-new concept for telescope
design and development,” emphasized Su.
“The idea to build LAMOST was proposed first by
academician Wang Shouguan. It involved a critical part
adopting a special kind of active optics, which integrates
the active optics applied to ESO and some observatories
in USA. Generally such technology is used to correct the
deformation of large mirrors due to gravity effects and
thermal fluctuations, to maintain a fixed aspherical surface
and eliminate aberrations. Here in LAMOST, besides
eliminating aberrations, active optics aims to achieve a
series of new aspherical surfaces, hence producing a series
of corresponding Schimdt optical systems in a single set of
telescope. In other words, every moment it forms a different
Schimdt telescope,” explained Su and Cui.
Sub-mirrors for the corrector MA. The 5.72m×4.40m corrector
consists of a total of 24 deformable super-thin sub-mirrors, which
can actively deform in real time to form an ideal off-axis nonspherical mirror, to eliminate the spherical aberration caused
by the 6.67m×6.05m primary mirror M B. More importantly the
corrector can adjust to achieve a series of ideal aspehrical
surfaces to form a series of corresponding Schimdt optical
systems. This is not only the first time ever a Schmidt optical
system is achieved in a reflecting telescope whose aperture is
larger than 1.34 m, but also the first time to get an variable optical
system by unconventional method—active optics.
“Both its primary mirror and corrector are segmented,
and both of the mirrors can reshape through accurate
positioning of the sub-mirrors to obtain ideal surfaces for
the optical system; and on top of this, the corrector can
deform actively to correct the aberrations due to gravity and
temperature changes,” advanced Cui.
“The whole design is ambitious, and very difficult to
realize. But Cui’s team made it,” Su smiled.
Creativity in other mammoth instruments
“LAMOST is just a miniature of the independent
innovation being made by Chinese scientists,” asserted
Cui: “Not just LAMOST, other large-scale astronomical
instruments also involve great, original creativity proposed
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by Chinese scientists and engineers. Good examples include
FAST, HXMT and the Antarctic Observatory underway.”
“First let’s look at the Five-hundred Meter Aperture
Spherical Telescope (FAST) under construction,” introduced
Cui: ‘Through adjustment controlled by the computer in line
with active optics, its primary surface can create parabolas
in the real illuminated area. After visiting LAMOST Prof.
Ekers, the President of International Astronomical Union,
said to me: “FAST follows LAMOST, i.e., both these two
telescopes use the active optics of Chinese characteristics.”
After completion in 2015, FAST’s capability, scientific
complications and technologies will reach a top-notch level
in the world.’
“So is the HXMT, the Hard X-ray Modulation Telescope.
It is also a unique instrument, adopting some creative data
processing method put forward by CAS Member LI Tipei
and Prof. WU Mei,” said Cui: “on completion, it will perform
properties better than its oversea counterparts.”
“Moreover,” added Su: “the Photoelectric Astrolabe
and the Solar Magnetic Telescope, both completed in the
decades between 1960 and 1980; and the planned Space
Solar Telescope are all invented by Chinese scientists.”
When asked about the Antarctic Observatory, which
has been suggested by astronomers to begin by 2015 or
more ideally 2013, and to complete in 10 years, Cui further
mentioned the independent creativity involved in the design.
“The 2.5m optical/infrared telescope to be set up
at the Antarctic Observatory will adopt the SYZ coude
system, an optical system named after its inventors, namely
Academician Su and Professors YU Xinmu and ZHOU
Bifang. It is a very special system, enabling observers
to switch between different foci without changing the
secondary mirror, when applied to the 2.16m telescope
located at the observatory station in Xinglong County in
suburban Beijing,” Cui introduced.
“What deserves mentioning is, when applying this
SYZ coude system to the 2.5m telescope to be situated at
the Antarctic Observatory, we tried to fit it to a new goal: to
obtain excellent image quality in a large field of view,” Cui
continued with sparkles of excitement in her eyes.
The Antarctic area boasts the best astronomical seeing
in the world, with the least turbulence in the atmosphere.
The opticists represented by Su and Cui are planning to
make the best from the ideal natural conditions characterized
by this area to achieve what would be impossible anywhere
else. For example, to obtain the best image, only next to
what produced by a space-based telescope, in a large field
of view through a ground-based one. “It will be very costwise to build such a telescope,” Cui could hardly hide her
happiness and excitement: “you know sending a telescope
to the space costs a lot of money, and it is very costly to
obtain a large field of view in the space.”
According to Cui, the new 2.5m telescope to be
installed at the Antarctic Observatory will be even more
capable than a 30m telescope located anywhere else of
this planet concerning near infrared observation. “In other
areas, you can only approach the diffraction limit through
adaptive optics in a very narrow field of view, if using a
30m telescope. Yet in the Antarctic area, probably we can
approach this limit in the whole field of view, only through
a very simple adaptive optics correction,” a big, simple
smile spread over the face of this happy explorer.
Approach of independent innovations to developing very large astronomical
instruments
When asked about the future strategy and prospects of
astronomical instrument development in China, Su and Cui
put much emphasis on independent innovations.
‘You see, in all these large-scale astronomical
instruments, LAMOST, FAST, HXMT and the Antarctic
Observatory, innovations originated from domestic scientific
research and applications play an important role. We are
now trying to build an innovation-oriented nation, making
efforts to transform our position in this field from a follower
to a leader. We are already taking the lead in some aspects,
including large-scale spectral sky surveying, active optics
and positioning of optical fibers; and in some others we are
“advancing with our own thoughts,” not just following the
steps of predecessors,’ argued Cui.
“Next step, we would like to build a 30m telescope
independently,” Su and Cui answered with steadiness and
confidence when asked what kind of large-scale instruments
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they would propose to build in the near future. “We do
not have substantial difficulties in accomplishing such a
horrible-looking project,” they emphasized.
“No subversive difficulty” in 30m telescope
development
Concerning why a 30m telescope would be the next
target, Su and Cui explained that LAMOST was designed
and built to meet the needs to conduct large-scale sky
surveys, spontaneously observing and recording multiple
objects. Massive data yielded by such surveys will serve
research in fields like large-scale structures of the cosmos,
structure of galaxies, origin and evolution of galaxies and
galaxy clusters. However, instruments for sky surveys are
not suitable for specific and precision observation locking
on a certain celestial body, which requires very high
accuracy for studies in great detail, and does not need a
Vol.25 No.3 2011
large field of view. Needed in such kind of observations are
large-aperture optical/infrared telescopes of general type,
which unfortunately are missing in China and even Asia.
Considering the reflective area of its main mirrors, the
effective illumination diameter of LAMOST is 7.8m, with a
6.67m×6.05m primary mirror and a 5.72m×4.40m corrector.
“It is equivalent to an 8/10m telescope in an optical sense,”
Su and Cui explained in response to doubts about its
aperture. “The scale of the project is also equivalent to that
of an 8/10m telescope. More importantly, LAMOST is a
large-field-of-view telescope, which suggests that it needs
to sacrifice the aperture in favor of a larger field of view,
because of the dilemma between field-of-view and aperture.
Therefore the relatively smaller aperture is not meaningful
here—it does not deny our ability to build telescopes
of larger apertures. Actually the segmented mirrors and
special active optics adopted in LAMOST made it even
more challenging to build than a general-type telescope of
the same aperture. In every way, LAMOST stands on the
frontiers of large-scale astronomical telescope development;
after completing this project, China is now at the same
scratch line for development of very large telescopes as
Western peers,” they argued.
“The technologies working in LAMOST are basically
the same as those needed to build a 30/50m telescope,”
answered Cui when asked about the difference between
them: “I would say, there exists no subversive difficulty for
us in building a telescope of such an aperture.”
The active optics and segmented mirrors adopted by
LAMOST, according to Su and Cui, are essentially the
same as those needed in a 30/50m telescope. “Of course a
30/50m telescope requires higher accuracy and more precise
automatic control, but no noticeable bottleneck is seen in
the way,” Cui insisted.
Generally speaking, active optics is a must for any
telescope bigger than 6m (or more strictly, 5m) in diameter,
because going beyond this threshold the deformations caused
by gravity and temperature fluctuation become intolerable
for a highly accurate astronomical instrument, explained Su
and Cui. LAMOST dealt with this problem very well, and
active optics can accommodate to the new situation when the
aperture reaches 30/50 meters, Cui introduced when digging
into more details on the technology.
According to Cui, when the aperture increases to
30~50 meters, the calculation of the vectors to determine
the positions of the sub-mirrors would have to involve 5
dimensions, in comparison with 3 dimensions in the case
of LAMOST. Because at this level deviations of other 2
degrees of freedom, whose influence is negligible for an
8/10m telescope, become too big to tolerate. However, the
accuracy requirement for these extra vectors is much looser
than that for the original 3 dimensions. “It is a difference of
several orders,” Cui depicted: “therefore it is not difficult
to cope with this problem, though we do need to take into
Interview
account these dimensions.”
“Concerning the assembly of segmented mirrors,”
advanced Cui: “things will not be too difficult for us
when the number of sub-mirrors increases from dozens
to hundreds. Potential challenges would emerge only in
the assembly of sub-mirrors to form a highly accurate
aspherical surface. However such potential challenges are
common for optical experts around the world.”
“We do have some difficulties in adaptive optics,” she
continued: ‘which is necessary in such a big telescope, and
there is a wide gap between us and the West. Still it is not
an impassable obstacle, however. On the other hand we
need to count in the huge inertia from the heavy system
when working on the automatic control part. Challenges
exist in some aspects, but I would insist that there is no
“subversive obstacle” in accomplishing a 30/50m telescope,
with experiences we gained in LAMOST,’ she repeated.
CFGT—30m telescope to be independently
developed by China
Su and Cui introduced that since 2000, Chinese optical
experts have dedicated themselves to exploratory research
for a very large telescope, named the Chinese Future Giant
Telescope (CFGT). According to a report on development
strategies for astronomy jointly made by the National
Natural Science Foundation of China (NSFC) and CAS,
this 30m telescope features a small secondary mirror and
a ring-shape design of mirrors which facilitates the easy
production of sub-mirrors in large batches. According to
the design, it will achieve excellent diffractive imaging in
a large field of view with an estimated total investment of
about 4 to 5 billion yuan.
So far, the exploratory research has won support from
the NSFC in the form of a Project for Young Scientists Fund,
as well as support from the Knowledge Innovation Program
run by CAS Headquarters as a priority project of orientation
importance. Meanwhile, scientists are also conducting
investigations in the west of China in search of a suitable site
for this extremely big telescope, according to Cui.
Open to international cooperation
“We need to further encourage independent scientific
innovations in China, but meanwhile we also welcome
international cooperation in the field of extremely large
telescope development, on a reciprocal basis,” Su and Cui
emphasized.
According to Cui, another LAMOST will be installed
in the Southern Hemisphere to conduct surveys of the
southern sky. “Chilean government has expressed their
interests in cooperation and would like to provide a site for
this instrument,” she introduced: “Chile has excellent natural
conditions for astronomical observation. If such cooperation
succeeds, it will greatly boost the astronomical research in
China. Moreover, it is also possible to set our CFGT there if
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we cannot find a suitable site for it in western China.”
“Development of extremely large telescopes in
nowadays China is just like the design and construction of
large-span bridges two decades ago in this country,” Su gave
a special story on the difficult course of major engineering
projects in China, based on a presentation given in 2010 by
Prof. FENG Maorun, the former General Engineer of the
Chinese Ministry of Transport. In the 1980s, it was planned
to build a big bridge over the Huangpu River in Shanghai.
At that time not many Chinese engineers were confident of
the domestic scientific buildup and some suggested inviting
Japanese bridge designers and engineering companies to
conduct the project. “At this critical time Prof. LI Guohao,
former president of the Tongji University and an expert in
bridge design and building, strongly insisted on designing
and building the bridge on our own,” Su recalled. Prof. Li
struggled to gain support from the government and eventually
Chinese engineers won the chance to design the first bridge
over the Huangpu River in Shanghai, the Nanpu Bridge. “The
bridge was completed in 1991. Two years later, another one,
the Yangpu Bridge was also completed. From then on, more
and more large-span bridges appeared in China. Before the
Reform and Opening up, there were only three bridges over
the Yangtze River, whereas by 2010 when Prof. Feng gave
his presentation, the number of such bridges had increased
to 129. Now a bridge spanning over the Yangtze River is no
more a miracle for Chinese.” Su raised his tone with great
confidence: “We built the 36km Hangzhou Bay Bridge on
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our own; and a further one connecting Hong Kong, Macao
and Zhuhai City is under construction and scheduled to be
finished in 2016. We are now even building bridges over sea
bays for western countries.”
“We are seeing valuable opportunities for boosting
technologies for extremely large telescopes in China.
We can not afford to miss this bus,” continued Su and
Cui. “More importantly, Chinese scientists have already
demonstrated their talents and capability in developing
very large astronomical instruments,” they added. “Aside
from LAMOST, HXMT, and FAST, Chinese scientists
invented lots of astronomical instruments the last century.
For example, the Solar Magnetic Telescope invented by
CAS Member AI Guoxiang, and the Photoelectric Astrolabe
by Profs. LI Dongming and HU Ningsheng, which were
completed during the period from 1960 to 1980, are still
in good operation. Furthermore, the 21 CentiMeter Array
completed in this century involved creative ideas put
forward by Prof. WU Xiangping, and the blueprint of
the Space Solar Telescope integrates the innovative ideas
proposed by academician Ai Guoxiang.”
“Such achievements,” they advanced: “particularly
the success in LAMOST have pushed us to the frontiers
of this field. We are now at the same scratch line as our
west counterparts. What we shall follow is an approach
of independent innovations to development of very large
astronomical instruments, mainly relying on domestic S&T
buildup, meanwhile open to international cooperation.”