Download New Generation Ground-Based Optical/Infrared Telescopes

New Generation Ground-Based
Optical/Infrared Telescopes
Alan T. Tokunaga and Robert Jedicke
Institute for Astronomy
University of Hawaii
Encyclopedia of the Solar System, 2nd edition
Editors: L. McFadden, T.V. Johnson, P.R. Weissman
Academic Press, 2006
1
TABLE OF CONTENTS
I.
II.
III.
IV.
V.
VI.
VII.
Introduction....................................................................................................3
Advances in the construction of large telescopes and in image quality.........4
Advances with detector arrays ......................................................................8
Advances in adaptive optics .........................................................................9
Sky survey telescopes ................................................................................10
Concluding remarks
Bibliography ................................................................................................12
DEFINING STATEMENT
The telescope is a crucial tool for astronomers. This chapter gives an overview of the
recent advances in ground-based telescope construction and instrumentation for visible
and infrared wavelengths, which have spurred extraordinary advances in our
understanding of the solar system. Although space-based observatories such as the
Hubble Space Telescope and the Spitzer Space Telescope have also immensely enriched
our understanding of the solar system we live in, the results from space observatories
are discussed elsewhere in this encyclopedia. Astronomers strive to build ever-larger
telescopes in order to collect as much light as possible. While cosmologists need the
large collecting area of telescopes to study the distant universe, solar system
astronomers need the large collecting area to study both nearby small objects and faint
objects at the limits of our solar system, and to exploit the high angular resolution they
provide. We discuss future telescope projects that promise to make further discoveries
possible in the next few decades and offer the prospect of studying solar systems other
than our own. Advances in instrumentation have in equal measure revolutionized the
way astronomy is done.
We discuss two major advances in this chapter: the advent of the large-format solidstate detector for visible and infrared wavelengths and the development of adaptive
optics. The development of large-format arrays has led to ambitious digital sky surveys.
These surveys allow searches for objects that may collide with Earth and are leading to
a fundamental understanding of the early history of our solar system. The development
of adaptive optics is reaching maturity and is allowing routine observations to be made
at the diffraction-limit at the largest telescopes in the world. Thus the limitation on
image sharpness imposed by the atmosphere since the invention of the telescope is now
removed with adaptive optics.
2
1. INTRODUCTION
The telescope has played a critical role in planetary science from the moment of its use
by Galileo in 1608. The observations that he made of the craters on our Moon and the
moons of Jupiter were the first astronomical discoveries made with a telescope. The
development of larger refracting and reflecting telescopes led to the seminal discoveries
of the rings of Saturn, asteroids, the outer planets Uranus and Neptune, new satellites of
Mars and the outer planets, and Pluto by 1930.
Although spacecraft missions have revolutionized our understanding of the solar
system (of which there are many examples in this encyclopedia), ground-based
telescopes continue to play a very important role in making new discoveries, and this is
the focus of this chapter. The discovery of the first Kuiper Belt Object (KBO) was made
in 1992 on the University of Hawaii 2.2-m telescope. Tremendous advances have been
made in detecting KBOs since then: presently over 900 KBOs have been discovered.
Using several of the largest telescopes in the world, it was recently found that the
largest KBO known, 2003 UB313, has methane ice on its surface and a moon (Fig. 1).
This finding has challenged our definition of what is considered to be a planet in our
solar system. Another recent result was the discovery of comets among the main-belt
asteroids. The most recent of these, asteroid 118401 was discovered by the 8-m GeminiNorth telescope. Two other comets in the main belt were detected previously by other
astronomers, and many more such comets are now thought to exist in the asteroid main
belt. If this is confirmed then such comets were likely the main source of water
delivered to the Earth during its formation. A final example is the Near-Earth Object
(NEO) designated 2004 MN4, which was discovered with the University of Arizona’s
2.3- m telescope. For a short time at the end of December 2004, this NEO had the highest
probability of any yet found for colliding with Earth (Fig. 3). These discoveries
demonstrate the importance of ground-based astronomy, and they will no doubt
provide the scientific motivation for future missions.
Solar system astronomers typically use telescopes built for other fields of astronomy.
However, during the 1970s, NASA constructed ground-based telescopes to support its
planetary missions. NASA funded the construction of the 2.7-m McDonald telescope,
the University of Hawaii 2.2-m telescope, and the 3.0-m NASA Infrared Telescope
Facility (IRTF) to provide mission support, but currently only the IRTF continues to be
funded by NASA for that purpose. NASA also provides funding for searches for NEOs
as part of a Congressional directive.
Telescopes are designed to collect and focus starlight onto a detector. While
conceptually simple, ground-based observers have to contend with limitations imposed
by physics, the atmosphere, and technology. First, the collecting area of a telescope is
limited in size. The largest optical telescope in the world presently has an equivalent
collecting area of an 11.8-m diameter mirror. Although larger telescopes could be built,
there are serious technical and financial difficulties to overcome. Larger telescopes not
only allow more light to be collected and put onto the detector, they also allow sharper
images to be obtained at the diffraction limit of the telescope. Second, the atmosphere
limits observations to specific observing “windows” where the atmosphere is
transparent, and the wavelength range 25 µm to 350 µm is largely inaccessible to
ground-based observers because of water absorption bands. Third, for infrared
observations, the thermal emission of the atmosphere at wavelengths longer than 2.5
3
µm greatly reduces the sensitivity of observations. To overcome the problems of
atmospheric absorption and thermal emission, it is necessary to go to high-mountain
sites such as Mauna Kea in Hawaii and Atacama in Chile, or to use balloons, aircraft, or
spacecraft. Fourth, atmospheric seeing typically limits the sharpness of images to 0.25–
0.5 arcseconds at the best high-altitude sites. To achieve diffraction-limited imaging, one
must employ special techniques that actively reduce it many times per second. One
such technique, called adaptive optics, is discussed later in Section 4.
Very large and low-noise visible and infrared detector arrays have been developed
in the past decade, and this advance has been as significant as improvement of
telescope construction in providing greater observing capability. An important
capability of large-format detector arrays has been to allow large sky surveys to be
undertaken. The key objectives of these sky surveys are to detect asteroids that may
present an impact hazard to Earth and to complete the reconnaissance of KBOs. The
major challenges of these survey projects are obtaining large enough detector arrays to
provide the field-of-view required, and analyzing and storing the tremendous amounts
of data that they generate.
In this chapter,we discuss very large telescopes that have been developed in the past
15 years to maximize collecting area, optimize image quality, and achieve
diffractionlimited imaging with techniques to reduce the atmospheric turbulence. We
also discuss sky survey telescopes that take advantage of the large-format detectors for
the detection of solar system objects.
2. ADVANCES IN THE CONSTRUCTION OF LARGE
TELESCOPES AND IN IMAGE QUALITY
The Hale 5.1-m telescope went into operation in 1949. It represented the culmination of
continual telescope design improvements since the invention of the reflecting telescope
by Newton in 1668. The basic approach was to scale up and improve design approaches
that were used previously. Figure 4 shows the increase in telescope aperture with time.
After the completion of the Hale telescope, astronomers recognized that building larger
telescopes would require completely new approaches. Simple scaling of the classical
techniques would lead to primary mirrors that would be too massive and an
observatory (including the dome enclosure) that would be too costly to build. Since the
1990s, a number of ground-breaking approaches have been tried, and the barrier
imposed by classical telescope design has been broken. Table 1 shows a list of telescopes
with apertures greater than 5 meters. Some of the telescopes listed in Table 1 are still
under development.
Major technical advances that have led to the development of large telescopes
include:
(1) Advances in computer-controlled hardware allows correction for flexure of the
primary mirror. This has permitted thinner mirrors to be used, reducing the mass of
the mirror and the total mass of the telescope. For example, the mass of the ESO
Very Large Telescope 8.2-m primary mirror is 23 tons with an aspect ratio (mirror
diameter to mirror thickness ratio) of 46. This is a very thin mirror compared with
the 5.1-m Hale telescope, which has a weight of 14.5 tons and an aspect ratio of 9.
4
(2) Altitude-azimuth (alt-az) mounts reduce the size of the required telescope
enclosure. An 8-m alt-az telescope can fit into the same size enclosure as a 4-m
equatorial telescope. An alt-az telescope requires computer controlled pointing and
tracking on two axes (whereas the traditional mount requires tracking on only a
single axis). The Hale telescope is the largest equatorial telescope ever built. All
larger and more recent telescopes use alt-az mountings. Figure 5 illustrates the basic
types of telescope mounts, and Figure 6 shows examples of the equatorial and alt-az
mounts.
(3) Advances in mirror casting and computer controlled mirror polishing allow the
production of larger primary mirrors with shorter focal lengths. A shorter focal
length allows the telescope structure to be smaller, thus lowering the weight and
cost of the telescope. It also greatly reduces the cost of the dome enclosure. The
stateof- the-art in short focal length primary mirrors are those with a focal length to
diameter ratio (f/no) of 1.14 installed in the Large Binocular Telescope. This can be
compared to the Hale telescope primary mirror that has an f/no of 3.3. The smaller
telescope structure with reduced mass requires less time to reach thermal
equilibrium, and its lower mass makes it easier to move. This is extremely important
in achieving the best image quality and to efficiently reposition in the telescope.
(4) Advances in reducing dome seeing led to significant improvement in image
quality. Dome seeing is caused by temperature differences within the dome,
especially differences between the mirror and the surrounding air. To reduce dome
seeing, it is necessary to flush the dome with outside air at night, refrigerate it
during the daytime, and cool the primary mirror to about 0.5º C below the ambient
air temperature. Dome seeing is so important that large telescope projects use wind
tunnel experiments to determine what type of dome design to employ. Careful
attention to dome design is critical in eliminating dome seeing and achieving the
very best seeing at the observatory site. Figure 6b shows an innovative approach to
providing dome flushing by providing slits in the dome.
(5) Advances in telescope construction have led to novel methods of reducing the
cost of building extremely large telescopes. For example, the 10-m Keck telescopes
have segmented mirrors to make up the primary mirror (Fig. 6c). Although this
technique had been used to build radio telescopes, the difficulty of making the
segments and the high-precision alignment at visible wavelengths presented
formidable obstacles. Fortunately, the problems of fabricating segmented mirrors
and aligning them were solved. The hexagonal mirror segments have a thickness of
75 mm, and so the aspect ratio of the 10-m primary is 133 and the total weight of the
glass required is 14.4 tons, about the same weight as the 5-m Hale telescope.
Another novel approach uses two 8.4-m primary mirrors on a single structure as in
the Large Binocular Telescope (Fig. 6d). A third approach involves building a
telescope with a fixed vertical elevation. Stars move past the prime focus and are
tracked for a limited time. This approach has limitations but is much less expensive
to build. Two projects (the Hobby- Eberly Telescope and the South African Large
Telescope) have adopted this design to achieve 9-m class telescopes at about 15–20%
of the cost of an equivalent alt-az telescope. An even less expensive approach is to
5
simply stare at the zenith with a liquid mercury mirror as demonstrated by the
Large Zenith Telescope.
Large telescopes generally employ one of three different types of primary mirror
fabrication. These are (1) Segmented mirrors. Each segment is figured appropriately and
all segments are aligned so as to act as a single mirror. (2) Thin meniscus mirror using
low expansion glass. Such mirrors are made as thin as possible to be light weight and to
have a short thermal time constant (thus coming into equilibrium with the atmospheric
temperature quickly). (3) Thick honeycomb mirror using borosilicate glass. The
advantage of using borosilicate glass instead of low expansion glass is that the former is
much cheaper. The disadvantage of borosilicate glass is that the mirror temperature
needs to be controlled more carefully. All of these types of primary mirror fabrication
approaches have been proven successful. Column (7) in Table 1 shows the type of
mirror used.
All large telescopes use active optics to control the shape of the primary mirror.
Active optics is the slow adjustment of a mirror to correct aberrations in the image.
These adjustments are not fast enough to correct for the atmospheric turbulence but
they can correct for flexure in the telescope structure and for temperature changes
(which will cause the telescope structure to expand and contract). The process for doing
this is illustrated in Figure 7. A star is required for the active optics system to be able to
compute the deformations on the primary mirror that are needed to correct the image.
Although Figure 7 illustrates the case for a single mirror, a similar approach is
employed for correcting the surface figure of a segmented primary mirror, although the
details are quite different.
Efforts to escape the harmful effects of the Earth’s atmosphere have led to telescopic
observations using balloons, aircraft, and rockets. Although we do not discuss space
observatories in this article, we note here that a major program undertaken by NASA
and the German Aerospace Center (DLR) is to fly a 2.5-meter telescope in the
stratosphere using a Boeing 747SP aircraft. At this high altitude it will be possible to
observe throughout the 25 µm to 350 µm wavelength range that is inaccessible from the
ground. This facility will provide long-term access to a critical wavelength range that
otherwise would only be exploited infrequently with spacecraft.
We do not know what ultimately will be the largest ground-based telescope to be
built (see Fig. 4). The limitations arise from the need to be diffraction limited, the
difficulty of building a suitable enclosure, and the cost. To be competitive with space
observatories, all large telescopes must work at the diffraction limit using adaptive
optics. But the need to be diffraction limited will ultimately cause adaptive optics
systems to be too complex on an extremely large telescope. An enclosure is necessary to
keep the disturbance by wind to acceptable levels, and the cost to build and operate the
telescope will be enormous. At some point, it may be more cost effective to go into
space, where gravity and the weather are not factors driving the design. This has been
estimated to be at approximately 70-m in diameter. This argument applies to fully
steerable telescopes, not to designs such as the Hobby-Eberly Telescope or the Large
Zenith Telescope.
6
The drive to build ever-larger telescopes is motivated by the need to collect as much
light as possible and thereby increase the signal-to-noise (S/N) ratio of observations.
One can derive that for a diffraction-limited telescope and a detector that is
background-limited, the S/N in a given integration time is proportional to:
S/N ! (A * !/")0.5/(FWHM),
(1)
where A is the area of the telescope, ! is the total transmission of the optics and the
detector quantum efficiency, " is the background emission, and FWHM is the full width
at half maximum of a stellar image. ! takes into account all of the light losses that occurs
from the reflection of the mirrors and transmission losses of lenses as light propagates
from the telescope to the detector. In order to minimize these losses it is necessary to
utilize high reflection coatings on mirrors and lenses as well as to minimize the number
of lenses. The detector quantum efficiency is the fraction of light that is absorbed by the
detector material. This is near the theoretical maximum of 1.0 at visual wavelengths and
about 0.8–0.9 for the 1–15 µm wavelength range. The background emission, ", arises
from the sky emission lines at visual wavelengths and thermal background from the
telescope and sky at wavelengths longer than 2 µm. To reduce the thermal emission
from the telescope, it is necessary to have the highest reflectivity mirrors available and
to reduce or eliminate the thermal emission from the secondary mirror. The latter is
often accomplished by forming an image of the secondary within the instrument and
then blocking it with a cooled metal plate. Then the infrared detector will only sense the
thermal emission from the sky and the object being observed.
After maximizing ! and reducing " as much as possible, one can only increase the
telescope area and reduce the FWHM to further increase the S/N. Reducing the image
FWHM requires decreasing the dome seeing to the absolute minimum, building on sites
that have good atmospheric seeing, and working at the diffraction-limit of the telescope.
Astronomical sites in Hawaii, Chile, and La Palma are prime locations for large
telescopes due to the good seeing they offer as well as having good weather conditions.
Figure 8 shows the advances in image quality that have been achieved. The
development of adaptive optics has led to the ability to work at the diffraction limit in
the near-infrared and to achieve improvements in S/N given by equation 1. Adaptive
optics is discussed in Section 4. The advances in constructing large telescopes coupled
with reducing dome seeing and adaptive optics have provided the means for studying
the surfaces of some KBOs and larger planetary satellites (see Fig. 1). Ground-based
telescopes provide the discoveries that pose new questions and motivation for future
planetary missions. This is likely to continue in the coming decades as the push to build
everlarger telescopes continues.
Several groups in the US are proposing the next leap in technology to a telescope in
the 20–30-m class, and the engineering studies have started. One proposal is the ThirtyMeter Telescope, an international consortium consisting of research groups in the US
and Canada (http://www.tmt.org/). This project proposes to build a telescope similar
in concept to the Keck telescopes that will have over 700 hexagonal segments
composing the primary mirror. As the name implies, the collecting area is equivalent to
a circular mirror 30 m in diameter. The other project is the Giant Magellan Telescope,
7
which is supported by a group of public and private institutions in the US
(http://www.gmto.org/). This telescope concept consists of seven 8.4-m mirrors to
create a single telescope with the collecting area equivalent to a 21.4-m circular mirror.
The European Southern Observatory is also considering an even larger telescope
concept (see http://www.eso.org/projects/owl/). Thus it seems inevitable that a
ground-based telescope larger than 10 m will be built.
3. ADVANCES WITH DETECTOR ARRAYS
Initial observations with telescopes were conducted solely with the human eye (still
much recommended for the nonprofessional), but the advantages of using photographic
plates to record and archive observations of the sky were quickly exploited beginning in
the 1850s. Photographic plates were eventually supplemented with electronic devices
like the photomultiplier tube, which amplified the signal from stars by about one
million. At infrared wavelengths, there were specialized detectors that employed
bolometers, photovoltaic devices, and photoconductive devices. However,
photographic plates were a necessity for recording high-resolution images of large areas
of sky and recording spectra with a wide wavelength range.
Images recorded by photographic plates depend on the chemical reaction that is
induced by a photon of light. Although the efficiency of the photographic plate in
converting a photon to an image is only a few percent, it allows quantitative
measurements to be made on the brightness of stars and the strength of spectral lines.
Most importantly, the information is archived on the photographic plate for future use.
This was absolutely necessary for the development of astrophysics.
The next technological revolution came with the invention of the charge-coupled
device (CCD) in 1973. CCDs are composed of millions of picture elements, or pixels.
Each pixel is a single detector and is capable of converting photons to electrons. The
accumulated electrons can then be sent to an amplifier to be “read out” and recorded by
a computer. CCD technology is employed in digital cameras, and just as digital
photography is gradually replacing photography, a similar transformation has taken
place in astronomy.
The impact of the CCD on astronomy was immediately apparent after its first use.
CCDs have two major advantages over the photographic plate: the capability to directly
record photons with an efficiency of 80–90% and to store data electronically. The stored
data can then be processed with a computer. Until recently, the main deficiency of the
CCD relative to the photographic plate was the relatively small amount of sky that
could be covered. However, the recent development of very large CCD mosaics now
permits larger areas of sky to be covered by a CCD than by a photographic plate. The
rapid development of computing power and disk storage has made it practical to use
large CCD mosaics. While astronomers have worked hard to develop CCD technology
that is optimized for astronomy, they are fortunate that the consumer market has driven
the development of the necessary computing power and storage. Figure 9 shows an
example of a state-of-the-art large format CCD.
There has been a similar revolution in the development of infrared arrays. The first
infrared arrays for astronomy were used in the early 1980s. While initially very modest
in size (32x32 pixels), infrared arrays now typically contain a million pixels. There are
several significant differences between CCDs and infrared arrays. One is that a CCD has
8
a single readout amplifier, while an infrared array has one readout amplifier per pixel.
The electrons in a CCD are transferred to a single readout amplifier (hence the origin of
the term “charge transfer”). Only a single readout amplifier is needed since the readout
electronics and the detector material are made out of the same semiconductor material.
In an infrared array, the detector material and the readout amplifier have to be made
out of different materials, so each pixel must have a separate amplifier. A second
difference is that the infrared arrays must be cooled to much lower temperatures. CCDs
can operate effectively at about "30 to "40º C. Infrared arrays must be cooled to liquid
nitrogen ("196º C) or liquid helium ("269º C) temperatures.
We show in Figure 10 an example of Saturn imaged at a wavelength of 18
micrometers. At these wavelengths, we are observing the thermal emission (heat) from
the planet. Thus temperatures can be measured in the atmosphere of Saturn and for the
dust particles in the rings.
The development of large-format CCDs and infrared arrays has enabled
astronomers to undertake large-scale digital sky surveys at visible and infrared
wavelengths, just as the use of large photographic plates enabled the first deep sky
surveys over 50 years ago.
4. ADVANCES IN ADAPTIVE OPTICS
Adaptive optics (AO) is a technique that removes the atmospheric disturbance and
allows a telescope to achieve diffraction-limited imaging from the ground. This is
critical in achieving the maximum S/N given in equation (1). The basic idea of AO is to
first measure the amount of atmospheric disturbance, then correct for it before the light
reaches the camera. A schematic of how this can be done is shown in Figure 11.
The effect of using AO is dramatic. It is like taking the telescope into space. An
impressive example of how AO can improve image quality is shown in Figure 12. AO
has been essential for detecting binary asteroids. With it over 60 systems have been
found, and the first triple system was recently found as shown in Figure 13.
AO requires a star or another object bright enough to use for rapidly and accurately
measuring the incoming wavefront. If the object of interest is not bright enough, then it
is necessary to use a nearby bright star. This limits the sky coverage, since not every
region of the sky will have a bright enough star nearby. If there is no nearby bright star,
then it is necessary to use a laser guide star. A laser is pointed in the same direction as
the telescope and is used to excite a thin layer of sodium atoms in the Earth’s
ionosphere (at an altitude of 90 km). This provides a point source that acts as an
artificial star for the AO system.
Figure 14 shows a laser guide star being used at the Keck Observatory. This laser
guide star system was used to detect the satellite of the largest KBO known (see Fig. 1).
With AO we can look forward to the exploration of other solar systems. Figure 15
shows a faint object next to a brighter object that is thought to have a mass 5 times that
of Jupiter—a planet. This is one of the first planetary-mass objects to be imaged. Most
planets are found by detecting radial velocity variations in the star they are orbiting.
About 160 planets have already been detected by the radial velocity method and there is
a possibility to detect Earth-mass planets around nearby low-mass stars. We can expect
future planetary systems to be discovered, and thus to be able to study the physical
9
characteristics of other solar systems for the first time. The study of extrasolar planets is
a key science area for all large telescopes.
5. SKY SURVEY TELESCOPES
Although large telescope projects tend to get a lot of attention, recently there has been a
corresponding quantum jump in the construction of visible and infrared survey
telescopes. This has been made possible by the availability of large-format CCD and
infrared arrays. In addition, the discovery of the Kuiper Belt has led to fundamental
advances in our understanding of how our solar system formed. There is a great need to
continue the survey of the Kuiper Belt because detailed knowledge of the size and orbit
distributions of these objects will allow us to test theories of the orbital migration of the
outer planets (Jupiter, Saturn, Uranus, Neptune), the origin of the short-period comets,
and the cause of the late heavy bombardment of the inner solar system.
There is also an increased awareness that it is important to identify asteroids and
comets that could collide with Earth (see Fig. 3). In 1998 the Congress of the United
States directed NASA to identify within 10 years at least 90% of NEOs larger than 1 km
that may collide with Earth. There are a number of scientific benefits that arise from the
NEO surveys, including determining the origin of NEOs, identifying interesting NEOs
that could be visited by spacecraft, improving our knowledge of the numbers and sizes
of the asteroids in the main asteroid belt, and the discovery of new comets.
The reason that the discovery of all NEOS larger than 1 km is important is because if
such an object collides with Earth the consequences will be catastrophic. If it is possible
to predict that there will be a collision, it may be possible to divert the asteroid so that it
misses Earth. The earlier such a prediction can be made, the more likely it is that the
diversion is possible. This is a case in which there is a practical use for astronomy, and it
is very fitting.
A number of programs are underway in the US and other countries that meet or
exceed the requirements set by Congress. Table 2 shows a partial list of sky survey
programs that are currently in progress or planned. Current productivity of various
programs is shown in Figure 16, which shows all NEOs discovered irrespective of size.
While the NASA directive is aimed at identifying NEOs larger than 1 km diameter,
many NEOs smaller than 1 km are also discovered due to the sensitivity of the search
programs and because small objects that come very close to Earth may be bright enough
to be detected. A recent NEO, 2005 WX, approached to within 1.3 million km of the
Earth and had an estimated diameter of only 10 m!
The number of known NEOS has been increasing due to the larger number of
funded survey programs and advances in detector arrays that have allowed much
larger areas of sky to be covered in a single exposure. The number of NEOs discovered
as a function of time is shown in Figure 16. Note that while the total number of
asteroids discovered is still increasing at a rapid rate, the number of new asteroids
larger than 1 km discovered each year is decreasing. This is a result of the fact that the
remaining unknown NEAs are intrinsically more difficult to detect. Their size and orbit
distribution is different from the known population due to observational selection
effects in the population of known objects. It is likely that existing survey programs (see
Table 2) will just miss the goal of discovering at least 90% of all near-Earth asteroids
larger than 1 km by 2008 as mandated by Congress. However, when the next generation
10
surveys (see Table 2) come online within the next decade they will quickly complete the
inventory of NEAs larger than 1 km.
There are three major ground-based sky surveys currently under development or
study (see Table 2). The Discovery Channel Telescope is a 4.2-m telescope that is under
construction near Flagstaff in Northern Arizona and should be operational by 2009.
Another survey telescope that is under development is Pan-STARRS, which consists of
four 1.8-m telescopes (with a combined aperture approximately equivalent to a 3.6-m
telescope) to perform rapid wide-field surveying of the entire sky on a weekly basis. It
is hoped that the full system will be operational by 2010, but a prototype single
telescope unit will be operational on Haleakala on Maui by the end of 2007. The
proposed Large Synoptic Survey Telescope is currently under engineering and design
study and is envisioned to be a monolithic 8.4-m wide-field telescope (with a collecting
area equal to a 6.7-m telescope). With its large diameter and fast focal ratio it should be
capable of reaching 24th magnitude in single 10-s exposures. Due to their extreme depth
and wide-field coverage each of these surveys should reach 99% completion for NEOs
larger than 1 km diameter within two years of beginning operation.
6. CONCLUDING REMARKS
Space does not allow coverage of all of the relevant subjects related to the vibrant topics
of novel telescope construction, optical fabrication techniques, advances in mirror figure
control, adaptive optics, and detector improvements at visible and infrared
wavelengths. The topics covered in this chapter can only hint at the tremendous
advances that have taken place in recent years and that carry on unabated. Since the
invention of the refractive and reflective telescopes by Galileo and Newton, the
construction of ground-based telescopes continues to challenge the very best minds in
physics and engineering. At the present time there are strong scientific drivers to build
larger telescopes in the 20–50 meter range. It seems only a matter of time before such
extremely large telescopes are built.
Solar system astronomy is driven by the need to have large telescopes in order to
study very faint objects in the Kuiper Belt and very faint NEOs that may present a
hazard to Earth. It is also necessary to have the highest spatial resolution possible by
working at the diffraction limit of large telescopes. This will enable researchers to study
the surface and atmospheric features of the outer planets, dwarf planets, and their
satellites. Large telescopes also allow the study of exo-planets, and thus bring about a
merging of studies of our solar system with those around distant stars.
Another driver of solar system astronomy is to detect and characterize NEOs that
may present an impact hazard to the Earth. Numerous sky survey programs are
underway to detect at least 90% of all NEOs larger than 1 km, and there is a push at the
present time to expand this program to detect at least 90% of all NEOs larger than 140
m. These survey programs will play a significant role in greatly expanding our
knowledge of the building blocks of our solar system— the asteroidal and cometary
bodies from the inner to the outer reaches of the solar system. These studies are likely to
profoundly affect understanding of the formation of our solar system and life itself.
11
We anticipate continuing growth in telescope and instrument development for at
least another generation. It is indeed a period great innovation—a renaissance in
telescope building and instrumentation—that we are fortunate to be able to witness and
participate in.
7. BIBLIOGRAPHY
Bely, P.Y. (ed.) (2003). The Design and Construction of Large Optical Telescopes. SpringerVerlag, New York.
Kitchin, C.R. (2003). Telescopes and Techniques. Springer-Verlag, London.
McLean, I. (1997). Electronic Imaging in Astronomy. John Wiley & Sons, Chichester.
Tyson, R.K. (2000). Introduction to Adaptive Optics. Soc. Of Photo-Optical
Instrumentation Eng., Bellingham.
NEO web site: http://neo.jpl.nasa.gov/programs/discovery.html. See also “Study to
Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller
Limiting Diameters”, a report of the Near-Earth Object Science Definition Team, 22
August, 2003, that can be downloaded from this site.
Zirker, J.B. (2005). An Acre of Glass: A History and Forecast of the Telescope. The Johns
Hopkins University Press, Baltimore
12
Table 1. Telescopes with Apertures Greater than 5 Meters.
(1)
(2)
Circular
(3)
(4)
Aperture
Aperture (m)
2 x 8.4
Equiv. (m)
11.8
11x9.4 Hexagon
(5)
(6)
(7)
(8)
Mirror
(9)
Date of
primary
Mirror
Aspect
Mounting
(10)
Telescope Name
Large Binocular Telescope (LBT)
Location
Mt. Graham, Arizona
Operation
(2006)
f/no
1.14
Type
Honeycomb
Ratio
9.4
Type
Alt-Az
Ref.
1
10.0
Keck I
Mauna Kea, Hawaii
1993
1.75
Segmented
133
Alt-Az
2
11x9.4 Hexagon
11x9.4 Hexagon
10.0
10.0
Keck II
Gran Telescopio Canarias (GTC)
Mauna Kea, Hawaii
La Palma, Canary Islands
1996
(2007)
1.75
1.65
Segmented
Segmented
133
125
2
3
11x10 Hexagon
9.2
Hobby-Eberley Telescope
Mt. Fowlkes, Texas
1997
1.4
Segmented
200
11x10 Hexagon
9.2
Southern African Large Telescope (SALT)
Sutherland South Africa
2005
1.4
Segmented
200
Alt-Az
Alt-Az
Azimuth
only
Azimuth
only
8.2
8.2
8.2
8.2
Subaru
Mauna Kea, Hawaii
1999
1.8
Meniscus
41
Alt-Az
6
8.2
8.2
Very Large Telescope (VLT) UT1 Antu
Very Large Telescope (VLT) UT2 Kueyen
Cerro Paranal, Chile
Cerro Paranal, Chile
1998
1999
1.75
1.75
Meniscus
Meniscus
46
46
Alt-Az
Alt-Az
7
7
8.2
8.2
Very Large Telescope (VLT) UT3 Melipal
Cerro Paranal, Chile
2000
1.75
Meniscus
46
Alt-Az
7
8.2
8.2
8.0
8.0
Very Large Telescope (VLT) UT4 Yepun
Gemini North
Cerro Paranal, Chile
Mauna Kea, Hawaii
2000
1998
1.75
1.8
Meniscus
Meniscus
46
40
Alt-Az
Alt-Az
7
8
8.0
8.0
Gemini South
Cerro Pachon, Chile
2000
1.8
Meniscus
40
Alt-Az
8
6.5
6.5
6.5
6.5
MMT Conversion
Magellan I - Walter Baade
Mt. Hopkins, Arizona
Cerro Manqui, Chile
1999
2000
1.25
1.25
Honeycomb
Honeycomb
9
9
Alt-Az
Alt-Az
9
10
6.5
6.0
6.5
6.0
Magellan II - Landon Clay
Large Zenith Telescope (LZT)
Cerro Manqui, Chile
Vancouver, Canada
2002
2005
1.25
1.5
Honeycomb
Liquid Hg
9
n/a
Alt-Az
Fixed
10
11
6.0
6.0
Bol'shoi Teleskop Azimultal'nyi (BTA)
Mt. Pastukhova, Russia
1977
4
Solid
6
Alt-Az
12
5.1
5.1
Hale
Mt. Palomar, California
1949
3.3
Honeycomb
8
Equatorial
13
References
(1) http:// lbto.org/, (2) http:// http://www.keckobservatory.org//, (3) http://www.gtc.iac.es/, (4) http://www.as.utexas.edu/mcdonald/het/het.html,
(5) http://www.salt.ac.za/, (6) http://www.naoj.org/, (7) http://www.eso.org/, (8) http://www.gemini.edu/, (9) http://www.mmto.org/,
(10) http://www.ociw.edu/magellan/magellan.html, (11) http://www.astro.ubc.ca/LMT/, (12) http://www.sao.ru/, (13) http://astro.caltech.edu/observatories/palomar/
1
4
5
Notes for Table 1.
This table is adapted from J.M. Hill’s web site: http://abell.as.arizona.edu/~hill/list/bigtel99.htm.
Column (1). The aperture is the diameter of the primary that can collect light. Unless specified, the number given is the
diameter of a circular aperture. The LBT consists of two 8.4-m mirrors that are on a single mount and the light from both
mirrors are combined to form a single image. The Keck, HET, and SALT telescopes have primary mirrors that are made
from hexagonal segments. The primary mirror has a hexagonal shape and the largest and smallest diameters of the
hexagon are given.
Column (2). This is the diameter of the equivalent circular aperture equal to the total light collecting area of the telescope.
For the HET and SALT telescopes this is the maximum equivalent circular aperture that is accepted by the prime focus
optics. The LBT, Keck, and VLT observatories can combine light from the mirrors for use as an interferometer. This mode
of observations is not considered in this table for the purpose of determining the equivalent circular aperture.
Column (5). Year that science operations started. Parentheses denote year science operations expected.
Column (6). Primary mirror f/no, which is equal to the focal length of the telescope divided by the mirror diameter.
Column (7). Honeycomb: Primary mirror that is lightened with a honeycomb structure in the back. Segmented: Primary
mirror is made out of hexagonal segments. Meniscus: Single thin concave mirror. Liquid Hg: Liquid mercury mirror.
Parabolic shape is obtained by spinning the mirror. Solid: Thick mirror with no light-weighting.
Column (8). The aspect ratio is the primary mirror diameter divided by the mirror (or segment) thickness.
Column (9). The azimuth only and fixed telescope mounts conduct observations by tracking object in the focal plane of
the telescope. For such telescopes the telescope is fixed but the instrumentation tracks the object.
2
Table 2 –
Summary of sky survey telescopes.
Survey
Status
Magnitude
limit
2.0
Field-ofview
(degree2)
1.3
21
Speed
(degree2
per hour)
20
CSS – Mt. Lemmon
operational
Aperture
(m)
1.5
f/no
Ref
CSS – Catalina Schmidt
operational
0.68
1.9
8
19.5
150
1
CSS – Siding Spring Uppsala
operational
0.5
3.5
4.2
19.5
75
1
LINEAR
operational
2 ! 1.0
2.2
2.0
19.4
1200
2
LONEOS (Schmidt)
operational
0.44
1.9
8.3
19.3
106
3
LONEOS (USNO)
in development
1.3
2.4
1.3
21.4
15
3
NEAT (Palomar)
operational
1.2
1.5
9.5
22.5
85
4
NEAT (MSSS)
operational
1.2
3.0
2.3
19.7
40.5
4
in development
1.2
2.5
9.4
~20.0
50
4
Spacewatch (Mosaic)
operational
0.93
3.0
2.9
21.5
160
5
Spacewatch (1.8m)
operational
1.82
2.7
0.32
22.5
8.9
5
Pan-STARRS (Hawaii)
in development
4!1.8
4
3.0
24.0
700
6
Discovery Channel Telescope
in development
4.0
2.2
3.1
21.8
110
7
proposed
6.9
1.25
7.0
24.0
2500
8
1
Schmidt
NEAT (Schmidt)
(Lowell)
Large Synoptic Survey Telescope
References: (1) Catalina Sky Survey, http://www.lpl.arizona.edu/css/, (2) Lincoln Near Earth Asteroid
Research, http://www.ll.mit.edu/LINEAR/, (3) Lowell Observatory Near-Earth-Object Search,
http://asteroid.lowell.edu/asteroid/loneos/loneos1.html, (4) Near-Earth Asteroid Tracking,
http://neat.jpl.nasa.gov/, (5) http://spacewatch.lpl.arizona.edu/, (6) Panoramic Survey Telescope & Rapid
Response System, http://pan-starrs.ifa.hawaii.edu/public/, (7) http://www.lowell.edu/DCT/, (8)
http://www.lsst.org/
Notes to Table 2.
1.
2.
3.
Field-of-view is the area of sky covered in a single exposure.
Magnitude limit is the faintest star recorded at visible wavelengths.
Speed is the rate at which observations can be carried out. One can see that of the operational facilities, LINEAR
covers the most sky per hour (1200 degree2/hour) but the faintest stars it can observe at this speed is 19.4 mag.
The Spacewatch (1.8 m) telescope can observe stars that are 3 magnitudes fainter but at a speed of only 8.9
degree2/hour).
3
(a)
(b)
Figure 1. (a) Image of KBO UB313 obtained with the 10-m Keck II telescope with a laser
guide star adaptive optics system. With a diameter estimated to be about 2400 km, it is
the largest KBO known and is slightly larger than Pluto. It was recently named Eris.
This image shows that UB313 has a satellite, as does Pluto. (b) A near-infrared spectrum
of UB313 and Pluto. The spectrum of Pluto was obtained with the 8-m Gemini North
telescope. Both objects have methane ice on their surface (methane ice absorption
marked with arrows), thus strengthening the idea that there is a common origin for
these objects. (Courtesy of M. Brown and C. Trujillo.)
Figure 2. Images of known comets in the asteroid main belt taken with the University of
Hawaii 2.2-meter telescope. These objects are known as the main-belt comets and are a
fundamentally new class of comets. The fuzzy appearance of these comets is due to
reflected light from dust particles that are ejected by a volatile material, most likely
sublimating water ice. (Courtesy of H. Hsieh and D. Jewitt.)
(a)
(b)
Figure 3. (a) Image of the asteroid 99942 Apophis. When it was discovered during its
last close approach to the Earth in 2004, it had a significant probability of striking the
Earth in the future. Subsequent observations show that it will pass within 5.6 Earth radii
of the Earth in 2029 (see panel b). However, the future trajectory of the asteroid cannot
be predicted well and the asteroid will have to be carefully monitored with groundbased telescopes. The diameter of the asteroid is about 250 m. Close passages by an
asteroid of this size are estimated to occur about once in 1300 years. (Courtesy of R.
Tucker, D. Tholen, and F. Bernardi.)
Figure 4. Increase in telescope area with time. Only the area of the largest telescopes at
each time period is shown, so this indicates the envelope of maximum telescope area as
a function of year. The time for the telescope area to double is about 26 years from the
invention of the telescope in 1608 to the current year. However the doubling time has
decreased from about 1900 to the present. The solid line shows a doubling of telescope
aperture about every 19 years. The next jump in aperture size is likely to be in the range
of 20–50 meters. For comparison the square symbol shows a 30-m class telescope in the
year 2020, and this indicates an even shorter doubling time. The increase in telescope
area is due to advances in telescope construction technology and the willingness of
society to bear the costs. How much longer can this increase in telescope area continue
on the ground? (See Racine 2004, Pub. Astron. Soc. Pacific, vol. 116, p. 77) for data on
the growth of telescope aperture with time.)
Figure 5. Schematic of different telescope mounts: (a) equatorial, (b) alt-az, (c) azimuthonly, (d) fixed. The Hale 5.1-m telescope was the last large telescope to be built with an
equatorial mount. The equatorial mount has one axis aligned to the rotation axis of the
Earth. (Note: there are many types of equatorial mounts. The Hale telescope uses a type
known as the horseshoe equatorial mount.) All fully steerable large telescopes utilize
the alt-az mount, such as the Keck, Gemini, VLT, and Subaru telescopes (see Table 1). In
the alt-az mount, the azimuth axis points to the zenith with a perpendicular altitude
axis. Two large telescopes built specially for spectroscopy use the azimuth-only
mount—the Hobby-Eberly and the South African Large Telescope. The telescope moves
only in azimuth and is fixed in declination. The only large telescope to date that uses a
fixed mount (the telescope points only to the zenith) is the Large Zenith Telescope, and
it uses a liquid mercury mirror.
Figure 6a: Hale 5.1 m telescope. The last large telescope to be built in the “classical
style” with an equatorial mount, a culmination of about 280 years of development of the
reflecting telescope. (c) 2005 Gigapxl Project
Figure 6b: 8-m Gemini South telescope. Instruments are mounted on the back of the
telescope. These instruments are on the telescope all of the time so that instrument
changes can be made very quickly. The dome has vents to allow flushing of the dome
by the night air. This allows the telescope and dome to quickly reach equilibrium with
the air temperature. (Courtesy of Gemini Observatory/AURA)
Figure 6c: 10-m Keck telescope. This image shows one of the two Keck telescopes. The
primary mirror consists of 36 hexagonal segments that are aligned to optical precision.
The instruments are located on a platform on two sides of the telescope facing the
declination bearings. Light from the two telescopes can be combined to provide
angular resolution equivalent to an 85 m telescope. (Courtesy R. Wainscoat.)
Figure 6d: Large Binocular Telescope consisting of two 8.4-m primary mirrors. First
light with a single mirror took place in in 2005 and the second mirror was installed in
2006. The light-gathering power of the two primary mirrors combined is equivalent to a
11.8-m telescope. Both mirrors are on a single structure and the light from both mirrors
is combined for imaging, spectroscopy, and interferometry. The combined light from
the two mirrors will have the angular resolution of a 22.8 m telescope when the LBT is
used as an interferometer. (Courtesy of the Large Binocular Telescope Observatory)
Figure 7. Schematic of an active optics system. Starlight from the telescope is sent to a
beamsplitter just in front of the focus that simultaneously sends light to the focus and to
a wavefront sensor. The computer analyses the output of the wavefront sensor and
sends control signals to the primary mirror to correct any aberrations in the image.
(Courtesy of C. Barbieri.)
Figure 8. Improvement in angular resolution at optical wavelengths. The development
of adaptive optics has permitted diffraction-limited observations from ground-based
observatories since 1990, largely eliminating the effects of the atmosphere. The dashed
line shows the theoretical diffraction-limited resolution for the telescope. The solid line
shows the seeing limit imposed by the atmosphere. Improvements were obtained by
going to very good seeing sites. The resolution of the Hubble Space Telescope is shown.
(From P. Bely, 2003.)
Figure 9. Large CCD mosaic installed in MegaCam, a prime focus camera at the
Canada-France-Hawaii. This mosaic consists of 40 CCDs, each with 9.5 million pixels. In
total the camera has 380 million pixels, the largest mosaic CCD currently in use. This
camera is capable of generating 100 billion bytes (100 gigabytes) per night. Larger
mosaic cameras are being planned. Each telescope of the Pan-STARRS survey telescope
will have a 1.4-Gigapixel camera and the Large Synoptic Survey Telescope will have a
single 3.2-Gigapixel camera. (Courtesy of CFHT)
Figure 10. Image of Saturn and its rings obtained in 2004 with the 10-m Keck I telescope
at a wavelength of 17.6 micrometers. This is a false color image, where higher signal
levels are shown lighter. At these wavelengths we are seeing the heat radiated by the
atmosphere and rings of Saturn. The South pole has an elevated temperature (–182 C)
compared to its surrounding. This is likely due to the fact that the South pole has been
illuminated by the sun for the past 15 years. (Courtesy of G. Orton, JPL).
Figure 11. Simplified diagram of an AO system. Light from the telescope is collimated
and sent to an adaptive or deformable mirror. If there were no atmospheric turbulence,
the wavefront of the light would be perfectly straight and parallel. The light is then
reflected to a beamsplitter, where part of the light is reflected to the wavefront sensor.
The wavefront sensor measures the distortion of the wavefront and sends a correction
signal to the adaptive mirror. The adaptive mirror is capable of changing its shape to
remove the deformations in the light wave caused by the atmospheric turbulence. In
this way the light with a corrected wavefront reaches the high-resolution camera, where
a diffraction-limited image is formed. (Courtesy of C. Max)
Figure 12. Images of Uranus with and without adaptive optics. This is a striking
demonstration of the effectiveness of adaptive optics in removing atmospheric
turbulence. One can also see that the signal-to-noise is greatly enhanced because light is
concentrated into a diffraction-limited image with adaptive optics, thus greatly
increasing the ability to detect faint spots and cloud structure. At a wavelength of 1.6
micrometers, we are seeing reflected light from low-altitude clouds while at 2.2
micrometers the high-altitude clouds are revealed. The planet is much darker at 2.2
micrometers due to absorption of methane gas in the atmosphere. This allows a much
longer exposure and for the rings to be seen clearly. The point-like cloud features at 2.2
micrometers show that in certain places turbulence is very strong and is pushing
material from lower altitudes into the stratosphere. (Courtesy of H. B. Hammel, I. de
Pater, and the W. M. Keck Observatory.)
Figure 13. Image of the asteroid 87 Sylvia showing its two satellites. This image was
taken with the European Southern Observatory 8-m Very Large Telescope at 2.2
micrometers with an adaptive optics system. The cross marks the location of the
asteroid and the scale bar shown is 0.25 arcseconds. The diameter of 87 Sylvia is about
280 km, and the diameters of the satellites are about 7 and 14 km. The orbits of the
satellites were measured in order to determine a density of about 1.2 grams/cm3 for 87
Sylvia—only 20% higher than the density of water. Thus 87 Sylvia is likely to have a
rubble pile internal structure with 20-60% of its volume being empty. (Courtesy of F.
Marchis.)
Figure 14. Sodium laser guide star in use at Keck II. The laser operates at a wavelength
of 5890 Angstroms (0.589 micrometers), and the laser light is propagated through a
smaller telescope attached to the Keck telescope. It excites sodium atoms in a layer in
the Earth’s atmosphere at an altitude of 90 km. The sodium atoms emit light at the same
wavelength as the laser and this is viewed as an artificial star by the telescope. (This is a
long exposure photograph. The laser guide star is barely visible with the naked eye
from this angle. The lights of the island of Hawaii are below the clouds. (Courtesy of
Jean-Charles Cuillandre.)
Figure 15. Infrared image of 2M 1207 (a brown dwarf and planet binary system)
obtained with one of the 8.2-m VLT telescopes. The brown dwarf (white) is 100 times
brighter than the planet (red) and both are emitting heat left over from their formation.
Their masses are estimated to be 25 and 5 Jupiter masses. In this image the infrared
colors at wavelengths 3.8, 2.2, and 1.6 microns are portrayed as red, green, and blue,
respectively. The separation of the objects in the sky is 0.78 arcseconds and this
corresponds to a physical separation of 55 AU. (Courtesy Gael Chauvin / ESO).
Figure 16. Cumulative discoveries of near-Earth asteroids. The total number of large
near-Earth asteroids (larger than 1 km) is increasing at a slower rate since most of the
easy-to-detect NEOs have already been discovered. The remaining unknown NEOs are
on orbits that are intrinsically more difficult to detect and therefore require a longer
time to discover. (Courtesy of Alan Chamberlin.)