1

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

2

TABLE OF CONTENTS I. Introduction....................................................................................................3

II. Advances in the construction of large telescopes and in image quality.........4

III. Advances with detector arrays ......................................................................8

IV. Advances in adaptive optics .........................................................................9

V. Sky survey telescopes ................................................................................10

VI. Concluding remarks VII. 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 solid-state 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.

3

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 Gemini-North 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

4

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.

5

(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

6

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.

7

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 Thirty-Meter 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,

8

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

9

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

10

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

11

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.

12

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. Springer-

Verlag, 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

1

Table 1. Telescopes with Apertures Greater than 5 Meters.

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

Circular Mirror

Aperture Date of primary Mirror Aspect Mounting

Aperture (m) Equiv. (m) Telescope Name Location Operation f/no Type Ratio Type Ref.

2 x 8.4 11.8 Large Binocular Telescope (LBT) Mt. Graham, Arizona (2006) 1.14 Honeycomb 9.4 Alt-Az 1

11x9.4 Hexagon 10.0 Keck I Mauna Kea, Hawaii 1993 1.75 Segmented 133 Alt-Az 2

11x9.4 Hexagon 10.0 Keck II Mauna Kea, Hawaii 1996 1.75 Segmented 133 Alt-Az 2

11x9.4 Hexagon 10.0 Gran Telescopio Canarias (GTC) La Palma, Canary Islands (2007) 1.65 Segmented 125 Alt-Az 3

11x10 Hexagon 9.2 Hobby-Eberley Telescope Mt. Fowlkes, Texas 1997 1.4 Segmented 200

Azimuth

only 4

11x10 Hexagon 9.2 Southern African Large Telescope (SALT) Sutherland South Africa 2005 1.4 Segmented 200

Azimuth

only 5

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 Cerro Paranal, Chile 1998 1.75 Meniscus 46 Alt-Az 7

8.2 8.2 Very Large Telescope (VLT) UT2 Kueyen Cerro Paranal, Chile 1999 1.75 Meniscus 46 Alt-Az 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 Very Large Telescope (VLT) UT4 Yepun Cerro Paranal, Chile 2000 1.75 Meniscus 46 Alt-Az 7

8.0 8.0 Gemini North Mauna Kea, Hawaii 1998 1.8 Meniscus 40 Alt-Az 8

8.0 8.0 Gemini South Cerro Pachon, Chile 2000 1.8 Meniscus 40 Alt-Az 8

6.5 6.5 MMT Conversion Mt. Hopkins, Arizona 1999 1.25 Honeycomb 9 Alt-Az 9

6.5 6.5 Magellan I - Walter Baade Cerro Manqui, Chile 2000 1.25 Honeycomb 9 Alt-Az 10

6.5 6.5 Magellan II - Landon Clay Cerro Manqui, Chile 2002 1.25 Honeycomb 9 Alt-Az 10

6.0 6.0 Large Zenith Telescope (LZT) Vancouver, Canada 2005 1.5 Liquid Hg n/a Fixed 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/

2

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.

3

Table 2 – Summary of sky survey telescopes.

Survey

Status

Aperture

(m)

f/no

Field-of-view

(degree2)

Magnitude limit

Speed (degree2

per hour)

Ref

CSS – Mt. Lemmon operational 1.5 2.0 1.3 21 20 1

CSS – Catalina Schmidt operational 0.68 1.9 8 19.5 150 1

CSS – Siding Spring Uppsala

Schmidt

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

NEAT (Schmidt) 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

(Lowell)

in development 4.0 2.2 3.1 21.8 110 7

Large Synoptic Survey Telescope proposed 6.9 1.25 7.0 24.0 2500 8

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. Field-of-view is the area of sky covered in a single exposure. 2. Magnitude limit is the faintest star recorded at visible wavelengths. 3. 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).

(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 ground-based 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) azimuth-only, (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.)