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New Generation Ground-Based Optical/Infrared Telescopes

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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. 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. 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 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 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,
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