New Generation Ground-Based Optical/Infrared Telescopes 721
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 are 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.)
diffraction-limited imaging, one must employ special tech-
niques 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 tele-
scope construction in providing greater observing capabil-
ity. 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 com-
plete 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 analyz-
ing 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 collect-
ing area, optimize image quality, and achieve diffraction-
limited 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.
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.)