New Generation Ground-Based Optical/Infrared Telescopes 729
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)
diffraction-limited imaging from the ground. This is criti-
cal in achieving the maximum S/N given in equation (1).
The basic idea of AO is to first measure the amount of at-
mospheric 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 wave-
front. 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 veloc-
ity 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 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 at-
tention, recently there has been a corresponding quantum
jump in the construction of visible and infrared survey tele-
scopes. This has been made possible by the availability of
large-format CCD and infrared arrays. In addition, the dis-
covery 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 be-
cause 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 impor-
tant 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, identi-
fying 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.