New Generation Ground-Based Optical/Infrared Telescopes 725
FIGURE6b 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)
adjustments are not fast enough to correct for the atmo-
spheric 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 cor-
recting the surface figure of a segmented primary mirror,
although the details are quite different.
Efforts to escape the harmful effects of the Earth’s atmo-
sphere have led to telescopic observations using balloons,
aircraft, and rockets. Although we do not discuss space ob-
servatories 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 25μmto350μm
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
FIGURE6c 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.)FIGURE6d 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)