The successes of speckle interferometry have firmly es-
tablished high-resolution astronomy as an important sci-
entific enterprise. Users of new, large telescopes are pay-
ing special attention to their interferometric applications
as well as to their light-collecting potential. Whereas not
long ago it was considered necessary to go into space to
achieve significant gains in resolution, there are now nu-
merous plans to further extend the boundaries of resolution
from the ground using single giant telescopes or arrays of
telescopes.
The 1990s saw the inauguration of several large tele-
scopes with apertures in the 8- to 10-m range. The Eu-
ropean Southern Observatory has built four 8-m tele-
scopes on Cerro Paranal in the Chilean Andes in a facility
known as the Very Large Telescope (VLT). The VLT has
the light-gathering power of a single 16-m telescope. A
consortium of institutions led by the University of Ari-
zona is building the Large Binocular Telescope (LBT) on
Mt. Graham, Arizona, consisting of two co-mounted 8.4-
m light-collecting mirrors. The VLT, LBT, and the twin10-m Keck telescopes represent a great advance in our
ability to collect light from the faintest objects in the uni-
verse. They also push resolution through speckle imaging
and adaptive optics applied to each of their large individual
apertures.
Many problems in astrophysics call for resolutions com-
parable to apertures of 100 m and larger. Individual tele-
scopes with such enormous dimensions are not likely to be
built in the foreseeable future, but the prospects for very
high resolution imaging are being realized through the
construction of synthetic large apertures using methods
analogous to those employed at radio wavelengths.
Arrays of telescopes, each of which may have a rela-
tively small aperture, can be distributed along the ground
to effectively synthesize an aperture hundreds of meters
across. Optical wavelengths are far more challenging to
this approach than are the much longer radio wavelengths,
and the technology suited to multiple-telescope optical
arrays matured significantly in the 1980s and 1990s. In-
terferometer arrays have been built in Australia, Europe
and the United States. The VLT and twin Keck telescopes
are both being equipped for “long baseline” interferome-
try by linking the large telescopes together with smaller
“outrigger” telescopes. Experience from speckle interfer-
ometry with single telescopes has gone far to improve our
knowledge of how the atmosphere will affect such arrays
as we strive for a gain of a factor of 100 over the resolution
now provided by speckle methods. Several dedicated op-
tical/interferometric arrays now in operation are expected
to provide important new data pertaining to stellar physics
as well as the first images of the surfaces of stars of a va-
riety of masses, diameters, and temperatures. These facil-
ities are likely to be the progenitors for a next-generation
array incorporating dozens of telescopes with apertures of
4–8 m synthesizing a kilometer-size aperture.
Space-borne interferometers should be operational this
decade and are expected to provide unprecedented in-
creases in our ability to directly measure distances to
stars and to add to the list of known extrasolar plane-
tary systems. Ultimately, space interferometry may yield
the first images of the surfaces of planets around other
stars.
High-angular-resolution astronomy is providing a revo-
lutionary approach to viewing the universe. This is remark-
able progress in the three de
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