The usefulness of adaptive optics for improving image resolution and quality of large optical telescopes has revolutionized the approach to ground based astronomy. One technique now made possible by adaptive optics is the coupling of several large telescopes to perform optical interferometry. Radio interferometry has been an important and widely used tool in astronomy, as evidenced by the VLA, the VLBA and other long baseline arrays of radio interferometers through out the world. But seeing limitations to the sensitivity of optical telescopes made an interferometric array of large optical telescopes ineffective, and so past efforts were limited to building small dedicated optical interferometers. With the ability to correct for the atmospheric seeing effects and the fact that optical telescopes tend to be clustered at a few good observing sites, the coupling of large telescopes into an interferometric array now appears to be feasible.
Interferometry uses the constructive and destructive interference of
radiation to determine the spatial properties of the source of the radiation.
In its simplest form, two telescopes separated by distance D, where D 
, the wavelength of the light, observe the same object. When the
two observations are correlated, an interference pattern occurs due to the
differences in the light's path length for each telescope. (Kitchin, 1991).
This is
equivalent to Young's double slit experiment. For Young's double slit
experiment,
where d is the difference in the path lengths and 
is the angle of the incident light on the telescope. For constructive
interference,
The double-slit interference pattern for a single source point is shown
in Figure 3.
(Heald & Marion, 1995; Hecht, 1987).
If a second point source is also observed, the structure of the superposition
of the two interference patterns depends on the separation of the two sources.
For two stars at the same position, the fringe pattern will simply have twice
the intensity as for one star. As the two stars move apart, the individual
fringe patterns will separate. When the angular separation of the two
stars is
the maximum of one fringe pattern will occur at
the minimum of the other fringe pattern and the fringes disappear. As sources
separate, the fringes will reappear. Therefore the fringe pattern is most
visible when 
and disappear for 
.
The resolution is therefore /2d and is improved with increasing d, the
baseline for an interferometer. (Kitchin, 1991).
Unlike for single telescopes, adaptive optics does not increase the angular
resolution for optical interferometry. Rather, it increases the sensitivity
of the interferometry by restoring the coherence of the incoming light over
the individual elements of the interferometer. The limiting magnitude
of an optical interferometer depends greatly on 
, the atmospheric
coherence length. For resolution limited by
the atmospheric conditions, the gain in sensitivity with
increased diameter of the individual elements (telescopes) is only
proportional to 
; the gain in magnitude with an increase in diameter
from 25 cm to 8 m is just 0.6 magnitudes. The efficiency of adaptive optics
is generally given by the Strehl ratio, the ratio of the central intensity
of the corrected image to the central intensity of a diffraction limited image.
For a partial correction of the images with adaptive optics with a
Strehl ratio 
0.3, the gain in sensitivity from a 25 cm diameter to
8 m is 6 magnitudes. (Mariotti, 1996; Mariotti et al., 1994).