Interferometric observations of pre-main sequence disks - Caltech ...

30 Introduction to interferometry
tion tens to hundreds of time larger than single aperture in the same wavelength domain.
They both use arrays of telescopes and, as described in the previous section, have to find
the most suitable telescope configuration to reach their objective. Moreover, both optical
and radio interferometry require to calibrate the measured complex visibilities.
Optical and millimeter interferometry differ, first of all, for the angular resolution
that can be achieved, defined by the fringe spacingλ/2B. Fixed the maximum baseline
B, the wavelength dependence implies that an interferometer observing in the near-
infrared may achieve an angular resolution three orders of magnitude higher than at
millimeter wavelengths. In practise, the existing millimeter interferometers are charac-
terized by antennas configurations more extended that the optical array and the differ-
ence is generally lower. For examples, while the VLTI has a maximum baseline of∼200
m, corresponding to a resolution of few milli-arcsec in the near-infrared, the Plateau the
Bure Interferometer can operate with a baseline of∼ 700 m and can achieve a resolution
of∼0.3 ′′ at 1 mm; for the Very Large Array the resolution is∼0.2 ′′ at 3 cm (B=36000
m). In future, with the Atacama Large Millimeter Array it will be possible to achieve a
resolution of∼0.01 ′′ at 1 mm with a baseline of 15000 m.
The atmosphere is the source of a second important difference. Due to the diver-
sity of the temperature between the ground and the upper layers of the atmosphere,
convection occurs and creates turbulent eddies, characterized by different refractive in-
dices. When looking to objects through the atmosphere, the rays of light are therefore
randomly deviated. The atmospheric turbulence is characterized by the spatial Fried’s
parameter r0, which corresponds to the spatial scale of the atmospheric turbulence, and
the coherence time t0, which define the temporal variation of the turbulence. At opti-
cal wavelengths, typical values of r0 and t0 are in the range 0.1–1 m and 10-100 ms
respectively; moreover, the physical analysis of the turbulence shows that both r0 and
t0 depend on the wavelength asλ 6/5 . At optical wavelengths, the dominant effect of
atmospheric turbulence is the corrugation of the wavefront both at the level of each in-
dividual aperture, seeing, and at the level of the interferometer, atmospheric piston. As
in the case of single optical telescopes, interferometers use adaptive optics systems to