The Size, Structure, and Variability of Late-Type Stars Measured ...

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Although a complete set of visibility data could be transformed back to the image
domain to obtain the intensity profile, this is rarely done. More often, the coverage of the
Fourier domain is too sparse to transform directly and is instead analyzed in terms of the
limitations it places on the image. For interpretation, a model having a small number of
variable parameters is usually transformed and fit to match the real visibility data. This
extraction of real astrophysical information from a set of interferometric measurements is a
challenging and important part of the data reduction process.
1.2 History of Interferometry
Using interferometry to obtain higher resolution for astronomical purposes was first
suggested by Fizeau in 1868. The first experimental use of interferometry on a star was carried
out by Stéphan in 1874 and he was unable to resolve any star using his 80 cm reflector
(Quirrenbach (2001) [83]). The first measurement of the size of a star was performed by
Michelson (1921) [70] using a 20 foot baseline within the 100” telescope at Mt. Wilson Observatory.
Betelgeuse (α Ori) was measured to have a diameter of 47 mas by Michelson. The
technologic requirements of interferometry did not allow the field to progress after the initial
measurements at Mt. Wilson and it wasn’t until the early 1970’s that fringes were formed
from separate telescopes using this type of interferometry. This was performed by Labeyrie
in the visible [83], and Johnson [53] in the mid-infrared. Since pathlengths in interferometry
need to be matched to within a fraction of λ, the strict instrumental requirements of optical
interferometry were not present at radio wavelengths. Successful radio interferometers have
been operating since the 1940’s and have progressed to the point where intercontinental
baselines are used to obtain resolution competitive with optical instruments. Within the
last 25 years, however, there have been several new optical and infrared interferometers
built. Most of these are two-telescope arrays using direct fringe detection, although very
recently, several multi-telescope arrays capable of phase closure are coming online. The current
state of optical interferometry is summarized in Quirrenbach (2001) [83] and a more
complete history can be found in Shao (1992) [93].