Optical Interferometry
Definition
Method of measuring distance changes with a precision down to sub nanometer scale.
Use of imaging techniques the acquisition of surface profiles.
The profile is derived from the relative or absolute phase of the reflected light for each profile point.
In case of a relative phase measurement, a so-called phase unfolding algorithm is used to reconstruct the absolute phase.
Motivations for Interferometry
Precise Position
Interferometry Measurements
High Spatial Resolution
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-Calibrate Cepheid distance scale
-Size of Accretion Disks around Supermassive black holes
-Measure Stellar Diameters
-Detect stellar “wobble” due to planets
-Track stellar orbits around SBHs
Stellar Suppression
-Detect zodiacal dust disks around stars
-Direct detection of exosolar planets
Interferometers can be thought of in terms of the Young’s two slit setup. Light impinging on two apertures and subsequently imaged form an Airy disk of angular width λ /D modulated by interference fringes of angular frequency λ/B. The contrast of these fringes is the key parameter for characterizing the brightness distribution (or “size”) of the light source. The fringe contrast is also called the visibility. 
Visibility is also measured in practice by changing path- length and detecting the maximum and minimum value recorded.
V=
Imax - Imin / Imax + Imin
Optical interferometers are homodyne, meaning incoming radiation is interfered only with light from other telescope. This requires transport of the light to a central station, without the benefit of being able to amplify the signal. However, homodyne interferometry allows large bandwidths to be used since the interfered light is detected directly.
One heterodyne optical interferometer (ISI) has been built to operate at 10 microns. The technique is feasible, but limited to bright sources.)
Types of interferometer
Image-Plane
Imaging interferometry is more typically implemented on a common-mount interferometer . An imaging interferometer can be designed to create high resolution images over a wide field of view.
Pupil-Plane
Pupil-plane interferometry is used in long-baseline interferometry.
Bracewell (1978) first suggested using this technique to null a stellar point source for detection of planets.
- An image of the object is formed with a resolution of λ/D.
- Visibility is measured by scanning the interferometer in pathlength.
- The flux seen in the image is the object flux multiplied by the transmission pattern, and convolved with the single aperture PSF.
An image of the object is formed with a PSF which is the FT of the entrance aperture.
PSF is invariant within the field.
The flux seen in the image plane is the object flux convolved with the PSF of the interferometer.