Long-baseline optical interferometry

Article By:

Elias, Nicholas Astrometry Department, Navy Prototype Optical Interferometer, U.S. Naval Observatory, Washington, DC.

Last reviewed:1998

DOI:https://doi.org/10.1036/1097-8542.YB980410

The phrase “to spatially resolve” refers to the ability to see structure in objects or to separate individual objects in an image. The maximum spatial resolution is given by Eq. (1), where θ is the minimum measurable separation in arc-seconds (an arc-second is 1/60 of an arc-minute or 1/3600 of a degree), λ is the wavelength of the light in nanometers (1 nm = 10⁻⁹ meter), and D is the size of the aperture through which the light travels in meters (1 m ≈ 3.3 ft). This resolution limit may be explained by the diffraction of light through the aperture, a process which smears images. (The maximum spatial resolution is also called the diffraction limit.) An alternative explanation is that some spatial information from the source is lost because only a fraction of its light gets through the aperture. The human eye is most sensitive to light at λ ≈ 500 nm (yellow light), and its pupil is typically D ≈ 2 mm in diameter, which translates to a spatial resolution of approximately 60 arc-seconds (1 arc-minute). Unfortunately, this resolution is not enough to produce images of stellar surfaces or to chart the orbits of binary star systems. To overcome the limitations of the human eye, astronomers since Galileo have constructed telescopes and instruments to increase resolving power (or, to increase the effective size of the eye's pupil).

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