Optical Interferometry: A Way Around the Rayleigh Criterion?

Invented by Albert A. Michelson during the 1870s, can optical interferometry be used effectively in circumventing the Rayleigh Criterion limitations of a typical reflecting telescope?


By: Ringo Bones


Albert A. Michelson was more famous for his work with Edward W. Morley in which they won the 1907 Nobel Prize in physics for proving that the mythical medium called the ether wind doesn’t exist, thus paving the way for Einstein’s theory of special relativity. Michelson invented the interferometer primarily as a way of accurately measuring the speed of light back in the 1870s. Albert A. Michelson also managed to earn the fame of being the first one to utilize optical interferometry as a very important astronomical instrument until 1920. It is this time when Michelson became the first ever person to measure a diameter of a star using an optical interferometer of his own design. He determined that Alpha Orion to be 260 million miles in diameter. A few years before, Michelson was also the first person to determine the diameter of Jupiter’s satellites. But how does optical interferometry works?

Optical interferometry depends on the light splitting and summing properties of an interferometer. An interferometer is an instrument that utilizes light interference phenomena for precise determinations of wavelength, fine structure of spectral lines, refractive indices of a given medium and very fine linear displacement of distant objects. By bringing together beams of starlight captured by two or more widely separated telescopes, a typical optical interferometer can achieve the equivalent resolving power of a single instrument equipped with a main mirror as large as the distance between the ganged telescopes.

When the starlight beams are combined, the light waves interfere with one another. Where the peak of one light wave meets the peak of another, they reinforce each other. When the peak of one light wave meets the through of another, they cancel out. An electronic detector or a video digital-to-analog-converter (video DAC) records the resulting pattern of dark and light areas – or interference fringes – which can then be analyzed by computer via digital signal processing to extract detailed information about the object being observed. If at least three telescopes are used, the fringes can be rendered into images hundreds of times crisper than even those obtained by the orbiting Hubble Space Telescope – at a much reduced expense, thus bypassing the Rayleigh Criterion limitations of constructing an ever bigger mirror of a typical astronomical telescope. Given its ability to improve reflecting telescope performance beyond their Rayleigh Criterion limitations, why is it that most astronomers are still mistrustful over optical interferometry?

There had been famous and amazing stargazing feats achieved by optical interferometry since the 1970s. In 1974, Kitt Peak National Observatory astronomers managed to penetrate the Earth’s atmospheric haze for the first time by discerning the features in the atmosphere of a star named Betelgeuse with the computer aided technique called speckle interferometry, while the Mark III Optical Interferometer on Mount Wilson Observatory in California had been in operation since 1986. Astronomers tend to be a conservative bunch and a lot of them consider optical interferometry to be “black magic” because even though they can measure the outlines of celestial objects millions – even billions - of miles away, optical interferometers cannot make true images of these objects.

If you remember the movie version of Tom Clancy’s Patriot Games when Langley analysts got a hard copy of a photo taken by a KH-11 reconnaissance satellite showing the cleavage of the female underwriters of the Ulster Liberation Army standing in the middle of the Libyan Desert. You’ll notice that it is monochromatic – i.e. black and white – yet managed to show features that according to Rayleigh Criterion on the KH-11 reconnaissance satellite’s mirror specifications cannot supposedly resolve that make it appear like a distinct black and white image of a woman’s cleavage seen from 160 miles up. That’s the power of optical interferometry put to use were the video signals are probably processed using the reconnaissance satellite’s built-in 10-bit video DAC that’s probably not more advanced than one’s found in a circa 1998 DVD player.

Image quality-wise, the resulting image is so “abstract” and “clinical” that free-spirited American under-aged teens who are frequent skinny-dippers have no fear having compromising photos taken by computer nerds who know how to re-task the US National Security Agency’s reconnaissance satellites - Largely because its image quality is far inferior in comparison to those photos taken by a typical paparazzi operating in the 90210 area code. Probably due to the fact that optical interferometry – a technique probably utilized by the KH-11 reconnaissance satellite to be able to read Soviet-era Pravda headlines and car license plates from 160 miles up - cannot make true images.