Francois Roddier of the Institute for Astronomy at the University of Hawaii answers:

"An optical interferometer is a device in which two or more light waves are combined together to produce interference. I assume the question is about an optical interferometer combining light from several telescopes. If two telescopes observe the same star and the light beams are superimposed, a phenomenon called interference occurs. The star's brightness will vary with the delay between the arrival times of the two light beams. If the arrival times are the same, the crest of one wave will add up to the crest of the other wave producing a new wave with twice the amplitude and four times the energy of a single wave. Yet as the earth rotates, one telescope becomes closer to the star than the other. When it is closer by half of a wavelength, a crest in one beam will correspond to a trough in the other beam, and the two light waves will exactly cancel each other, making the star disappear.

"The star reappears and disappears every time the delay between the telescopes is a multiple of the wave period. In other words, the star looks as if it were moving behind a dark picket fence. The 'pickets' are called interference fringes. The wider apart the telescopes are, the smaller the fringe spacing (the tighter the picket fence). Two telescopes only a few meters apart produce fringes as close as the diameter of a giant star like Betelgeuse. In this case, the star is larger than the size of a picket. It can no longer hide behind it, that is, it no longer disappears.

"This is how Albert A. Michelson and Francis Pease detected and measured the apparent diameter of Betelgeuse in 1920. This was the first direct measurement of a stellar diameter. Instead of using two telescopes, they used two apertures on the same Mount Wilson telescope, which was easier to implement. Betelgeuse is the largest star in the sky. Most other stars require much larger separations and therefore separate telescopes, which is much more difficult to implement because of the very stringent mechanical tolerances. Another difficulty in doing such observations at optical wavelengths (on the order of 10-6 meters) comes from the inhomogeneities in the air's refractive index, which produce random delays in the light arrival time. As a result, the star seems to move randomly behind the picket fence, making measurements more difficult.

"For these reasons, interference with separate telescopes first became commonly used in radio astronomy at centimeter wavelengths. They are now commonly used at millimetric wavelengths. An indirect radio technique called intensity interferometry was first used in the visible spectrum in 1957 by Hanbury-Brown and Twiss but was limited to very bright stars. In 1975 Antoine Labeyrie was the first to observe fringes directly with two independent optical telescopes. Although several optical interferometers have now been built around the world, the technique is still under development. It is best used for astrometric purposesthat is, for accurate measurements of distances between stars and of stellar diameters.Yet, by using a sufficient number of telescopes or by moving the telescopes in a sufficient number of directions and distances, one can reconstruct images of the source with an extremely high angular resolution. Progress in instrumentation such as adaptive optics and infrared detector arrays now make interferometry a very promising technique for use with optical telescopes operating in the infrared."

Peter Lawson of the Mullard Radio Astronomy Observatory (MRAO) in Cambridge, England, adds:

"The interferometer was invented by Albert A. Michelson in about 1880. It is an optical instrument that has been redesigned in numerous forms and has many applications in optics where precision measurements are required. Michelson originally designed the interferometer for ether-drift experiments to prove the existence of the medium, which was thought to explain the propagation of light. He also used the interferometer to define the International Standard Meter in terms of the red wavelength of cadmium light, to study the fine structure in spectral lines, to determine the degree of rigidity and elasticity of the earth, and to measure the angular diameters of the satellites of Jupiter and the diameters of several of the largest stars. In recognition of his development of precision measurement techniques using the interferometer, Michelson was awarded the Nobel Prize in 1907. Interferometers are now widely used for spectroscopy, the study of thin films, the testing of precision optics, measurements of refractive indices, and both radio and optical astronomy.

"An optical interferometer samples the wavefronts of light emitted by a source at two or more separate locations and recombines the sampled wavefronts to produce interference fringes. The fringes are a striking example of the wave nature of light: the wavefronts add constructively or destructively, depending on the path difference between the wavefronts, and produce fringes that appear as bright and dark bands, with the bright bands being brighter than the sum of intensities in the two separate wavefronts. If the pathlength in one arm of the interferometer is changed by even a fraction of a wavelength, the fringes will appear to move. This extreme sensitivity to minute path variations has made interferometry a powerful tool with a wide range of applications.

"The advantage of interferometry for optical astronomers is that it can provide measurements of stars with a higher angular resolution than is possible with conventional telescopes. An astronomical long-baseline interferometer is composed of an array of several separate telescopes, which redirect starlight to a central location where interference fringes are formed. The available angular resolution depends only on the telescope separations, which may in principle be arbitrarily large. Most observations with optical interferometers have used telescope separations of less than 100 meters, although instruments exist that will eventually use separations several times larger. In marked contrast to this, conventional telescopes have a resolution restricted by their aperture diameter, which is unlikely ever to be much larger than 10 meters. Interferometry is therefore clearly the future of high-angular resolution astronomy.

"Interferometers are used as a tool for stellar astrophysics, principally for the measurement of the angular diameters of stars and for the measurement of binary star orbits. An optical interferometer does not produce direct images of stars and typically has only a rudimentary ability to make images. The observations consist of measurements of the fringe contrast or position. An infinitesimally small source will produce interference fringes that have a high contrast. If, as viewed with a given telescope separation, the source has any appreciable size, the fringe contrast is reduced. The loss in contrast is usually calibrated with observations of stars that are known to be unresolved. In this way, by fitting mathematical models to the data, it is straightforward to determine stellar diameters and the parameters of binary star orbits. It is also possible to determine stellar radii, surface fluxes and effective temperatures. More difficult still is the modeling of limb-darkening, stellar rotation and the detection of surface features on stars. These are more challenging because accurate measurements must be made of low-contrast fringes, and many of the most interesting stars are notoriously variable.

"It is also possible to measure the relative positions and proper motions of stars, using interferometers designed for astrometry. It is hoped that interferometric measurements will be able to maintain the catalogue established by the Hipparcos satellite. Most interestingly, narrow-angle astrometry is being developed for the detection of extrasolar planets.

"A good overview of long-baseline stellar interferometry is contained in the following review articles:

  • Stellar Optical Interferometry in the 1990s, by J. T. Armstrong, D. J. Hutter, K. J. Johnston and D. Mozurkewich in Physics Today, Vol. 48, No. 5, pages 42-49; 1995.
  • Measuring the Stars, by J. Davis in Sky and Telescope, Vol. 82, pages 361-365; 1991.
  • The Quest for High Resolution, by J. Davis in Sky and Telescope, Vol. 83, pages 29-33; 1992.
  • Long-Baseline Optical and Infrared Stellar Interferometry, by M. Shao and M. M. Colavita in Annual Review of Astronomy and Astrophysics, Vol. 30, pages 457-498; 1992. (This is a more comprehensive review of technical aspects of interferometry, with an emphasis on astrometry.)

Stephen T. Ridgway is a visiting scientist at the Center for High Angular Resolution Astronomy at Georgia State University. He offers the following reply:

"The most active frontier of astronomy usually falls in areas of very difficult measurements of very faint stars and galaxies and fine details in the structure of distant objects.

"To study faint stars and galaxies, large telescopes are required for their light-gathering power. Hence, the great excitement about inventions (mirror mosaics, thin mirrors, spin-cast mirrors), which can produce telescope apertures of eight to 10 meters.

"Large telescopes are also valuable for their ability to measure fine detail. For example, the 10-meter Keck I telescope has recently resolved stars separated by only 0.05 arc second (about 100 times better than possible with the human eye) and further improvements of two to five times are expected. Yet for many purposes, this resolution is still not sufficient. A resolution 100 times better is required, for example, to resolve spots on a typical solar-type star.

"Nobody envisions scaling up conventional telescopes to 100- or 1,000-meter diameter. Fortunately, it isn't necessary. An array of telescopes can be operated synchronously (as an interferometric array) so as to achieve the resolving power of a single telescope having a diameter equal to the largest spacing between the individual telescopes.

"Now how does this work? We feel that we can intuitively understand the formation of an image by a lens (or mirror) because we are accustomed to handling themin binoculars, magnifiers and so on. Also, on closer look, the rays of light (for example, in a science hall museum exhibit of a telescope or a textbook ray trace) are seen to propagate in a simple fashion from the source to the image. Still, the simplicity of geometric optics hides the complexity of electromagnetic-wave propagation. Examined in detail, image formation is a subtle process involving the interference of light waves that propagate by different paths through space and through the telescope(s). Understanding this process, we can carry out image formation by a combination of light collection (multiple telescopes), interferometry (bringing the signals together) and analysis (from multiple measurements, reconstructing by computer the image that would have been formed with a single ultralarge telescope). This method is difficult and has severe limitations, but it allows us to achieve 'aperture synthesis' and greatly exceed the resolution of any telescope.

"There is an important 'gotcha,' however. An array of telescopes must be combined with the same severe tolerances that apply to manufacture of a single telescope mirror--that is, to about 1/10 wavelength of light. This is not so difficult in the radio wavelengths (tolerances of one millimeter to one centimeter or so), and arrays of radio telescopes have been providing high-quality radio images for decades. Through most of the history of astronomy, this has not been possible at visible wavelengths. Michelson's classic demonstration of stellar interferometry early in the century was facilitated by the clever use of a single telescope with multiple apertures--attempts to generalize to larger, separate apertures, failed. The technique lay dormant until it was reinvented by the French astronomer Antoine Labeyrie in the 1970s.

"It is now possible (even relatively easy) to build arrays of optical telescopes to required optical tolerances (the well-known 1/1,000 of a hair). More than a dozen optical arrays have been built. Most were prototypes that served their purpose and were phased out. Permanent optical/infrared arrays are now operating in the U.S. on Mount Hopkins and Anderson Mesa (both in Arizona) and on Mount Palomar and Mount Wilson (both in California), as well as in Australia and France. Major new projects (with five or more telescopes each) are under construction on Mount Wilson and in Chile, and a major array is planned for Mauna Kea (in Hawaii). Astronomers hope these facilities will generate the technical demonstrations and scientific momentum required to bring interferometric arrays into the mainstream of astronomical research.

"The array projects under way and planned will make contributions in such areas as the measurement of stars (their sizes, shapes, spots, atmospheres, shells), of binary stars (orbits), detection of exoplanetary systems (by the cyclical motion of the central star caused by orbiting planets) and of many other primarily bright star observations. Dramatic advances in stellar astrophysics should rapidly follow as we obtain the first detailed views of stars other than the sun.

"Astronomers are also eager to study sources that are small, faint and complex. Faint sources still require large telescopes, and complex sources require an array with many telescopes, so the success of optical interferometry with a few small telescopes will naturally lead to the planning of an array of many large telescopes--a facility consisting of 20 to 30 telescopes, each of three- to four-meter aperture, has been suggested as a likely concept for the early 21st century.

"Meanwhile the advantages of interferometry are not limited to ground-based astronomy. NASA is intensively planning a Space Interferometry Mission (SIM), which will directly measure the distances to stars on the other side of our galaxy and the orbits of stars in nearby galaxies. The scientific return in understanding of galaxies and their evolution will be immeasurable. SIM could fly within five years. The Terrestrial Planet Finder, employing array interferometry, could detect terrestrial planets and scrutinize their atmospheres (spectroscopically) for trace gases indicative of life. The TPF could launch within 10 years. (And enthusiasm for these opportunities is worldwide: the European Space Agency has parallel studies of similar or analogous missions.)"

Francesco Paresce is project scientist for the European Southern Observatory's Very Large Telescope Interferometer. He responds:

"An optical interferometer is a device that allows astronomers to achieve the highest possible angular resolution with conventional telescopes. There are two essential attributes of an astronomical telescope: sensitivity and angular resolution. The first allows a telescope to detect but not necessarily to resolve the faintest possible objects in the sky and, therefore to look back the furthest in time. This characteristic is proportional to the collecting area of the telescope or to the square of its diameter. It is usually measured by the limiting magnitude of an object that can be clearly detected above the background; it is currently hovering around the 28th or 29th visual magnitude for the most powerful existing telescopes. Thus, the bigger the diameter, the better, because the telescope will be able to collect more photons from the source. This is analogous to the water-collecting power of a reservoir: the bigger its area, the more rain water it is able to collect.

"Angular resolution, on the other hand, is different in that it is the ability to distinguish accurately or separate two sources that are very close together on the sky, either because they are physically very close to each other or because they are very far away from the observer. It is measured by astronomers in seconds of arc, or arcseconds for short. A resolution of one arcsecond corresponds roughly to the ability to distinguish from the earth a person standing on the moon. This characteristic is, of course, of great importance in a very wide range of applications, from the determination of diameters of stars, the direct detection of extrasolar planets and the unraveling of the structure of the nucleus of an active galaxy. Like sensitivity, resolution depends (in principle) on the diameter of the telescope. So again, the bigger the better!

"But resolution has a strategic advantage over sensitivity. And this is where interferometry comes in. We don't actually need to make one huge telescope of enormous diameter to get the highest possible resolution. (Sensitivity does require such behemoths.) We can achieve the same effect by placing two or more small telescopes at large distances from each other and appropriately combining their output beams together in one common spot. What counts in this particular application is the distance between separate collecting elements, and these do not have to be physically connected. This is, in essence, what interferometry is all about, and it is called optical interferometry if the light being combined is in the visible range of the electromagnetic spectrum.

"The real trick, of course is to combine the beams in phase with each other after they have traversed exactly the same optical path from the source through each telescope and down to the beam combination point. This has to be done to an accuracy of a few tenths of the wavelength, which in the case of visible light is of order one thousandth of a millimeter. For visible light, telescopes separated by several hundred meters must have precisions maintained to within several parts in a billion! That's why we're still struggling to make this work. But enormous progress has been made recently so in a few years this technique should be routine. Then we will have effective angular resolutions approaching one milliarcsecond at one micron, which could only be attained by single telescopes of 250-meter diameter! (The biggest one currently, the Keck, has a diameter of only 10 meters.) This is likely to remain the highest angular resolution achieved by humankind for quite a while or at least until we can figure out how to do this from space with individual telescope separations of hundreds of kilometers.