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| Since the invention of the telescope, astronomers have been striving for clearer definition of the objects they observe; this has led to the development of larger and larger telescopes. As we increase the size of a telescope, faint objects become easier to see and the detail seen in the object improves as well. The physical effect that limits the resolution of a telescope is called diffraction with the amount of diffraction decreasing with increasing telescope size and vice versa. To illustrate this behavior let's consider a pair of stars lying close together on the sky. One such pair could be the binary star system Mizar, visible to the naked eye and found in the handle of the Big Dipper. In the figures to the right, we have shown the hierarchy of stars in the Big Dipper to aid the reader in understanding the Mizar system. Mizar lies very close to Alcor, another naked eye star, and both may be seen as distinct stars by the unaided eye. This visual pair of stars is not physically connected by gravity. Mizar itself is a pair of stars, Mizar A and Mizar B, that are separated by about 14 seconds of arc, too close to be seen by the naked eye as a distinct pair. The eye, which has a resolution of about 2 minutes of arc, discerns the Mizar star system as a single star. However, with modest optical aids (binoculars, for example) it is resolved into a close pair. |
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We have illustrated this difference in the computer simulation, seen in the figures to the left. In the upper panel, the pair of stars as seen with the naked eye appears indistinct; in the lower panel it is clearly two stars when seen through binoculars. As we noted above, Mizar A, the brighter of the two stars, is itself a binary that is so close that giant telescopes are required to resolve them. Observations of binary stars represent the only direct means to measure the masses of stars. In the Interesting scientific results page we show the contribution of the Navy Prototype Optical Interferometer (NPOI) to understanding this system. |
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An important by-product of increased resolution is
the ability to point the interferometer very accurately to the position of a
star. If we can accurately point from one star to another, we can determine the
relative positions of the stars. This is the principal of astrometry, the
measurement of star positions. Surveying the interferometer position relative
to bedrock is a key function of the laser metrology system at the NPOI. This
metrology system allows us to determine the accurate angular differences between
the stars, thereby measuring their positions on the sky.
Measuring accurate star positions is one of the historical mandates of the Navy and was a strong motivation to finance the development of the NPOI. Accurate star positions are useful in traditional forms of navigation (before GPS). When the interferometer is fully functioning as a precision astrometric instrument we expect to be able to measure star positions from the ground with an accuracy of about 1/1000 of a second of arc. These measurements will provide an important demonstration for space-based interferometers that may increase that accuracy manyfold. These high precision measures will also yield interesting scientific results. Following the work of the Hipparcos satellite, we should be able to correct measures of the motions of stars in the galaxy that will help to establish distance scales and to understand the structure of our galaxy. |
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Another interesting scientific problem for today's astronomer is the direct observation of surface features on stars. Now, we only have high resolution photographs of the Sun in which we may see spots, prominences, flares and other structure that reveal activity on and below the surface. We do not yet have comparable photographs of any other stars than our Sun. If we tried to observe alpha Centauri, our nearest solar-like star at a distance of 1.3 parsecs, its disk would have a size of 7 milliseconds of arc, almost 270,000 times smaller than the apparent size of the Sun! As our astronomer friend to the left notes a very big telescope is necessary to resolve the star's disk, let alone observe star spots on its surface. In fact, to see the surface of alpha Centauri in visible light we would need a telescope with a mirror diameter of 14 meters, larger than the Keck telescopes in Hawaii. To resolve spots would require a telescope at least 100 times larger than that. Such a large telescope is well beyond our present day technology, if we try to construct one using a single mirror. |
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| However, it is within our present capability if we use interferometric techniques. Invented by Albert Michelson in the 19 th century, the interferometer makes use of separate telescopes that are widely spaced rather than on a single large mirror. While the interferometer does not capture as much light as a single mirror of the same diameter it can achieve the resolution. Interferometers first became practical in the mid-1970s and are now under development in several parts of the world. Most visitors to an optical interferometer will not recognize the array of optical elements as a normal telescope, as our visitors on the right note. |
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One example of an interferometer is the NPOI on
Anderson Mesa outside Flagstaff, Arizona. More details of this interferometer
may be found on the other pages of this web site. Descriptions of other operating ground-based
optical and infrared interferometers may be found at the following links:
CHARA
-- The Georgia State University project
Also, please visit OLBIN , Peter Lawson's extensive interferometry page at the Jet Propulsion Laboratory for more general information about interferometry and interferometers. |