by Nat White
[cover story, Lowell Observer, Autumn 1999, Issue 44]
Lowell Observatory Dark-Sky Site and NPOI by Jay L. Inge
A new, strange looking telescope is nearing completion at the Lowell Observatory dark-sky observing site. The telescope, called the Navy Prototype Optical Interferometer (NPOI), looks more like a small oil refinery with pipes and tanks spreading over ten acres on the top of Anderson Mesa. However, with vacuums, mirrors, lasers, and computers, this instrument will provide unsurpassed detailed images of bright stars.
A three-way collaboration between the U.S. Naval Observatory, the Naval Research Laboratory, and Lowell Observatory has made the design, funding, and construction of this unique instrument possible. Since the late 70's, the Navy has built three successively larger optical interferometers in their effort to push the capabilities of an earth-based interferometer to its limit. Completion of the telescope on Anderson Mesa, the largest and most versatile of its kind, is expected within the year.
A conventional telescope used a large, precisely curved mirror to capture the beams of starlight and focus them on a single plane, such as a photographic plate or more commonly now, an electronic detector. By "focus" we mean that the curved mirrors reflect and direct the light from each part of the source to a corresponding place in the focal plane. A sharp image will only appear when all the beams forming a given point in the image travel the same distance to within a few millionths of an inch. When that precision is accomplished, an image of the light source, such as a field of stars, appears. If the light paths were gradually made unequal by a mirror with a rough surface, for example, the image would lose sharpness and turn into a smudge of light.
Besides high precision mirrors, astronomers argue for bigger mirrors because they gather more light beams and therefore can image fainter objects or brighter objects faster. However, the diameter of the mirror also determines how much detail can potentially be seen at the focal plane. The larger the mirror diameter the more detail, or angular resolution, is possible. It is this latter fact that drives the effort to build large optical interferometers.
In 1890, A.A. Michelson pointed out mathematically that by collecting light from two separate, small flat mirrors precisely adjusted to bring the reflected beams together, one could measure very small details. In fact, the detail obtained would be equivalent to that obtained by a single, gigantic mirror with a diameter equal to the separation or baseline of the two small mirrors. The instrument was called an interferometer because the image details showed up as interference patterns. It has taken over 100 years for astronomers and engineers to develop the lasers and computers necessary to build and control large optical interferometers to the required high precision of a few millionths of an inch, the same precision required for the mirror surfaces of conventional telescopes.
NPOI has four major components shown in the cover diagram [click graphic above to enlarge]: the array, the vacuum pipes, the optical laboratory, and the control building. The "Y" shaped array marks the location of the small, widely separated flat mirrors that collect the light beams. The size of the array will enable the recording of image detail equivalent to seeing a dime 3,000 miles away. A single conventional telescope mirror would have over 400 meters — or a quarter of a mile — in diameter to record the same detail. A single mirror that large would be impossible to build and move.
The straight lines in the diagram indicated pipes that contain a vacuum so that the captured light beams can be directed to the optical laboratory building without further atmospheric distortion. Critical light paths are equalized and brought to a focus in the optical laboratory. In order to reduce all vibrations, operation of the entire instrument is from the control building.
Although not yet complete, NPOI is already producing results that are attracting attention in scientific journals. By using the completed inner part of the array, scientists have been able to make extraordinary observations. For example, imagine how carefully the brightest stars in the well-known constellation Orion have been studied over the centuries and then imagine the excitement when NPOI determined that one of the very brightest stars in the constellation is actually two very close, very bright stars. No other conventional telescope in the world could have resolved the stars, an observation that will provide information about their masses.
Another set of observations has shown the variation of light across the disc of a star called "limb darkening," an effect of the temperature gradient of the star's atmosphere. The angular diameters of dozens of stars have already been measured. One star revealed evidence of cyclic changes in its diameter; a phenomenon called stellar pulsation — an observation related to dynamics deep below the star's surface. Data of this type confirm astrophysical theories in some cases and raise new questions in others.
Another extraordinary capability will be the 100-fold improvement of earth-based measurements of star positions. All positions on the earth, including those made by the popular Global Positioning System (GPS), trace their accuracy to the position of distant stars. In refining stellar positions for practical reasons, the interferometer will be providing new information on the distance to stars (closer stars move, more distant stars appear stationary). The high precision measurements may even show star wobble, indicating a dark unseen companion — a planet.
The establishment of the interferometer is not only a benefit to the astronomical community but also a benefit to Flagstaff. Some 10 to 15 new scientists, technicians, and student interns have moved to Flagstaff and are working daily to complete this state-of-the-art instrument, with much of the infrastructure built by local firms. The science being done and the potential for extraordinary measurements and discoveries will only bring positive recognition. Stay tuned.