Optic Interferometry Essay, Research Paper

Optic Interferometry The telescope is the astronomer’s key to the gateway of cosmic information. One of the most important aspects of a telescope is its resolution. That is, the degree to which fine details in an image are separated, or resolved. Resolution used to be limited by the size of the mirror or lens used in the telescope, but now because of advanced technology and computer assistance astronomers have developed a technique call interferometry. By combining light from telescopes tens or hundreds of meters apart, an array of telescopes can mimic a single telescope that has a gigantic mirror. The actual size of this virtual mirror is equal to the distance between the telescopes. This distance is called the baseline. Instead of the image a usual telescope would collect, this array of telescopes, called an interferometer, collects light and dark bands, that are created when beams from the separate telescopes merge. When the light source is point-like, a small or distant star, the beams meet “coherently,” to produce crisp, clear bands. When the source is not a point, as in binary or giant stars, the coherence decreases by an amount that depends on the object s apparent size along the baseline. As the Earth rotates the baseline is readjusted, by observing the source repeatedly during this time, an interferometer can obtain the full dimensions of the source. One problem with telescopes is atmospheric turbulence. This turbulence degrades the coherence of the beams by alternating the wave fronts of the light before it enters the telescopes. Correcting for the turbulence requires readjusting the lengths of the light s path within the device every few milliseconds. The accuracy of correction must be within a fraction of a wavelength. Radio astronomers succeeded in this decades ago because the radio waves have much longer wavelengths. Doing so with light waves looked impossible, until recently. “Ten years ago, people said you could never [meet such tolerances],” says Gerard van Belle of Jet Propulsion Laboratory (JPL). “Now it s run-of-the-mill.” Another use of interferometry is its ability to null the image of bright stars so that faint, nearby planets can be seen more clearly. Because a star can be up to ten billion times brighter than its surrounding planets, this technique is instrumental in the study of nearby planets. This technique works by offsetting the light waves of just one telescope by one-half of its wavelength. Therefore the peaks of one telescope are superimposed on the troughs of the other, this wipes out the brightness of the star. Any object that is off center in the image is not completely canceled and still emits a measurable signal. A team in Tucson, Arizona used this technique to cancel out the image of the star a-Orionis, which showed a dust nebula surrounding the star. Normally this nebula would be lost in the glare of the star. Starlight nulling “is the only way we can directly detect planets in the foreseeable future,” says Malcolm Fridlund of the European Space Agency.

One project that will be using these new found techniques in optic interferometry is the Palomar Testbed Interferometer (PTI). Located on Mt. Palomer near San Diego, two 40-centimeter telescopes are set 110 meters apart and collect near-infrared light which is passed onto movable mirrors. These mirrors keep the beams as coherent as possible by rolling back and forth on rails, vibrating like a speaker, and jiggling on specially made mounts, which quickly swell or shrink in response to electric fields. The end result is an effective resolution of 1 milliarcsecond (mas), that is, 1/1000 of a second of arc.A few other projects include the Sydney University Stellar Interferometer, which is almost ready to begin taking data with a 200 meter baseline, and will ultimately have a baseline as long as 640 meters. This will enable it to make measurements as accurately as 0.05 mas. The Navy will also have an optical interferometer, located near Flagstaff, Arizona, with as many as six telescopes spread along intersecting baselines up to 437 meters long. Like the Navy, Cambridge, UK and the Center for High Angular Resolution Astronomy (CHARA) also have multiple baseline interferometers. These interferometers are capable of reconstructing actual images, rather than the limitation of simple features that single baseline interferometers face.PTI is only a testing ground for more aspiring and productive interferometers. There are plans for an interferometer that would link the Keck I and Keck II telescopes on Mauna Kea in Hawaii, with four smaller telescopes. So far, current interferometers can see faint objects because they are built with small telescopes. The two, 10 meter Keck telescopes, separated by 85 meters, will change all that.Looking further into the future, JPL s Michael Shao, leader of the PTI team, is heading up the plans for NASA s Space Interferometry Mission (SIM). The spacecraft of this mission will carry an array of telescopes that will watch stars for “planet-hinting wobbles” as small as 0.001 mas. Bibliography 1. Abell s Exploration of the Universe 7th ed., Saunders College Publishing, 1995. 2. The Concise Columbia Electronic Encyclopedia, 3rd ed., Columbia University Press, 1994. 3. Glanz, James, ASTRONOMY: Intimate Views of the Stars, American Association for the Advancement of Science, 1998. 4. Watson, Andrew, PLANETARY SCIENCE: Interferometry: Getting More for Less, American Association for the Advancement of Science, 1998.