SIM PlanetQuest and the Terrestrial Planet Finder - Interferometer (TPF-I) use measurements made with an instrument called an interferometer to learn about planets, stars, and the motions of stars in galaxies. An interferometer makes its measurements by collecting light from two or more widely separated telescopes and combining their light.
Unlike a single telescope, an interferometer can take advantage of the wave nature of light to find the locations of stars very, very accurately. Adjustments are made to find the point where the interferometer's mirrors combine the light exactly correctly to make a large, bright signal, something like this, called constructive interference:
When the light is combined improperly, the result is destructive interference and there is darkness:
In the interferometer, the reality is more complex and measurements are based on a brightness signal that looks like this, with the group of high peaks and valleys called fringes:
Fringes, made by a related process, can be observed at home using a laser and a slit that is easy to construct. Fringes much like those that will be measured by SIM PlanetQuest and by TPF-I can be seen in a telescope by constructing a simple, two-holed mask.
SIM PlanetQuest will make very precise measurements of the positions of thousands of stars, based on the highest peak in the fringe group of each star. Over time, changes in the apparent position of a star can be used to determine if planets are orbiting it, how massive there are, and how far they orbit from their parent star. Similar measurements can be used to study stars in pairs and motions of stars in star clusters and in the Milky Way and other galaxies.
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