Progress in four-beam nulling: results from the terrestrial planet finder planet detection testbed

The terrestrial planet finder interferometer (TPF-I) is a large space telescope consisting of four 4 meter diameter telescopes flying in formation in space together with a fifth beam combiner spacecraft. The planet detection testbed has been developed to study the combination of the four beams of light from the telescopes in a twin nulling beam combiner which emulates the functionality of the TPF-I beam combiner. Recent results from the testbed include nulling of 4 input beams simultaneously at null depths of 250,000 to one and planet detection at a contrast ratio of one 2 millionth of the intensity of the star. For successful detection of exoplanets, the space interferometer will need to reach null depths of one million to one and demonstrate high stability during long observing periods. Testbed program goals include the same one million to one null depths and stability of the null comparable to the levels required in space (2 times 10^-7)