Experimental Progress and Results of a Visible Nulling Coronagraph

The crux of visible exoplanet detection is overcoming significant star-planet contrast ratios on the order of 10^-7 to 10^-10 - at very small angular separations. We are developing a nulling interferometer coronagraph designed to achieve a 10^-6 contrast ratio at a working science bandpass of 20% visible light using a pseudo-achromatic phase flip. Recent results yield contrast ratios at the 10^-6 level with a 15% visible bandpass averaged over three seconds. Further broadening of the bandpass will require hardware modifications, although the argument is made that broadening the bandpass should not adversely affect the null depth until beyond 20% visible light. The current testbed configuration has reached monochromatic null depths of 1.11times10^-7 (lambda = 638 nm) averaged over three seconds. This paper will describe the experimental approach for achieving broadband nulls, as well as error considerations and limitations, and the most recent results of the nulling interferometer testbed.