LABORATORY PERFORMANCE OF THE KECK INTERFEROMETER NULLING BEAM COMBINER

摘要

Now that regular visibility squared measurements are routinely achieved, mid-infrared nulling is the next observing mode to be implemented on the Keck Interferometer (KI). This mode's main objective is the characterization of exo-zodiacal dust disks around nearby main sequence stars (Kuchner & Serabyn, 2003) in support of the TPF space mission. Keck Nuller also shares numerous characteristics with an interferometric TPF, and will then serve as a technical precursor for this mission. We report here the results obtained in the laboratory with the KI mid-IR nulling beam combiner, which is based on a dual polarization Modified Mach Zender (MMZ) combiner (Serabyn & Colavita, 2001), and dispersion compensation and achromatic nulling through zinc-selenide (ZnSe) dielectric plates (Koresko et al., 2002). Best broad-band dual polarization transient rejection levels obtained so far on thermal sources range from 7500:1 over a 29% bandwidth (FWHM from 9.20 to 12.35 microns) to 17000:1 with an 18% bandwidth (FWHM from 9.70 to 11.65 microns). These rejections are basically one order of magnitude better than the limit set by the stellar finite angular size, which is about 1000:1 at 10 microns for a 1 mas diameter star observed with KI 80 m baseline. These results then validate the optical concept chosen for the nulling back-end, which now needs to be properly integrated with the mid-IR nulling camera and the KI beam train.