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This work is funded by ESA under ESTEC/Contract No. 14827/00/NL/CK. Astrium Germany has been awarded this first ESA breadboarding towards nulling interferometry. Interferometric nulling devices are essential ingredients in the TPF and DARWIN missions for suppressing the star light by a factor of 106 over a wide wavelength range in the mid infrared. The current DARWIN baseline concept comprises six telescopes. The coherent combination scheme in the nulling mode operation foresees three nulling assemblies in parallel. The breadboard serves to demonstrate the deep and stable null required for an operational instrument. The demonstrator operates in the near infrared to save costs but its principle is fully applicable to the mid infrared. The nulling device is based on an autobalancing Sagnac core offering just one critical beam combiner. Two different ways of achieving the required π phase shift are implemented:
a) arbitrary phase shift by dispersive phase shifter plates
b) phase shift of π using periscopes (image flip)
The target simulator features two point sources of adjustable radiometry and angular separation, representing a strong star and a weak planet. In addition, the sources can be also used to simulate a double star for demonstrating the basic DARWIN imaging mode. The simulator can be operated in two styles, namely as wavefront dividing star/planet source and, alternatively, as an amplitude dividing source, providing highly symmetric wavefronts to both interferometer arms. Because of its representativity for the DARWIN situation, the latter mode is the preferred simulator for quantitative nulling experiments. The breadboard design has been finalized in January 2002 and verified by detailed simulations. The entire hardware has been manufactured by end of July. Currently, nulling and imaging measurements are in progress to validate the per-formance of the selected approach. The project is part of ESA´s technology preparatory program for DARWIN, paving the way for a collaborative ESA/ESO guest instrument at ESO's VLTI with scientific implications.
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