The measurement of exozodiacal dust is challenging. The features of the problem that drive the measurement technique are the strong light from the central star and the relatively weak exozodiacal signal. However, as the disk is best detected in the mid-IR, the measurement technique needs to accommodate the strong thermal background. The planned approach uses interferometric nulling to cancel light from the central star to reduce the required dynamic range, and fast modulation to accommodate the mid-IR background.

The scales of the problem match those of the instrument: at a distance of 10 pc, the diameter of the central star is 1 mas, while the diameter of a (1 AU radius) dust disk is approximately 200 mas; thus the light from the target will be collected in the Lambda/D = 200 mas beam of a single Keck, while the interferometer resolution Lambda/B at 10 µm of 25 mas allows exploitation of interferometric methods. The measurement technique is shown schematically in the figure at the right. Each Keck aperture is split into two halves, each of which is sent separately to the beam combining lab. Interferometer nullers cancel most of the light from the central star on two 85-m baselines. The outputs of these nullers are then combined in a conventional beam combiner which uses fast fringe scanning to demodulate the exozodiacal signature in the presence of the 10 µm background.