The NASA Terrestrial Planet Finder Interferometer (TPF-I) and ESA Darwin missions are designed to directly detect
mid-infrared photons from earth-like planets around nearby stars. Until recently, the baseline TPF-I design was the
planar stretched X-Array, in which the four collectors spacecraft lie on the corners of a rectangle with the combiner
spacecraft at the center, all in the plane normal to the direction to the target star. The stretched X-Array has two major
advantages over other configurations: the angular resolution is very high, and the ability to eliminate instability noise. A
direct consequence of the latter is that the null depth requirement is relaxed from 10-6 to 10-5. Implementation of the
planar configuration requires a significant number of deployments, however, including large sunshades and secondary
mirror supports. ESA had been pursuing a non-planar configuration with 3 collector telescopes. Dubbed the 'Emma'
architecture (after the wife of Charles Darwin), this approach brings the combiner spacecraft up out of the plane of the
collectors, and offers significant simplifications in the collector design with minimal deployments. The Emma X-Array
combines the best aspects of each design, bringing together the 4-collector stretched X-Array collector configuration
with the out-of-plane combiner of the Emma geometry. Both the TPF-I and Darwin missions have now adopted the
Emma X-Array as the baseline design, moving a step closer to a single, joint TPF/Darwin mission.
In this paper we assess the planet-finding performance of the Emma X-Array. An optimized completeness algorithm is
used to estimate the number of Earths that can be found as a function of collector diameter. Other key parameters − the
inner and outer working angles and the angular resolution − are also addressed.