TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 1
angled Three Telescope Nuller with a dedicated beam combiner spacecraft to alleviate the complexity of
the beam relay, and sufficient asymmetry to improve the imaging properties.
The focus at NASA was on improving the imaging properties of the array, which is important for
separating the contributions from multiple planets, determining the orbit, and discriminating against
lumps in the exozodiacal emission. This led to a rearrangement of the collectors in the Linear DCB to
produce the X-Array – a configuration in which the nulling baselines lie along the short side of the
rectangle and the imaging baselines (which determine the angular resolution) along the long dimension.
The beams are relayed in a single hop from each collector to a central combiner. The decoupling of the
nulling and imaging baselines makes the X-Array more flexible than other configurations. This flexibility
was subsequently exploited to eliminate ‘instability noise’ with the ‘Stretched X-Array’ design. Instability
noise—an analog of speckle noise in the coronagraph—arises from fluctuations in the path lengths,
pointing, dispersion, etc. of the instrument, and drives the requirement on the null depth down to 10 -6 . The
long imaging baselines of the Stretched X-Array give the planet signals a unique spectral signature that
can be effectively separated from the instability noise, and they also greatly improve the angular
resolution of the instrument.
The configurations above are defined by the relative location of the collectors. Until recently, the
combiner spacecraft was always located in the same plane as the collectors, normal to the direction to the
target star. ESA then proposed the ‘Emma’ architecture, in which the combiner is moved out towards the
star by about 1 km, and the collectors are reduced to a single spherical mirror. Most of the nulling
configurations already described can be implemented in either the classic planar format or the out-ofplane
Emma format. The Emma design offers significant advantages which are presently being studied
independently by ESA and NASA. Preliminary results of these studies were first reported very recently,
only in the later half of 2006, and are therefore not within the scope of this document. The appeal of the
Emma design is primarily in its simplification of the telescope optics, eliminating the need for any
deployables, and also in the design of the sunshields, which become folded into a hard shell—thereby
reducing the risk of catastrophic failure. However, the simplification of the telescope optics increases the
complexity of the beam combiner, and it currently is not known to what extent this will reduce the overall
cost and risk of the mission. What is clear is that there exists obvious agreement in design principles
between researchers at NASA and ESA, and the architectures for both TPF-I and Darwin appear to be
converging in 2007.
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