TPF-I SWG Report - Exoplanet Exploration Program - NASA

D ESIGN AND A R C H I T E C T U R E T RADE S TUDIES
of constant integration time; targets to the left are limited by local zodiacal emission, while those to the
right are limited by stellar leakage.
This technique can also be extended to predict the number of targets that can be spectroscopically
characterized in a given amount of time. This depends on the prevalence of Earth-like planets in the
Habitable Zone. If planets are rare then the average distance to the systems being characterized is high,
the integration times are long, and only a few systems can be accommodated in the time available. If
planets are very common, however, then we will be able to characterize a much larger number of nearby
systems. The simple algorithm to account for this is described in Dubovitsky and Lay (2004).
A more sophisticated model to predict and optimize the program completeness is now underway. Based
on similar analysis for the TPF-C mission, the algorithm assesses the observability for each of 1000
planets around each star. For each week of the mission, only the most productive stars are selected in
order to maximize the number of planets found.
4.3 Nulling Configurations
We define an architecture by the combination of nulling configuration, collector aperture diameter, beam
routing between spacecraft, beam combiner design, number of launches, and type of launch vehicle. The
nulling configuration includes the number and relative locations of the collectors, and the amplitudes and
phases with which each collector beam is combined. All are significantly constrained in geometry by the
need for equal optical path lengths from each collector to the combiner.
The six basic architectures compared in this study are listed in Figure 4-8, ranging from three to five
spacecraft. The first four are all part of the Dual Chopped Bracewell (DCB) family, in which the four
apertures have phases of 0, π/2, π, and 3π/2 radians. The Linear DCB (Beichman et al. 1999) can be
phased in two ways, with either separated or interleaved nulling baselines. In the analysis we choose the
optimal case for each observing scenario. The X-array (Lay and Dubovitsky 2004) chosen for study has a
fixed 2:1 aspect ratio (a tunable aspect ratio is discussed in Section 7). The Diamond DCB and Z-Array
were both proposed by Anders Karlsson (ESA) as a means of reducing the number of spacecraft; the
hatched circle in the schematic indicates a spacecraft that functions as both a collector and combiner. The
Z-Array uses multiple relays between the collectors to balance the path lengths. The four DCB
architectures have identical beam combiners. The Triangle and Linear 3 are based on a three-way nulling
strategy with phases of 0, 2π/3, and 4π/3 (Karlsson et al. 2004). The other three-spacecraft designs have
been proposed by Serabyn and Mennesson (2004). In all cases, the spacecraft are confined to a plane
perpendicular to the target star direction for thermal reasons. The beams must be routed such that the path
lengths from the star to the combiner are equal; this is achieved with a single hop from the collector to the
combiner in the X-array, two hops for the Linear DCB, Diamond DCB, Triangle and Linear 3, and as
many as four hops for the Z-Array.
Many nulling configurations were not considered in the trade study. The single Bracewell nuller is
attractive for its simplicity (only two collectors), but it cannot accommodate a phase chop, making it
extremely vulnerable to slow systematic effects (a variation of 1 photon per second of scattered light can
look like a planet, for example). There are also a number of configurations with higher-order nulls, in
which the central null of the response on the sky has a broader null than the quadratic profiles seen in
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