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
4 Design and Architecture Trade Studies
There are many different ways in which a mid-IR nulling interferometer can be implemented, each with
strengths and weaknesses. This chapter describes the main issues involved and the trade study that was
conducted to select the best configuration. The first section describes the principle of operation of a
nulling interferometer observing planets around nearby stars. Section 4.2 introduces the performance
models that are used to assess the capability of each design. The configurations considered are outlined in
Section 4.3. Subsequent sections address key elements of the trade: aperture diameter and array size
(Section 4.4), co-planar vs. non-co-planar arrays (Section 4.5), beam combiner design (Section 4.6), the
impact of stray light (Section 4.7), the nature and mitigation of instability noise (Section 4.8), and the
imaging properties of these phased arrays (Section 4.9). These elements are all combined in the trade
study that was conducted in 2004 to select the best architecture as summarized in Section 4.10.
4.1 Principle of Operation
In this section we describe how planet photons can be isolated from their parent star using a nulling
interferometer.
Figure 4-1 shows the simplest nulling interferometer – the single Bracewell configuration – proposed by
Bracewell in 1979. This comprises two collecting apertures separated by baseline length B, phased such
that the light from an on-axis source is canceled in the single-mode spatial filter at the beam combiner
output. This is the nulled or dark output port; all the on-axis photons exit from the bright port to the left of
the figure. To implement this scheme requires that a phase difference of π, independent of wavelength,
be introduced between the two arms. The corresponding response of the interferometer on the sky is
shown in both upper panels. It is a sinusoidal corrugation with a null running through the star at the
center, and an angular periodicity of λ / B. If the array is rotated about the line of sight to the star, then
this corrugated pattern rotates with respect to the star and the offset planet. While the star remains on the
null, the planet follows the circular locus and the detected planet photon rate (lower right) rises and falls
as the peaks and troughs of the response sweep through the location of the planet. The main disadvantage
of the single Bracewell configuration is that the response on the sky is symmetric. As a result there is
ambiguity in the location of the planet, the exozodiacal dust emission can have a similar signature to the
planet, and (most important) it is not possible to implement an effective chopping scheme.
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