TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
4.5
4
3.5
Response
3
2.5
2
1.5
1
0.5
0
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
θ x / μrad
0 π
0.70
1 2
Beamtrain
optics
Single-mode
spatial filter
Planet photon rate / s -1
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0 1 2 3 4 5 6
-0.10
Array rotation angle / radians
Figure 4-1. Single Bracewell configuration. The schematic of the interferometer is shown, lower
left. A cross-section through the fringe response on the sky is shown, upper left. The fringe
pattern is rotated around the star (hidden behind a central null) to detect a planet. The Planet
follows the red locus as the array is rotated about line of sight to star, upper right; the
corresponding photon rate vs. rotation angle is shown at lower right.
These disadvantages are overcome with the Dual Bracewell configuration, an example of which is
illustrated in Figure 4-2. There are now four collecting apertures. In this case, they are deployed along a
line with equal spacing, phased as indicated. This configuration is essentially two single Bracewell
baselines, which are then cross-combined with a third beam combiner with a relative phase shift of π / 2.
The resulting response on the sky of this four-element phased array is shown in the top panel. The
structure is more complex than before, and there is a clear left–right asymmetry. We will refer to this as
the ‘left’ chop state, since there is a large peak in the response immediately to the left of the star.
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