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
18
16
14
Response
12
10
8
6
4
2
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/2 +π +3π/2
0.70
1 2 3 4
Beamtrain
optics
Planet photon rate / s -1
0.60
0.50
0.40
0.30
0.20
0.10
Single-mode
-0.10
spatial filter
0.00
0 1 2 3 4 5 6
Array rotation angle / radians
Figure 4-2. Dual Bracewell configuration. Lower left – schematic of interferometer; upper left –
section through response on the sky; upper right – response on sky showing star at central null
and planet offset. The Planet follows the red locus as the array is rotated about the line of sight to
the star; the corresponding photon rate vs. rotation angle is shown at lower right.
By negating the relative phases of the collectors, we obtain the mirror image response on the sky, as
shown by the dashed line in Figure 4-3. This is the ‘right’ chop state. By switching the phasing of the
instrument back and forth between these two states, the response on the sky is chopped from left to right
and back. Taking the difference of the photon rates obtained gives the ‘chopped’ response denoted by the
heavy line in Figure 4-3 (upper left panel) and the 3D view shown in the upper right. The chopped
response is purely asymmetric, and the chopped photon rate has both positive and negative excursions. It
is now possible to distinguish the side of the star on which the planet is located, and to discriminate
against any symmetric sources of emission (e.g., star, exozodiacal dust). Any source of noise (e.g., stray
light) that contributes equally to the left and right chop states is also removed.
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