TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 5
a)
c)
b)
d)
IWA = 43 mas OWA = 280 mas
IWA = 9.6 mas OWA = 280 mas
Figure 5-18. Response to planet emission at 10 μm for the stretched X-array. (a) Sky response for the
minimum array size of 120 × 20 m at λ = 10 μm. (b) Section through the sky response for minimum
array size. The IWA is 43 mas. The OWA is 280 mas. (c) Sky response for maximum array size (612
× 102 m). (d) Section showing IWA of 9.6 mas.
At the center of each response is the dark hole that represents the effect of the central null. The Inner
Working Angle is defined to be the offset angle from the star at which the signal response (blue solid line)
first crosses the asymptotic response (green dashed line), for a wavelength of 10 μm. This IWA scales
approximately as B null -1 . At the minimum array size of 120 × 20 m, the IWA = 43 mas; at the maximum
array size of 612 × 102 m, the IWA = 9.6 mas.
Figure 5-18 shows the response to planet signal photons; this is not the same as the sensitivity to planet
photons which depends on the signal-to-noise ratio. The noise sources described in Section 4.2 result in a
statistically uniform distribution in the rms noise over the map. The noise level integrates down with time,
so that the effective contrast that can be achieved scales with T int 1/2 . This is different from the
coronagraph, which has a fixed contrast floor introduced by the instrument that ultimately limits the
performance. For the interferometer, the powerful combination of fast phase chopping and spectral-fitting
(described in Section 4.8) should ensure that any residual noise will be random in nature.
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