TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
4.8.2 Stretched X-Array and Spectral Filtering
In the stretched X-array and spectral filtering approach, we use the spectral dependence as the means to
distinguish between the planet signal and the instability noise. To make this effective, the array must be
stretched significantly with respect to previous designs. The X-array is the natural choice, since it can be
stretched along its long dimension while preserving the short nulling baselines needed to minimize the
stellar leakage. In principle, all instability noise can be eliminated. The nulling requirement can be relaxed
from 10 -6 to 10 -5 , while at the same time the sensitivity is improved and the angular resolution of the array
is significantly increased. The technique is described below. A more complete analysis can be found in
Lay (2006).
Figure 4-18a shows the stretched X-array geometry, consisting of four collectors located on the corners of
a rectangle and a central combiner, observing a star normal to the plane of the page. The aspect ratio has
been stretched to 6:1, with 35-m nulling baselines. The corresponding instrument response at a
wavelength of 10 μm, projected onto the plane of the sky, is shown in Fig. 4-18b. This is the “chopped”
response – the phasing of the array is switched rapidly between two states, and the difference is taken.
The star is located in the middle of the central, vertical null stripe (mid-grey); on either side the response
Figure 4-18. (a) Stretched X-array configuration. The aspect ratio is 6:1 in this example. (b) Chopped
response of the stretched X-array configuration on the sky for a wavelength of 10 μm. The target star is
located on a null (mid-grey); white and black represent positive and negative regions of the response. The
planet has an offset of 250 nrad, or ~ 50 mas. (c) Chopped planet photon rate as the array is rotated. Peaks
and valleys correspond to the white and black regions encountered along the circular locus shown in (b).
(d) Chopped planet photon rate (grey-scale) as a function of optical frequency (left axis) or wavelength
(right axis) and array rotation azimuth. Similar wavelength-azimuth plots are shown for (e) photon noise
and (f) instability noise.
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