TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
Table 4-2. Impact of Null Depth on SNR (after instability noise removed)
Null depth @ 10 μm Broadband SNR (relative) Ozone SNR (relative)
10 -6 1.00 1.00
10 -5 0.97 0.92
10 -4 0.80 0.60
Figure 4-19a shows vertical cuts through the wavelength-azimuth plot at an azimuth of 30 degrees, and
depicts both the planet signal and an example of instability noise. The instability noise is a smooth, slowly
varying function within the two halves of the spectrum, whereas the planet signal oscillates with
wavelength. We exploit this difference to remove the instability noise. Removing a low-order polynomial
fit from each half of the spectrum gives the curves shown in Fig. 4-19b. In each case, the instability noise
signature has been almost totally removed, and the signal remains largely intact, although somewhat
modified. The impact of the fitting on the planet signature is predictable and can be corrected. Some of
the planet signal is removed by the fitting process (which impacts the sensitivity), and it is this that
motivates the need for a stretched array. As the array size is reduced, there are fewer oscillations of the
planet signal across the spectrum, and more of the planet signal is removed by the low-order fit. The 6:1
aspect ratio described here is a compromise between the array size and the amount of planet signal that is
lost.
With the effective removal of the instability noise, it is possible to relax the required null depth.
Table 4-2 lists the SNR obtained for both broad-band detection and ozone spectroscopy, relative to the
SNR with a 10 -6 null depth. In the absence of instability noise, the SNR is determined by photon noise,
with principal contributions from stellar size leakage, local zodiacal dust, and stellar-null floor leakage.
Only the stellar-null floor leakage depends on the null depth. Relaxing the null depth to 10 -5 has only a
small impact on the SNR. At a null depth of 10 -4 the stellar-null floor leakage is becoming the dominant
source of photon noise. But even with this relaxation by a factor of 100, the mission is still viable, albeit
with reduced sensitivity.
A significant added benefit of the stretched array is that the angular resolution is improved by a factor of
~3. The benefits of this will be described in Section 4.9.
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