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An Earth-like planet around a Sun-like star would appear about 10,000,000 times fainter than the star. To reach an Earth-like planet, a series of steps must be taken:
- The star's apparent intensity must be reduced relative to the planet by a factor of 100,000 through interferometric nulling.
- The interferometer array must be rotated around the line of sight to the star to search the whole region around the star for a characteristic planet signature. During this rotation, stable nulls need to be maintained.
- The planet signal is modulated against the bright background of zodiacal and exozodiacal light. This is done using phase chopping. The combination of rotation, phase chopping, and averaging over time reduces the noise level by a factor of 100 to 1/10,000,000th of the stellar intensity.
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- The technique of spectral fitting uses correlations between null fluctuations across the spectral band to reduce the instability noise. This yields a further factor of 10 in reduction of the noise level.
Thus, the combination of these four techniques yields the necessary performance.
Broadband Starlight Suppression
Results obtained in the lab have met the Pre-Phase A requirements for a flagship mission. Laboratory work with the Adaptive Nuller has demonstrated mid-infrared rejection ratios (inverse of the null) of 100,000:1 with a bandwidth of 34% and a mean wavelength of 10 microns; this demonstrates that at the subsystem-level the nulling performance for a flagship mission is near TRL 4 (cryogenic testing would be needed for TRL 5). These results also show that the achromatic phase shifters, the Adaptive Nuller, and the mid-infrared single-mode fibers that are contributing to these results are now mature technology. These results are shown in Figure 1. Others results of note are included in Figure 2, which illustrates the experiments undertaken since 1999.
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Figure 2. Chart of null depths achieved since 1999. The blue shaded area highlights the nulling performance needed to be demonstrated in the lab in preparation for a flagship mission. (P. R. Lawson, JPL) ( Download EPS file) |
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These results were achieved by Adaptive Nulling and was the goal of the project's Milestone #1 and Milestone #3. No further room-temperature demonstrations are necessary in broadband nulling. The greatest advance would now be to repeat this demonstration in a flight-like environment.
Planet Detection with Chopping and Averaging
Further noise suppression is required in addition to starlight suppression. Nulling can reduce the glare of starlight to a level fainter than the warm glow of local zodical dust (surrounding our Sun) and exo-zodiacal dust (around the target star), but the planet itself may still be 100 times fainter.
The first step is to suppress the response to any thermal emission that is symmetrically distributed around the star. In principle this will remove the detected glow of local and exozodiacal dust. By rotating the array and averaging the response, the planet signal can be further enhanced. The beam combiner for TPF/Darwin therefore combines two pairs of beams to null the starlight, and the beam combining system modulates the response on the sky (keeping the star nulled) by chopping back and forth between these nulled pairs. This milestone (Milestone #4) has been detailed by Martin et al. (2008).
Laboratory work is well advanced to demonstrate the detection of planet light with the Planet Detection Testbed. Simulated planets two million times fainter than a star have been detected in early trials. Although these tests represent the full complexity of beam combination, this research is being conducted at room-temperature in air, and the noise properties of the testbed are therefore unlike those that would be met in flight. We expect this milestone will be completed by the Planet Detection Testbed in 2009.
Systematic Noise Suppression
An additional step is required to suppress the noise down below the typical brightness of an Earth-like planet. After nulling and chopping, the dominant source of noise is due to residual instabilities in the null depth, which generates a 1/f-noise of similar intensity to the planet signal. This noise can be suppressed by appropriate choice of interferometer baselines and by filtering the measured data from a complete rotation of the array. We expect this milestone (Milestone #5) will be completed in 2009-2010.
Additional technology progress beyond this milestone would depend on cryogenic testing of components and systems.
Cryogenic System Testing with a Flight-like Interferometer
The final demonstration of the feasibility of nulling for a flagship mission would be to integrate all the necessary components in a vacuum cryogenic testbed. This would demonstrate the full system complexity and include flight-like servo systems and brass-boards. The path towards that goal will entail future cryogenic testing of components and subsystem.
Mid-IR Spatial Filters
It would be greatly advantageous to improve the throughput of the mid-IR spatial filters that have been tested to date, to test them throughout the full wavelength range they are intended for, and to test them cryogenically. Spatial filter technology would then be at TRL 5. It would, furthermore, be advantageous to implement mid-infrared spatial filters and beam combiners using integrated optics, so as to reduce the risk associated with the complexity of the science instruments.
Cryogenic Adaptive Nulling
The Adaptive Nuller was designed to correct phase and intensity variations as a function of wavelength in a nulling interferometer. This has not only allowed nulling with the performance required for flight, and reduced the tolerances on the interferometer optics. Adaptive nulling is straightforward to generalize over a full science band, and it should be demonstrated within a cryogenic vacuum, bringing the technology to TRL 5. This would necessitate the successful validation of cryogenic spatial filters (above) and the testing of a cryogenic deformable mirror.
Updated 04 April 2009
References
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