Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT
Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT
Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT
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unfettered by the Earth’s atmosphere [8].<br />
1.1.2 Planet Detection<br />
The second main goal of the Origins program is to search for life in other solar<br />
systems. This aim requires the detection and study of extra-solar, Earth-like planets.<br />
Currently, planet detection is done by ground-based telescopes that measure the<br />
motion of a star due to an orbiting planet. This technique has been successful in<br />
locating a number of Jupiter-sized planets, but is not sensitive enough to find smaller<br />
planets <strong>with</strong> mass comparable to that of the Earth. Therefore, the concept of a<br />
single instrument that detects Earth-like planets through their radiated heat and<br />
then performs a spectroscopic analysis to find signatures of carbon dioxide, water<br />
and ozone has been proposed [6].<br />
There are two main challenges associated <strong>with</strong> this type of detection scheme. First,<br />
it is necessary to distinguish the radiated heat from that of the star. If the observing<br />
instrument is located on Earth, the heat from the planet is entirely swamped by the<br />
radiation from the Earth’s atmosphere and the ambient temperature of the optical<br />
elements. However, even if the instrument is placed in space, removing these sources<br />
of noise, the contrast ratio of the star to the planet is on the order of 10 7 . It is<br />
necessary, then, to find a way to separate the star radiation from that of the planet.<br />
A second challenge arises due to the small angular separation between the planet and<br />
the star. As discussed above, a high-angular resolution instrument is necessary to<br />
resolve the two sources.<br />
A nulling interferometer operating in the infrared (IR) has been proposed as<br />
the solution to the planet detection problem. The concept was first introduced by<br />
Bracewell [20]. He and his colleagues suggested using an interferometer to destruc-<br />
tively interfere star radiation captured by two apertures separated by a baseline [21].<br />
The destructive interference results in the cancellation of the stellar flux over a broad<br />
waveband. If the instrument is designed correctly, the planet emission, which is<br />
slightly off-axis from the star, constructively interferes and is reinforced allowing the<br />
detection of the planet despite the stronger stellar radiation. Designs for nulling IR<br />
20