Report - School of Physics
Report - School of Physics
Report - School of Physics
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average out on a long-exposure image. At the wavelengths <strong>of</strong> interest they should<br />
not, however, occur more than 10 milli-arcsec from the image centre, corresponding<br />
to 0.1 AU at a distance <strong>of</strong> 10 pc. Additionally, techniques such as simultaneous<br />
differential imaging may completely cancel these super-speckles. Nevertheless, their<br />
noise contribution remains even after subtraction, and is typically the strongest noise<br />
source in the 5–15 λ/D region. This can only be reduced by improving the Strehl<br />
ratio <strong>of</strong> the adaptive optics system.<br />
Detection <strong>of</strong> a terrestrial-like planet in the presence <strong>of</strong> the stellar glare is made<br />
possible in principle by the relatively large angular separation between the image<br />
<strong>of</strong> a habitable planet and the central diffraction peak <strong>of</strong> its parent star. Thus at<br />
separations <strong>of</strong> 100 milli-arcsec (1 AU at 10 pc) only the sky background, the wider<br />
scattering components <strong>of</strong> the intrinsic point-spread function, and the adaptive optics<br />
halo contribute to the background. The orbital radius at which an exo-planet is in<br />
practice detectable is then bounded by two effects. At the inner extreme, the bright<br />
inner structures <strong>of</strong> the stellar image will drastically reduce sensitivity at angular<br />
separations below 10–20 milli-arcsec, corresponding to an orbital radius <strong>of</strong> 1 AU<br />
at 50 pc or 5 AU at 250 pc, so that beyond these distances only self-luminous exoplanets<br />
(‘young Jupiters’) could be detected. At the outer extremes, the brightness<br />
<strong>of</strong> the starlight reflected by the planet falls <strong>of</strong>f with increasing orbital distance from<br />
the parent star, even though it is well separated from it. Some relevant scales are<br />
shown in Table 6.<br />
Various techniques to increase the contrast <strong>of</strong> the images are at different stages <strong>of</strong><br />
development, from theoretical concepts to working prototypes. The most promising<br />
are: (i) classic coronography, which involves masking the star in the focal and pupil<br />
planes; advanced Lyot stop studies taking into account the segmented mirror suggest<br />
that contrasts <strong>of</strong> 10 −9 are achievable (neglecting diffusion by dust, mirror microroughness<br />
and the atmosphere); (ii) nulling interferometry (using the coherence <strong>of</strong><br />
the star light to eliminate it interferometrically), (iii) extreme adaptive optics and<br />
multi-conjugated adaptive optics (which result in a higher Strehl ratio and cleaner<br />
point-spread function), (iv) simultaneous differential imaging (using the contrast <strong>of</strong><br />
the target between nearby spectral bands and/or the polarisation <strong>of</strong> the target).<br />
Some <strong>of</strong> these methods can be combined to reach yet higher contrast (Codona &<br />
Angel, 2004).<br />
The wavelength range where a broad range <strong>of</strong> planets may best be detected and<br />
studied with OWL is probably the near infrared J-band at 1.25 µm (where achievable<br />
Strehl ratios should approach 90% and where strong spectral features <strong>of</strong> water are<br />
available as diagnostics), and the far red Z and I bands extending down to 700 nm<br />
(less favourable for adaptive optics, but containing the critical O 2 B-band absorption<br />
complex at 760 nm). In the centres <strong>of</strong> the stronger absorption bands saturated lines<br />
will obscure the signal from an exo-Earth. However, in the wings numerous narrow<br />
unsaturated but detectable lines will move in and out <strong>of</strong> coincidence as the two<br />
planets move around their respective suns with a modulated Doppler shift <strong>of</strong> up to<br />
50 km s −1 . As discussed in Section 1.2, free oxygen in the exo-planet atmosphere<br />
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