Report - School of Physics

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