Report - School of Physics

3 The Period 2015–2025
3.1 Ground Observations: 2015–2025
3.1.1 OWL/ELT
The 100-m diameter Overwhelmingly Large Telescope (OWL) is being studied by
ESO. S/N = 10 will be reached at 35 mag (t = 1 hr) for imaging, and at 30 mag
for R = 1000 spectroscopy (t = 3 hr). The resolution, λ/D ∼ 1 milli-arcsec in the
V-band, is a factor 40 improvement over HST. Detection of Earth-mass exo-planets
is part of the scientific case for the 100 m OWL/ELT project (see the on-line case
for the European Large Telescope, ELT, at www-astro.physics.ox.ac.uk/∼imh/ELT,
from which some of the following has been taken).
This section focuses on the science case and technical issues for Earth-mass planet
detection. For this, OWL must be equipped with advanced adaptive optics systems
to compensate for atmospheric seeing and the production of a near-diffraction limited
image or image core, expected to deliver Strehl ratios S (the peak image brightness
relative to that in a near-perfect image) from several to many tens of percent. In
the near-infrared (1–5 µm), values near 90% may be attainable, and are needed for
planet detection. Ultimate detection capabilities depend sensitively on whether such
high Strehl ratios can be delivered in practice (such advanced AO systems do not
yet exist). In addition, the telescope design (e.g. number and shape of elements)
has an impact on the final image quality, even with co-phasing techniques, so that
the exo-planet objectives must be considered early in the project.
Applied to a nearby solar-type star, adaptive optics produces an image with several
components. A central ‘spike’ resembling a diffraction-limited image contains some
S per cent of the total light, less a modest fraction diverted into the other, widerspread
components of the telescope point-spread function. The spike is surrounded
by a residual halo containing the remaining light distributed like an unmodified seeing
disc, i.e. with a generally Lorentzian distribution. This halo overlies, and within
the seeing disk generally dominates, the fainter wings of the telescope point-spread
function. These wings combine the light diffused by the small-scale imperfections of
the optics and the accumulated dust, as well as the light diffracted by the central
obstruction and geometric edges (mirrors, and supporting structures in the beam).
The diffraction pattern is strongly dominated by the secondary mirror supporting
structure, and by the edges of the mirror segments, which also produce strong secondary
peaks.
The diffracted light can locally strongly dominate the uncorrected seeing light. The
detailed structure of the halos in adaptive optics-corrected images is still being explored.
At large radii they are composed of the rapidly varying diffraction-spot-sized
‘classical’ speckles. Less well-understood ‘super-speckles’ occur closer to the first 2–
3 diffraction rings; larger, brighter, less rapidly variable and therefore less likely to
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