Report - School of Physics

Table 7: Distance limits and numbers of stars searchable by ELT/OWL as a function of planetary
mass and primary mirror diameter (estimated by Hainaut). The detailed assumptions (noise
sources, exposure times, etc.) have not been documented, and these results should be taken as
indicative and preliminary. They also rest on the feasibility of achieving the high Strehl ratios
referred to in the text.
D(m) Earth-mass Jupiter-mass
Imaging Spectroscopy Imaging Spectroscopy
30 d(pc) 10 – 70 5
n(star) 22 0 6800 3
60 d(pc) 22 – 120 18
n(star) 210 0 35 000 170
100 d(pc) 40 15 500 35
n(star) 1200 67 2 500 000 860
The limiting distances at which imaging and spectroscopic observations can be performed
must take into account the photon noise from the star, sky and planet, the
speckle noise for a realistically high Strehl ratio, and slowly varying aberrations
that contaminate the image subtraction. From these distances, the number of host
candidates are obtained from star catalogues (e.g., keeping only single G and early
K stars). Results are shown in Table 7. The actual number of planets which will
be discovered is a function of the (unknown) fraction of planets per star. The conclusions
of the OWL studies are that the number of stars accessible to a 100 m
telescope is large enough to secure spectroscopic measurements of a large number
of planets. In the case of spectroscopy of Earths at 1 AU, however, the number
of accessible stars is just large enough to guarantee that a few planets should be
observable. These observations will be difficult (but hopefully feasible) for a 100 m
telescope, but are out of reach of smaller telescopes. These preliminary conclusions
clearly all require further evaluation.
After the mechanical assembly of the OWL telescope is completed, the mirror cell
will start to be populated with segments. It is expected that that phase will take
a few years (∼ 3), during which the telescope will already be available for scientific
observations, although with a reduced collecting area. The configuration of the segments
in the cell during this filling phase is still under discussion, but an attractive
option is to place them in such way that OWL could be used as a ‘hyper-telescope’,
i.e. an interferometer with densified pupil, assuming that proper phasing can be
achieved in such a scheme. In such configuration, the resolution and imaging characteristics
are very similar to that of a filled aperture telescope with the full diameter
(cf. Riaud et al. (2002) and Appendix B), but with a very small accessible field (of
the order of λ/d of a single segment), i.e. ∼ 0.1 arcsec, which is very suitable for a
planet detection. Science time during the mirror assembly phase could then be used
to perform a broad survey for planets around nearby stars. Assuming 100 nights
of observations per year for 2–3 years, 1 hr per single observation, and 8–10 epochs
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