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