Report - School of Physics

Fourier-Kelvin Stellar Interferometer (FKSI) is a concept under study at
NASA GSFC (Danchi et al., 2003). It is a space-based mid-infrared imaging interferometer
mission concept being developed as a precursor for TPF. It aims to
provide 3 times the angular resolution of JWST and to demonstrate the principles
of interferometry in space. In its minimum configuration, it uses two 0.5-m apertures
on a 12.5-m baseline, and predicts that some 7 known exo-planets will be directly
detectable, with low-resolution spectroscopy (R ∼ 20) possible in favourable cases.
Optical Planet Discoverer (OPD) is a concept midway between coronography
and Bracewell nulling (Mennesson et al., 2003).
Phase-Induced Amplitude Apodization (PIAA, Guyon (2003)) is an alternative
to classical pupil apodization techniques (using an amplitude pupil mask). An
achromatic apodized pupil is obtained by reflection of an unapodized flat wavefront
on two mirrors. By carefully choosing the shape of these two mirrors, it is possible
to obtain a contrast better than 10 9 at a distance smaller than 2λ/d from the optical
axis. The technique preserves both the angular resolution and light-gathering capabilities
of the unapodized pupil, and claims to allow efficient detection of terrestrial
planets with a 1.5-m telescope in the visible.
Occulting masks are another approach to tackle in a conceptually simple manner
the basic problem of how to separate dim sources from bright ones, and have been
considered as precursor missions to Darwin/TPF. Interest in this approach at NASA
level currently appears limited.
UMBRAS (Umbral Missions Blocking Radiating Astronomical Sources) refers to
a class of missions (Schultz et al., 2003), currently designed around a 4-m telescope
and a 10-m occulter, with earlier concepts including a 5–8 m screen (CORVET),
or as NOME (Nexus Occulting Mission Extension) a modification to Nexus, itself
foreseen as a test of technologies for JWST.
BOSS (Big Occulting Steerable Satellite (Copi & Starkman, 2000)) consists of a
large occulting mask, typically a 70 × 70 m 2 transparent square with a 35 m radius,
and a radially-dependent, circular transmission function inscribed, supported by a
framework of inflatable or deployable struts. The mask is used by appropriately
aligning it with a ground- or space-based observing telescope. In combination with
JWST, for example, both would be in a Lissajous-type orbit around the Sun-Earth
Lagrange point L2, with the mask steered to observe a selected object using a
combination of solar sailing and ion or chemical propulsion. All but about 4 × 10 −5
of the light at 1 µm would be blocked in the region of interest around a star selected
for exo-planet observations. Their predictions suggest that planets separated by
as little as 0.1–0.2 arcsec from their parent star could be seen down to a relative
intensity of 1 × 10 −9 for a magnitude 8 star. Their simulations indicate that for
systems mimicking our solar system, Earth and Venus would be visible for stars out
to 5 pc, with Jupiter and Saturn remaining visible out to about 20 pc.
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