TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 6
TPF-I is designed with the capability of arranging the telescope array into a non-redundant geometry, then
an alternate beam combiner could be used that maximized the Fourier coverage and, thus, the imaging
capabilities.
There are possibilities for giving TPF-I truly wide-field imaging capabilities, extending beyond a single
primary beam. Wide-field imaging is best obtained using homothetic pupil mapping (Beckers 1986),
which is difficult to implement in practice for a reconfigurable array geometry, although formation flying
offers a possible context for application. An FOV intermediate between a single primary beam but smaller
than the optical vignetting limits can be attained through pupil densification, so-called hyper-telescope
concept (Labeyrie et al. 2000, 2003)
Novel optical arrangements have also been proposed that, when combined with special OPD modulation
schemes, promise to provide a wide-field imaging capability simultaneous with high spectral resolution
(e.g., the SPECS concept; Leisawitz et al. 2000). Recently, these schemes have been partially validated in
the laboratory, clearly offering attractive features, and further research in this area should be pursued
(Rinehart et al. 2006).
6.4.5 Double Fourier Interferometry
The mid-IR is rich in spectral features from source components in the solid state and gas phase. The
spectrum includes atomic hydrogen recombination lines, ionized neon (Ne), argon (Ar), and sulfur (S)
forbidden lines, H 2 rotational lines, and silicate, SiC and polycyclic aromatic hydrocarbon (PAH)
features. These features appear on a continuum of thermal-dust emission. A wealth of information is
available from the mid-IR spectrum, such as the radiation field intensity and hardness, the dust
temperature, and the chemical state of the medium. Kinematic information is potentially available as well,
but only when the available spectral resolution is ~10 4 for galaxies or ~10 5-6 for protostars and objects of
similar size.
The spectral resolution of TPF-I can be increased by increasing the stroke of the delay line. R = 1000 at
10 μm requires only a 1-cm stroke, but R = 10 6 requires a 10-m delay-line stroke, a packaging challenge.
Such very high resolutions probably require the addition of a grating.
The wide field-of-view double Fourier technique (e.g., Mariotti and Ridgway 1988) can enable a TPF
mid-IR interferometer to provide high spatial resolution, high spectral resolution observations of spatially
extended astrophysical sources. TPF-I could map protostars, debris disks, extragalactic star-forming
regions, and protogalaxies on relevant spatial scales and simultaneously provide the spectroscopic data
that would enable deep insight into the physical conditions in these objects. table 6-5 below shows
desired measurement capabilities for a variety of targets and indicates that 10 mas is an interesting
angular resolution and that ~1 arcmin is an interesting FOV for a wide variety of applications.
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