Report - School of Physics

3.2.3 Terrestrial Planet Finder (TPF)
NASA’s TPF roughly parallels the ESA Darwin study, with close discussions taking
place between the two teams. TPF was conceived to take the form of either
a coronograph operating at visible wavelengths or a large-baseline interferometer
operating in the infrared (Beichman, 2003). There are two aspects of this choice
which should be distinguished: (a) the scientific aspect: whether reflected (visible
and near IR) light or thermal emission (mid-IR) is the best regime to characterise
planets (albedo, temperature, colour, key species that can be identified e.g. CO 2 etc.;
see Schneider (2003) for a recent discussion); (b) the instrumental aspects: whether
an interferometer or a coronograph is the best. Here, the NASA Technology Plan
for TPF stated that ‘Technology readiness, rather than a scientific preference for
any wavelength region, will probably be the determining factor in the selection of a
final architecture’. In May 2002, two architectural concepts were selected for further
evaluation: an infrared interferometer (multiple small telescopes on a fixed structure
or on separated spacecraft flying in precision formation and utilising nulling), and
a visible light coronograph (utilising a large optical telescope, with a mirror three
to four times bigger and at least 10 times more precise in wave-front error than the
Hubble Space Telescope).
In April 2004, NASA announced that it would embark on a 6×3.5 m 2 (more recently
changed to 8 × 3.5 m 2 ) visual coronograph in 2014 (TPF-C), with a wavelength
range 0.6–1.06 µm, and targeting a full search of 32 nearby stars and an incomplete
search for 130 stars (more recently quoted as 80). A free-flying interferometer, in
collaboration with ESA, would be considered before 2020 (TPF-I).
A visible light system can be smaller (some 10 m aperture) than a comparable infrared
interferometer, however advances in mirror technology are required: mirrors
must be ultra-smooth (∼ λ/15 000; a number stated in the TPF documentation,
although values an order of magnitude inferior appear more plausible) to minimise
scattered light, and in addition active optics would be needed to maintain low and
mid-spatial frequency mirror structure at acceptable levels. Infrared interferometry
would require either large boom technology or formation flying, typically with separation
accuracies at the cm-level with short internal delay lines. For the detection
of ozone at distances of 15 pc and S/N∼25, apertures of about 40 m 2 , and observing
times of 2–8 weeks per object, are indicated.
Many ideas for scientific and technological precursors for TPF have been examined.
The many possible solutions involve combinations of adaptive wavefront correction,
coronographs, apodization, interferometers, and large free-flying occulters.
The main contenders are summarised in Appendix A for completeness, although
with the recent (April 2004) NASA announcement on TPF strategy, it is not clear
whether any of these concepts will be developed further.
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