TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 6
that would expand the general astrophysics capability of the current baseline design. Although such
hardware is not currently funded for development within the TPF-I project, its importance has been
emphasized by the TPF-I SWG for funding in future years.
6.4.1 Off-Axis Phase Referencing
The most critical hardware requirement for maximizing general astrophysics will be the addition of phase
referencing using an off-axis guide star, a capability not needed for the planet finding activity where the
primary star acts as the phase reference. This capability is crucial to obtain the theoretical sensitivity limit
that is needed for very faint objects, since current baseline designs require an external reference for
monitoring OPD drifts internal to the spacecraft and perhaps for monitoring array geometry.
Having the capability of using an off-axis star, and hence an additional beam transport system, for phase
referencing may not pose a problem if it is incorporated into the design process from the beginning, but it
surely will require substantial modifications to the optics at the collector telescopes. TPF-I is a system
optimized for low thermal background and off-axis phase referencing may require non-trivial changes to
the current design of a “dual-star” module that is used to select the off-axis reference star while passing
the on-axis light directly as is done with VLTI/PRIMA (Quirrenbach et al. 2004) and the Palomar Testbed
Interferometer (PTI) (Colavita et al. 1999). Another possible approach would be to include a “fast
switching” capability to the feed system or secondary mirrors on the collectors, to alternate between a
guide star and the target star as is currently done in ground-based radio and mm-wave interferometry. A
final option would be use an efficient dichroic to select a star in the field as a phase reference while
allowing the mid-IR beam train to contain a different target. Further study is necessary to develop
optimal designs for off-axis guiding that are consistent with the nulling requirements.
6.4.2 Wavelength Coverage
We must also consider the broadest possible spectral coverage when optimizing scientific return for
general astrophysics. Based on the science case for terrestrial planet finding, the short wavelength side of
TPF-I is generally restricted to ~5 μm due to: a) planets in the habitable zone have temperatures near
room temperature, giving a peak in blackbody emission near 10 μm, and hence limited emission at short
wavelengths; b) the performance requirements for the optics and wavefront quality that are driven for
maximum performance around 10 μm to provide sufficient starlight suppression (deep nulling) to detect
planets in the habitable zone.
Because these limitations are not relevant for general astrophysical observations, the final TPF-I design
should consider incorporating a secondary beam combiner optimized for general astrophysics. While this
could add significant cost and complexity to the system, one option might be to design the phasereferencing
combiner for this purpose. This idea is further explained below.
It is likely that a ‘fringe tracking’ system will be incorporated into any baseline TPF-I design to control
internal and external optical path difference (OPD) drifts. Most such designs operate by using off-band
(near-infrared) light from the target itself (i.e., an “on-axis” guide star). In principle, this subsystem could
be designed with its own separate beamcombiner that might function for example in the 1–5 μm band, to
be used in parallel with the 5–15 μm beam combiner that is optimized for nulling and other lower-spatialresolution
astrophysics.
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