TPF-I SWG Report - Exoplanet Exploration Program - NASA

T E C H N O L O G Y R OADMAP FOR TPF-I
6.4.3 Spectral Resolution
Spectral resolution plays an important role both in allowing
robust spectroscopy and also in aperture synthesis imaging,
where the bandpass must be restricted to avoid “bandwidth
smearing” distortion. Bandwidth smearing sets a minimum
spectral resolution for imaging all objects within a single
‘primary beam’ (truly, the minimum field-of-view
requirement): the requirement on spectral resolving power, i.e.,
R ~ λ / Δλ ≥ B / D, where λ is the wavelength, Δλ is the
bandpass, B is the baseline length, and D is the diameter of the
telescopes in the array. For 200-m baselines (12.5 mas
resolution at 10 μm) and 4-m telescopes (primary beam of
FWHM ~ 0.6”), we see that R > 40. This resolution can be
obtained with a simple prism or grism that would likely be part
of any detection system. Even extending the baseline to 1 km
(2.5-mas resolution) would only require R > 200 spectrograph
for imaging.
While this minimum spectral resolution for imaging is easy to
achieve, higher spectral resolution in the infrared would be
invaluable for many other scientific goals. A special combiner
for general astrophysics (suggested in the last section) could
host a spectrograph with multiple levels of dispersion
depending on the science goal.
Figure 6-11. Beam combiner for pairwise
combination.
While it may difficult to obtain high spectral resolution (R>20000) for space-based telescopes due to
weight and payload size limitations, the science gains from resolving ro-vibrational lines themselves
could be immense by combining this kinematic information with the high angular resolution of the
interferometer. As an alternative to deploying an echelle spectrograph, the interferometer could be used
in a Fourier-transform spectrometer (FTS) mode to allow high-spectral-resolution interferometric
observations, a potential gold-mine of information to probe the many mid-IR transitions of diatomic and
polyatomic molecules associated with star formation and the extragalactic interstellar medium.
6.4.4 Beam Combination and Field-of-View (FOV)
There are readily available and proven techniques for imaging the entire field-of-view of the primary
beam. However, this is a very small beam for TPF-I (~0.6″ at 10 μm) and thus would impose large costs
in observing time to image a large FOV (such as Hubble Deep Field or nearby galaxies). Despite this
limitation, the science case laid out in Chapter 3 is clearly transformational, resolving otherwise point-like
objects across our Galaxy and indeed across the Universe. In this section we discuss options for imaging
with TPF-I and explore ways to optimize the potential.
With a symmetric rotating array of four telescopes arranged in either a line or a rectangle, there are only
three fully independent baselines. All of the necessary beam combinations can be generated with a
straightforward two-level four-beam combiner, as shown in Figure 6-11. Indeed, one of the outputs is in
fact redundant (allowing for application of this output to the lowest signal-to-noise combination). If
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