  Space-based direct imaging provides prospects for detection and spectral
characterization of exoplanets at optical and near-infrared wavelengths.
Integral field spectrographs (IFS) have been historically baselined for these
mission concepts. However, multiple studies have revealed that detector noise
is a serious obstacle for such instruments when observing extremely faint
targets such as Earth-like planets. Imaging Fourier transform spectrographs
(iFTS) are generally less sensitive to detector noise, and have several other
compelling features such as simultaneous imaging and spectroscopy,
smaller-format detector requirements, and variable spectral resolution. To
date, they have not been studied as options for such missions.
  In this work, we compare the capabilities of integral field spectrographs and
imaging Fourier transform spectrographs to directly obtain spectra from an
Earth-like planet using analytic and numerical models. Specifically, we compare
the required exposure time to achieve the same signal-to-noise ratio of the two
architectures over a range of detector and optical system parameters. We find
that for a 6-meter telescope, an IFS outperforms an iFTS at optical
wavelengths. In the near-IR, the relative efficiency of an IFS and iFTS depends
on the instrument design and detector noise. An iFTS will be more efficient
than an IFS if the readout noise of near-IR detector is above 2-3 e-/pix/frame
(t_frame=1000s), which correspond to half to one-third of detector noise of the
state-of-art. However, if the readout noise is further reduced to below this
threshold, the performance of an IFS will experience a substantial improvement
and become more efficient. These results motivate consideration of an iFTS as
an alternative option for future direct imaging space missions in the near-IR.
