  Many biologically produced chiral molecules such as amino acids and sugars
show a preference for left or right handedness (homochirality). Light reflected
by biological materials such as algae and leaves therefore exhibits a small
amount of circular polarization that strongly depends on wavelength. Our Life
Signature Detection polarimeter (LSDpol) is optimized to measure these
signatures of life. LSDpol is a compact spectropolarimeter concept with no
moving parts that instantaneously measures linear and circular polarization
averaged over the field of view with a sensitivity of better than 1e-4. We
expect to launch the instrument into orbit after validating its performance on
the ground and from aircraft.
  LSDpol is based on a spatially varying quarter-wave retarder that is
implemented with a patterned liquid-crystal. It is the first optical element to
maximize the polarimetric sensitivity. Since this pattern as well as the
entrance slit of the spectrograph have to be imaged onto the detector, the slit
serves as the aperture, and an internal field stop limits the field of view.
The retarder's fast axis angle varies linearly along one spatial dimension. A
fixed quarter-wave retarder combined with a polarization grating act as the
disperser and the polarizing beam-splitter. Circular and linear polarization
are thereby encoded at incompatible modulation frequencies across the spectrum,
which minimizes the potential cross-talk from linear into circular
polarization.
