  The soft MeV gamma-ray sky, from a few hundred keV up to several MeV, is one
of the least explored regions of the electromagnetic spectrum. The most
promising technology to access this energy range is a telescope that uses
Compton scattering to detect the gamma rays. Going from the measured data to
all-sky images ready for scientific interpretation, however, requires a
well-understood detector setup and a multi-step data-analysis pipeline. We have
developed these capabilities for the Compton Spectrometer and Imager (COSI).
Starting with a deep understanding of the many intricacies of the Compton
measurement process and the Compton data space, we developed the tools to
perform simulations that match well with instrument calibrations and to
reconstruct the gamma-ray path in the detector. Together with our work to
create an adequate model of the measured background while in flight, we are
able to perform spectral and polarization analysis, and create images of the
gamma-ray sky. This will enable future telescopes to achieve a deeper
understanding of the astrophysical processes that shape the gamma-ray sky from
the sites of star formation (26-Al map), to the history of core-collapse
supernovae (e.g. 60-Fe map) and the distributions of positron annihilation
(511-keV map) in our Galaxy.
