  This paper is one of three companion papers presenting the results of our
in-depth analysis of the interstellar neutral helium (ISN He) observations
carried out using the IBEX-Lo during the first six Interstellar Boundary
Explorer (IBEX) observation seasons. We derive corrections for losses due to
the limited throughput of the interface buffer and determine the IBEX spin-axis
pointing. We develop an uncertainty system for the data, taking into account
the resulting correlations between the data points. This system includes
uncertainties due to Poisson statistics, background, spin-axis determination,
systematic deviation of the boresight from the prescribed position, correction
for the interface buffer losses, and the expected Warm Breeze (WB) signal.
Subsequently, we analyze the data from 2009 to examine the role of various
components of the uncertainty system. We show that the ISN He flow parameters
are in good agreement with the values obtained by the original analysis. We
identify the WB as the principal contributor to the global $\chi^2$ values in
previous analyses. Other uncertainties have a much milder role and their
contributions are comparable to each other. The application of this uncertainty
system reduced the minimum $\chi^2$ value 4-fold. The obtained $\chi^2$ value,
still exceeding the expected value, suggests that either the uncertainty system
may still be incomplete or the adopted physical model lacks a potentially
important element, which is likely an imperfect determination of the WB
parameters. The derived corrections and uncertainty system are used in the
accompanying paper by Bzowski et al. in an analysis of the data from six
seasons.
