Inferring contributions from unresolved point sources to diffuse
  emissions measured in UV sky surveys: general method and SOHO/SWAN case study
  In observations of diffuse emissions like, e.g., the Lyman-$\alpha$
heliospheric glow, contributions to the observed signal from point sources
(e.g., stars) are considered as a contamination. There are relatively few
brightest point sources that are usually properly resolved and can be
subtracted or masked. We present results of analysis of the distribution of
point sources using UV sky-survey maps from the SOHO/SWAN instrument and
spectrophotometry data from the IUE satellite. The estimated distribution
suggests that the number of these sources increases with decreasing intensity.
Below a certain threshold, these sources cannot be resolved against the diffuse
signal from the backscatter glow, that results in a certain physical background
from unresolved point sources.
  Detection, understanding and subtraction of the point-source background has
implications for proper characterization of diffuse emissions and accurate
comparison with models. Stars are also often used as standard candles for
in-flight calibration of satellite UV observations, thus proper understanding
of signal contributions from the point sources is important for the calibration
process. We present a general approach to quantify the background radiation
level from unresolved point sources in UV sky-survey maps. In the proposed
method, a distribution of point sources as a function of their intensity is
properly integrated to compute the background signal level. These general
considerations are applied to estimate the unresolved-point-sources background
in the SOHO/SWAN observations that on average amounts to $28.9$ R. We discuss
also the background radiation anisotropies and general questions related to
modeling the point-source contributions to diffuse UV-emission observations.
