  Simple and fast analytic method to find out optimal combinations of sizes of
neutron moderator and optical system entrance, allowing for the full sample
illumination with minimum to none background, is developed. In the case of
employing low-dimensional para-hydrogen moderators with enhanced neutron beam
brilliance, the method allows to determine the minimum size of moderator, which
provides the highest sample flux while keeping the sample fully illuminated.
  This method can be used during the design of new neutron sources, upgrades of
neutron optical systems and moderator replacements.
