  Resolving the separation between two incoherent optical sources with high
precision is of great significance for fluorescence imaging and astronomical
observations. In this paper, we focus on a more general scenario where two
sources have unequal brightnesses. We give the ultimate precision limit with
respect to separation by using the quantum Fisher information. Through the
calculation of the classical Fisher information, we analyze and compare several
specific measurement schemes including direct measurement, Gaussian mode
measurement and zero-photon measurement. The results indicate that Gaussian
mode measurement is the nearly optimal for a small separation. Our work
provides a positive complement to the aspect of superresolution imaging of
incoherent sources.
