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We derive angle differences between two null geodesics, propagating fromlight sources to an observer, on the de Sitter spacetime with multi-lensingobjects. Assuming the lensing objects are mass monopoles on the de Sitterbackground, we derive the metric tensor by solving the Einstein equationperturbatively. On that spacetime, we solve a null geodesic parametrized by thecoordinate time. Using the null geodesics, we define the angle differences in acoordinate invariant way. We take in the relativistic effects up to the firstorder of perturbation and clarify the magnitude of approximation errors. Wefind that the rest observer, who sees the isotropic cosmic space, implicitlyobserves the effect of the cosmological constant on the angle differencesthrough the positions of the light sources. As a practical application, weregard the massive black hole at our galactic center (Sgr A*) and the solarsystem as the lensing objects, further a star and a flare around Sgr A* as thelight sources. We write the angle differences between these light sources usingtheir spatial coordinates. We find that deflections by Sgr A* remain in theangle differences while deflections by the solar system cancel out up to thefirst order of perturbation. The deflections by Sgr A* amounts around 10microarcseconds, which is detectable in the near future observations.

Light deflection in Observation: Angle differences between two null geodesics on the de Sitter spacetime with multi-lensing objects