Illustration of the Pluto/Charon photocenter problem: two Gaussian PSFs, f_a(x) = A·exp [ −(x − x_a0)²/ 2σ² ] and f_b(x) = B·exp [ −(x − x_b0)²/ 2σ² ] (full lines), are separated by a distance d. They overlap each other and result in the dashed line f_a(x) + f_b(x), which is the observed mixed PSF. This PSF depends on the brightness ratio k and has a peak at a distance x_phot from the Pluto Gaussian which is centered at the origin. This term x_phot is the correction that we need to find to recover the Pluto photocenter from the measured photocenter of the mixed PSF. Here, as an illustration, A = 1, B = 0.2, σ = 1, x_a0 = 0, and x_b0 = d = 1.