[en] Stupakov has used a perturbation method to estimate the transverse impedance at zero frequency of a rectangular collimator having characteristic taper length (ell), half-width w, and average vertical half-aperture b₀, under the condition b₀ << w << (ell). We use the boundary perturbation method to approximate the transverse impedance of a flat, slowly tapered chamber in the complementary limit, w = ∞