= Angular deflection kernel for cosmological lensing
{title2=$\alpha^i=2\int_0^{\chi_s}\frac{\chi_s-\chi}{\chi_s}\partial_\perp^i\Psi\,d\chi$}
For a source at comoving distance $\chi_s$ in a spatially flat universe, define deflection as apparent minus unlensed source direction. The transverse <Weyl lensing potential> gradient along the ray receives the distance weight $(\chi_s-\chi)/\chi_s$. Integrating the transverse photon acceleration twice and dividing the source displacement by $\chi_s$ gives the formula. Reversing the definition of the deflection reverses its sign. Its angular lensing potential is $2\int_0^{\chi_s}(\chi_s-\chi)\Psi/(\chi_s\chi)\,d\chi$.
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