In this null tetrad, the spin coefficients are κ κ = σ σ = τ τ = 0 , ν ν = λ λ = π π = 0 , ε ε = 0 ρ ρ = − − 1 r , μ μ = − − r + 2 M ( u ) 2 r 2 , α α = − − β β = − − 2 cot ⁡ ⁡ θ θ 4 r , γ γ = M ( u ) 2 r 2 .