Fℓ⁡(η,ρ)∼(2⁢ℓ+1)!⁢Cℓ⁡(η)(2⁢η)ℓ+1⁢(2⁢η⁢ρ)1/2⁢I2⁢ℓ+1⁡((8⁢η⁢ρ)1/2),