The invention provides an optical coupling, comprising: a first coupling portion and a second coupling portion, the first and second coupling portions configured to mate in kinematic constraint via a first mounting interface and at least two further mounting interfaces, wherein: the first coupling portion comprises a first mounting element and the second coupling portion comprises a socket, and the first mounting element is receivable in the socket at the first mounting interface such that the first mounting interface constrains lateral displacements of the first coupling portion relative to the second coupling portion when mated; and at least one optical channel for transmitting abeam of light along an optical axis through the optical coupling, the optical channel comprising complementary light-transmitting ports in the first and second coupling portions, wherein the optical axis is not laterally removed from the first mounting interface by more than half the distance between the first mounting interface and the nearest of the further mounting interfaces.