An optical demultiplexer has a coarse filter 41,43,45 and a plurality of arrayed waveguide gratings 49 connected to respective outputs of the coarse filter 41,43,45. For example, a system may include a lens sleeve defining an interior lens cavity. kinematic viscosity of from 5 to 15 mm2s; and one type or a mixture of two or more types of ester-based base oil having a 40\xb0 C. In certain embodiments, the wall repair compound comprises one or more glycol ether smoothing agents.