Mirrors Definition and 252 Threads

I heard about a technique where mercury or some other liquid is spun in a dish until it assumes a parabolic shape. Would this work in space? Could a flat membrane of liquid metal do just as well? It could be liquified by an electric current and start out flat like soapy water in a bubble wand...

I know that a telescope system is supposed to be at least 1/4 wave or less (for both primary and secondary ratings combined), but I am not quite sure what the wave rating refers to. Can someone explain what this is measuring.