Abstract: A coupler for joining optical devices. It receives a peripheral flange. The coupler includes a socket to receive the flange. Each of a plurality of radially movable retainer studs has a shoulder which can project through a respective aperture in the side wall of the socket to overhang and engage a shoulder on said flange, thereby to hold together optical devices that are in turn integral with or fastened to the flange and to the coupler. Means is provided for moving the retainer studs in the apertures, both into and out of the socket.