We are developing a setup that will allow us to measure the propagation of magnons with femtosecond time resolution and (sub)-300 nm spatial resolution. These experiments will be performed at cryogenic temperatures in the liquid helium regime, as this will allow us to explore the phase diagram of many materials. Therefore, we are developing a magneto-optical, laser-based cryogenic microscope. The goals are to measure and manipulate the propagation of coherent magnons on both a local and global scale. The ultimate goal is to establish the concept of a magnonic waveguide at these extreme scales.