Here’s a picture of the Odin submillimeter wave radiometer before it was mounted onto the rest of the Odin satellite. At the top left you can see several mirrors which redirect the radiation from the main telescope into the detectors which sit inside the box on the right-hand side (underneath the big orange cables). The components on the top right corner of the picture are the Local Oscillators which provide a reference signal to the detectors. The target signal, which can be a star or nebula for example, or the Earth’s atmosphere, is mixed with the reference signal giving us highly accurate frequency information about the target.