Overview of the HPA measurement setup in front of the cryostat window, (a) Sketch of the cryostat and HPA measurement setup. In green, we plot the two signal generators SG₁ and SG₂. The harmonic mixers are depicted in yellow. The beam splitter, shown in pink and denoted BS, is situated such that its faces are illuminated by both mx₁ and mx₂. A lens, depicted in red, is placed in between mx₂ and the beam splitter to increase the coupling. The modulated signal U_m then travels through the cryostat to the ISS. (b) Photograph of the HPA measurement setup in front of the cryostat window. The components are colour-coded in the same fashion as in panel a. The mixer mx₁ is located inside the yellow disk on the left. The material in which mx₁ is embedded is a sheet of radiation-absorbing material. This prevents standing waves from generating in between mx₁ and the cryostat window. In this photograph, the x-axis depicted in Fig. 1a points out of the cryostat window into the beam splitter and scanning plane. The scanning plane is oriented along the z- and y-axes. The diagonal horn containing mx₂ is also shown in yellow and is visible at the bottom of the image, below the lens, which is shown in red.