Effect on the system temperature of the windows used in the experiment. The receiver temperature was measured between 41.5 and 49 GHz. The quartz windows, which are best suited for high vacuum, have a poor performance and introduce a significant loss of sensitivity. The Upilex windows have a very good behaviour concerning transmission but are much more sensitive to stress when the chamber is evacuated. The thick lines correspond to the predicted receiver noise temperature by the model described in the text.