Cooled or uncooled?

The main advantages of cooled systems are sensitivity and stability. It would also be easy to provide a cold reference load. There is however some concern about how one would calibrate out losses in the Dewar window, especially if there is a possibility of getting dirt or water on it. External optics would almost certainly still be required for the pointing system and it might be possible to introduce some additional calibration signal there. With cooled systems, the radiometer will essentially take up one complete slot in a Dewar and the development path will interact strongly with the main receiver programme. It will also take up some of the cooling power budget (IF amps, windows, connections, etc.) and there would be greater likelihood of LO power leakage. An uncooled system is clearly simpler, and should cost less to develop and build. Uncooled Schottky mixers can be obtained commercially and are robust and stable. We therefore believe that an uncooled system should be adopted as the baseline. Assuming, however, that the goal of correcting the pointing is confirmed, there is some question as to whether sufficient sensitivity can be obtained with an uncooled system. Until this is established the cooled option should be kept open as the backup. Digression on cooled systems: