Beam Offsets

It is clearly important that the radiometer samples the same path through the atmosphere as the incoming astronomical signal. It is in fact not possible for these to match absolutely perfectly. (For one thing the radiometer signal is incoherent emission from the water molecules and is therefore sampled by the intensity pattern of the antenna, which is always positive. The path length change is a coherent effect and therefore depends on the amplitude pattern. Molecules in certain locations will not contribute to the phase delay and some will even produce an advance!) The question of how well the beams need to overlap depends on how much small-scale structure there is in the water vapour and how far away it is in front of the aperture. We need more data on the height of the fluctuating layers to make quantitative statements on this. It is however clear that it is desirable to keep the radiometer close to the astronomical feeds but this is not likely to be a very critical parameter because most of the phase fluctuation is in scale that are considerably larger than the beam. If we can place the radiometer feed in the centre of the ring or cluster of feeds, then the beam offsets are likely to be in the range 3 to 10 arc minutes. This corresponds to distances of 1 to 3 metres at a distance of 1 km, i.e. a modest offset compared to the dish diameter. To illuminate a suitable area on the subreflector to be able to do the pointing correction would require a feed about 75 mm in diameter. It is more likely that a much smaller feed (or group of feeds) would be used which would be reimaged onto the subreflector by an optical relay. The final mirror of this could then be in the central position and it would be advisable to allow about 100 mm clear diameter to accommodate it. The baseline should be to keep the radiometer beam within 10 arc minutes of the astronomical ones and, if it is practical to do so, make this offset smaller than that for the higher frequency channels.