Candidate Schemes

There are five schemes that are usually considered for this purpose. The simplest is the on/off pointing method. Here one points at the source for a short integration, perhaps 10-30 seconds, and then at blank sky for the same time, and then takes the difference. The rest of the map results from a sequence of such measurements. For spectral line observations with narrow band widths, the receiver noise is usually large enough that it dominates both the atmospheric brightness fluctuations and the noise due to receiver gain fluctuations in this method, and it works. The second scheme is to use rapid frequency switching for spectral line observing, and this also works. Neither of these procedures will work for continuum measurements. That's obvious for the second method. For the first, the wider bandwidth means that the receiver noise is lower than that due to either the atmospheric brightness fluctuations or the effects of receiver gain fluctuations. For continuum total power observations, there are three schemes that can be used. The most common method is to employ a nodding secondary mirror. A related alternative is the focal plane chopper. The third idea is On-The-Fly mapping (Emerson, Klein, and Haslam, 1979, A&A, 76, 92). In all three cases, differential ground spillover will be a problem and will probably set the fundamental limit to the accuracy and depth of the continuum single antenna maps. The nodding secondary, giving a rapid comparison, avoids gain drifts. Its main weakness is that it is difficult to get a throw of more than a few arc minutes. While this will be adequate for some science programs, there are situations where one needs to chop to an ``off" position that is 10-20 (or more) minutes away. This is especially the case at the shorter wavelengths where, in the Milky Way, the background dust emission is bright. The focal plane chopper, on the other hand, can only throw large angles, typically 10 minutes or more. Other disadvantages for our application are that it is often difficult to have a good balance between the ``on" and ``off" and the mechanism would probably have to be mounted on each receiver separately, which could be an annoying complication for the ALMA antennas with their many receivers. The On-The-Fly (OTF) method looks to be the most flexible and simplest, and we summarize its properties and requirements below. Whatever scheme is chosen, the maximum throw will limit the maximum spatial scales in the resulting maps.