Observing the Sun with the Total Power array is significantly different to TP observations of other sources. In the latter case the TP antennas perform a raster scan of the source, observing along successive rows with increasing vertical offsets until a rectangular area has been covered. For the Sun though, the TP observations must use so-called “fast” scanning in order to map this large source quickly enough such that the changing properties of the solar disc can be properly resolved in time.
For early cycles of solar observing it was only possible to produce TP maps of the full solar disc, even though the interferometric data only covers a small region on the Sun's surface. From Cycle 9 though it has been possible to also request TP observations of this region, in addition to the full solar disc. How the TP observations are carried out is specified in a special panel (“TP Regional Mapping”) where the user can, for example, indicate whether they want just the full solar disc or maps of the interferometric region as well.
At the moment, in regional mode it is only possible to observe a circular area mapped using a so-called “double-circle” pattern - for this the user only needs to define the diameter of the circle which should be somewhat larger than the interferometric mapping area. More options will be added in the future.
The ALMA OT Team, 2024 May 21