Wide Field Imaging

Visibilities cannot be fully corrected to compensate for direction dependent effects. Corrected visibilities are valid for one direction only. By contrast, images can be corrected in a direction dependent fashion. For example, an image can be adjusted to correct for the direction dependent sensitivity pattern of a primary antenna beam. However, this image-plane correction can only be properly applied if the beam is not time dependent. Effects that are both time and direction dependent can only be corrected by the imager, during the process of inversion from the visibility to the image domain. A-projection is one example of an algorithm that can apply these corrections efficiently. For LOFAR data, the AWimager software implements this algorithm. This chapter describes the A-projection algorithm, its implementation in AWimager, as well as tips for its usage and performance tuning.