5 Off-axis polarimetric purity

The off-axis polarimetric response of an antenna is an important consideration in polarimetric mosaicing. The main concern is the leakage of total intensity into the polarized intensities as a function of position in the primary beam. Understanding off-axis response is as important in polarimetric mosaicing as understanding the total intensity primary beam response is for traditional mosaicing. Appropriate design of the antenna system can produce good off-axis polarimetric purity, and, indeed, this is the case with the data presented below. Additionally, the process of mosaicing will tend to average down the impure response, as a pixel is the result of a number of pointings, and so the off-axis response is averaged over a number of different positions in the primary beam. Furthermore, if there is significant parallactic angle rotation during an observation (e.g. long observation with an alt-az mount), this too will tend to average down the impure response.

However, off-axis impurity can still be a significant consideration in many instances. If so, software techniques can be used to reduce it - see Cotton (1994) and Sault & Ehle (1996b) for some approaches.