The problem due to non co-planarity discussed above are for mapping the sky within the primary beam of the antennas (which are assumed to be identical). In this section we discuss the techniques used to handle the problem of mapping fields of interest which are larger than the primary beam of the antennas. The approach used is similar to that used for mapping with a single dish, namely to scan the source to be mapped. The fact that we are using an interferometer to synthesis the ``lens'' (or the a ``single dish'') adds some more complications.
These techniques are useful for mapping with interferometers operating in the millimeter range where the size of the primary beams is less than an arcmin and at meter wavelengths where the primary beams are larger but so is the extent of emission. For example, the primary beam of GMRT antennas at 327 MHz is and there are mapping projects which would benefit from mapping regions of the sky larger than this (for example, in the Galactic plane).