  W projection is a commonly-used approach to allow interferometric imaging to
be accelerated by Fast Fourier Transforms (FFTs), but it can require a huge
amount of storage for convolution kernels. The kernels are not separable, but
we show that they can be closely approximated by separable kernels. The error
scales with the fourth power of the field of view, and so is small enough to be
ignored at mid to high frequencies. We also show that hybrid imaging algorithms
combining W projection with either faceting, snapshotting, or W stacking allow
the error to be made arbitrarily small, making the approximation suitable even
for high-resolution wide-field instruments.
