Measuring line-of-sight dependent Fourier-space clustering using FFTs
  Observed galaxy clustering exhibits local transverse statistical isotropy
around the line-of-sight (LOS). The variation of the LOS across a galaxy survey
complicates the measurement of the observed clustering as a function of the
angle to the LOS, as fast Fourier transforms (FFTs) based on Cartesian grids,
cannot individually allow for this. Recent advances in methodology for
calculating LOS-dependent clustering in Fourier space include the realization
that power spectrum LOS-dependent moments can be constructed from sums over
galaxies, based on approximating the LOS to each pair of galaxies by the LOS to
one of them. We show that we can implement this method using multiple FFTs,
each measuring the LOS-weighted clustering along different axes. The N log(N)
nature of FFTs means that the computational speed-up is a factor of >1000
compared with summing over galaxies. This development should be beneficial for
future projects such as DESI and Euclid which will provide an order of
magnitude more galaxies than current surveys.
