Schematic diagram illustrating the estimation of the mtNUFFT periodogram described in Section 2.2.3. The left panel shows the Kepler-91 time series for which we compute the power spectrum estimate. The middle panels show three DPSS or Slepian tapers with time-bandwidth product NW = 2 and order k = 0, 1, 2 (number of tapers K = 2NW − 1), their corresponding tapered time series, and the single-tapered NUFFT periodograms. The rightmost panel shows the multi-tapered NUFFT or mtNUFFT periodogram that is constructed by averaging the three single-tapered estimates with adaptive weights d _k (f).