In order to distinguish real secondaries from artifacts of the filtering process, the data is filtered on either side of the primary frequency. The filtered data sets are then reanalyzed for the secondary frequency. If the secondary frequency shows a dependence with respect to the filtering frequency, then the signal is an artifact of the filtering process and cannot be considered real. The frequency dependence of the residual is clearly visible. This secondary frequency is clearly independent of the filtering process.
Problems can arise if frequencies are too close together.
Filtering of this central peak will produce strong secondaries at, or near, the resolution limit of the periodogram producing a multitude of closely spaced signals. These spectra allow for the initial evaluation of periodic structure. If no structure is apparent, then periodograms for each color are generated and examined. If periodic structure is visible, then the frequency ranges over the peaks are taken and the peak values, from the normalized periodograms, are determined for each color. If a frequency group lies near the orbital period, then that group is selected.