After the neutron data is downloaded from NMDB/NEST, some post-processing is applied to make the time series data more useful for supsequent operations:
Extreme oulier rejection is supposedly performed by the operating station, so it is not done in SET operations.
The Butterworth Filter is a type of signal processing filter in the frequency domain designed to have a frequency response as flat as possible in the passband. It is particularly useful for extracting the signal out of time series that has a very large high-frequency component (noise). It is a commonly used in electronic resonant circuits.
Below are two comparisons, showing how data with a one-hour boxcar average compares to a Butterworth filtered time series.
Note the differences between the two methods of smoothing the data: The Butterworth filter results in better end-point estimates, and there is an improvement in reducing high-frequency noise. However, in all cases tested, the overall differences between the filtered and mean time series were on the order of 1%.