low-pass filter for non-uniformly sampled data

Adrian Price-Whelan (Flatiron) and I used the new fiNUFFT non-uniformly-sampled fast Fourier tranform code to build a low-pass filter for stellar spectra today. The idea is: There can't be any spectral information in the data at spectral resolutions higher than the spectrograph resolution. So we can low-pass filter in the log-wavelength domain and that should enforce finite spectral resolution. The context is: Making features to use in a regression or other machine-learning method. I don't know, but I think this is a rare thing: A low-pass filter that doesn't require uniformly or equally-spaced sampling in the x direction or time domain.