  We provide a detailed description of the Herschel-SPIRE Fourier Transform
Spectrometer (FTS) Spectral Feature Finder (FF). The FF is an automated process
designed to extract significant spectral features from SPIRE FTS data products.
Optimising the number of features found in SPIRE-FTS spectra is challenging.
The wide SPIRE-FTS frequency range (447-1568 GHz) leads to many molecular
species and atomic fine structure lines falling within the observed bands. As
the best spectral resolution of the SPIRE-FTS is ~1.2 GHz, there can be
significant line blending, depending on the source type. In order to find, both
efficiently and reliably, features in spectra associated with a wide range of
sources, the FF iteratively searches for peaks over a number of signal-to-noise
ratio (SNR) thresholds. For each threshold, newly identified features are
rigorously checked before being added to the fitting model. At the end of each
iteration, the FF simultaneously fits the continuum and features found, with
the resulting residual spectrum used in the next iteration. The final FF
products report the frequency of the features found and the associated SNRs.
Line flux determination is not included as part of the FF products, as
extracting reliable line flux from SPIRE-FTS data is a complex process that
requires careful evaluation and analysis of the spectra on a case-by-case
basis. The FF results are 100% complete for features with SNR greater than 10
and 50-70% complete at SNR of 5. The FF code and all FF products are publicly
available via the Herschel Science Archive.
