Finally, the normalized power of each channel is compared to a signal-to-noise (S/N) threshold value. Those channels exceeding the threshold (S/N $>$ 30) are recorded in the FITS file, including the signal time, frequency, detection power, mean power, telescope pointing, and other information. Each channel recorded is called a hit.

Nebula for Radio frequency interference removal {#sec:4}
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Radio frequency interference (RFI) removal has always been a crucial part of radio telescope data analysis. There has been much work done to date on the problem of RFI removal, including SumThreshold method [@2010MNRAS.405..155O], Singular value decomposition (SVD), Surface fitting and smoothing [@2007AN....328...68W] and Sky-Subtracted Incoherent Noise Spectra (SSINS) [@2019arXiv190601093W].

Nebula is a complete off-time data analysis system, including data cleaning, RFI removal, candidate selection, and scoring. Here we just give a brief introduction of RFI removal part with the example from FAST data.

Narrow-band RFI {#sec:4.1}
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Narrow-band RFI is the most common kind of RFI coming from artificially engineered signals on the Earth, especially within the FAST electromagnetic environment. In Nebula, we separate the narrow-band RFI into two kinds. Narrow-band RFI that is stable in frequency (called zone RFI in Nebula) and Narrow-band RFI that drifts in frequency (called drifting 