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UV_PREVIEW cannot identify lines if there are two few channels. It will only display the spectra in this case. A minimum of 32 channels is required, but confused spectra may also prevent a proper line recognition.
The line detection is based on the current specified velocity. If this is incorrect, lines may appear shifted and considered as not detected. PREVIEW%TOLERANCE indicates the matching precision in frequency (default is 2 MHz).
If variable REDSHIFT exists, the frequency scale is corrected for that source Redshift, so that line identification remains possible.