Is it better to combine multiple IDFs and extract a single spectrum or to extract the spectra separately and cross-correlate them?
The standard advice is that, for bright targets, you want to optimize spectral resolution, so should extract and cross-correlate the individual spectra. For faint targets, you want the best possible background subtraction, so should combine the individual IDFs into a single file. There’s an implicit trade-off between spectral resolution and background fidelity, but the presumption is that resolution is less important for faint targets. For complete instructions, see the FUSE Tools in C web page. If you want to have your cake and eat it, too, you can shift the photons in the individual IDFs before combining them. For example, you could extract the individual spectra, determine the appropriate shifts for each, apply them to the IDFs, then run idf_combine. John Grimes has written a tool to shift spectra within an IDF. See the FUSE IDL Tools web page.
