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tokens: 69 Å) per pixel for the G130M (G160M) grating. The resolving power of each individual extraction is quoted as R ∼ 18 000 to 20 000, which corresponds to 15–17 km s −1 for a Gaussian LSF. However, the COS FUV LSF shows the time-dependent non-Gaussianity and the resolving power degraded with time. The spectral resolution can be approximated to ∼19 km s −1 for the COS non-Gaussian LSF at Lifetime Position 1 (see more details in Section 3.2 ). The wavelength regions contaminated by strong Galactic ISM lines are discarded. The typical COS FUV limit is ∼ 13.0. The CalCOS flat-field correction corrects strong wire grid shadow features greater than ∼20 per cent in intensity, but not weak (≤10 per cent in intensity) fixed pattern noise (FPN) produced by the hexagonal pattern of the fiber bundles in the COS FUV micro-channel plate known as MCP Hex (Dashtamirova et al. 2019 ). MPC Hex is supposed to be fixed in the detector pixel space, but not in the wavelength space. In practice, the position of MCP Hex and its intensity change along the pixel space. This sometimes produces false, equally spaced weak absorption-like features in the high-S/N region of the coadded spectrum (Fig. 3 ). It is the most conspicuous when a high-flux Ly α emission region falls on the longer wavelength edge of Segment B of the detector. Due to FPN, the noise is not Gaussian and the conventional way to quote noise as the reciprocal of 1 r.m.s. of the unabsorbed region underestimates true noise (Keeney et al. 2012 ). Since only an individual extraction with 12 shows distinct FPN and the majority of our individual extractions has a lower S/N, we did not correct for MCP Hex (Fitzpatrick Spitzer 1994 ; Savage et al. 2014 ; Wakker et al. 2015 ). Figure 3. Examples of sawtooth-line MCP Hex fixed pattern noise (FPN) in coadded COS spectra. The red vertical ticks spaced at ∼0.4 Å mark the position of Hex FPN. The blue thick tick in the upper panel marks the position of the Milky Way Cl i λ1347.23. When assumed as a H i Ly α, FPN has log N FPN ∼ 12.5. If a weak IGM H i absorption falls on FPN, the apparent increases. 2.6 <italic>FUSE</italic> data Available FUSE spectra (917–1187 Å) were used to obtain a reliable column density of saturated COS FUV H i Ly α lines, since FUSE spectra cover high-order Lyman lines at z ≤ 0.12. The 8th column of Table 3 lists whether the COS AGN has a corresponding FUSE spectrum. The FUSE spectra used in this study are the same ones analysed by Wakker ( 2006 ). They are sampled at ∼0.0066 Å per pixel, weakly dependent on the wavelength. As they are oversampled, we binned the FUSE spectra by 3, 5, or 7 pixels to increase the S/N. The S/N in general increases towards longer wavelengths, i.e. more reliable Ly β profiles than Ly γ profiles. The S/N per resolution element at ∼1050 Å is listed in parenthesis in the 5th column of Table 3 . Since wavelength regions with S/N 5 are not very useful to deblend saturated lines reliably, we excluded these low-S/N