Spectral flux density distribution for pixels in the trough of five DLAs with ⟨ Noise ⟩  = 0.233 × 10^-17  erg  s^-1  cm^-2  Å^-1. A Gaussian fit to the spectral flux density distribution (solid green line) gives σ = 0.232 × 10^-17  erg  s^-1  cm^-2  Å^-1 (dashed magenta lines), which is in excellent agreement with the measured average noise. The zero-level spectral flux density correction has been applied to these DLAs.