Example of a path integral of the area of overlap of one PSF pixel. Left: overview of an area of 3 × 3 dector pixels, showing the area covered by a PSF pixel, dispersed over a spectral interval [t₀ − t₄] (red/green), and the detector pixel under consideration (gray). Right: detail showing the path, which is split up in four different segments. The types of overlap are: none (t₀ − t₁), top + left limited (t₁ − t₂), top limited (t₂ − t₃), and full overlap (t₃ − t₄).