Geometry of a simple slitless spectrograph in the plane orthogonal to the CCD (top) and parallel to the CCD (bottom). A convergent beam with incident angle θ₀ is focused on a CCD at position r₀ = (x₀,y₀), but also passed through a grating with N_eff grooves per millimetre at a distance D_CCD above the CCD. The beam is deflected at an angle θ(λ) along the mean dispersion axis u, which forms an angle α with respect to the CCD x axis, but is focused somewhere above the sensor. Atmospheric refraction adds a supplementary dispersion along and transversally to the mean dispersion axis.