Assumed instrumental response for the individual H.E.S.S. I telescopes. (a) indicates the total bandpass, where ɛ(λ) is defined as the ratio between detected and incident photons. For comparison, the night sky spectrum from Benn & Ellison (1998) is shown in red. (b) indicates the effective area for a pixel close to the optical axis. (c) indicates the effective area for an off-axis pixel. This suggests noticeable asymmetry due to optical aberrations. The white hexagons in (b) and (c) indicate the pixel size when projected onto the reference frame of the telescope.