The two types of EPIC camera are fundamentally different. This does not only hold for the geometry of the MOS chip array and the pn chip array (Figs. 17, and 19), but other properties as well, like e.g., their readout times. The readout of the pn chips is much faster than that of the MOS cameras, because each pixel column has its own readout node. Another important difference is that the MOS chips are front-illuminated, while the pn CCDs are back-illuminated, which affects the detector quantum efficiencies decisively (§ 3.3.5).