  The European XFEL, currently under construction, will produce a coherent
X-ray pulse every 222 ns in pulse trains of up to 2700 pulses. In conjunction
with the fast 2D area detectors currently under development, it will be
possible to perform X-ray Photon Correlation Spectroscopy (XPCS) experiments on
sub-microsecond timescales with non-ergodic systems.
  A case study for the Adaptive Gain Integrating Pixel Detector (AGIPD) at the
European XFEL employing the intensity autocorrelation technique was performed
using the detector simulation tool HORUS. As optimum results from XPCS
experiments are obtained when the pixel size approximates the (small) speckle
size, the presented study compares the AGIPD (pixel size of (200 $\upmu$m)$^2$)
to a possible apertured version of the detector and to a hypothetical system
with (100 $\upmu$m)$^2$ pixel size and investigates the influence of intensity
fluctuations and incoherent noise on the quality of the acquired data.
  The intuitive conclusion that aperturing is not beneficial as data is 'thrown
away' was proven to be correct for low intensities. For intensities larger than
approximately 1 photon per (100 $\upmu$m)$^2$ aperturing was found to be
beneficial, as charge sharing effects were excluded by it.
  It was shown that for the investigated case (100 $\upmu$m)$^2$ pixels
produced significantly better results than (200 $\upmu$m)$^2$ pixels when the
average intensity exceeded approximately 0.05 photons per (100 $\upmu$m)$^2$.
  Although the systems were quite different in design they varied in the signal
to noise ratio only by a factor of 2-3, and even less in the relative error of
the extracted correlation constants. However the dependence on intensity showed
distinctively different features for the different systems.
