# Analysis Strategies for MHz XPCS at the European XFEL
## 4 Discussion  
The situation presented in the previous paragraph is a straightforward example of the advantages that come with the \"photonization\" step that eliminates correlated noisy pixels. In sequential XPCS, the choice of the criteria that determine the identification of a photon is not very critical since, as long as the mistaken photons are purely random, they will contribute to the correlation functions only for \(t=0\). Conversely, the choice of the parameters for the photonization is of primary importance for other classes of correlation experiments such as XSVS [35], where the contrast from single frames is used to evaluate the dynamics at very short timescales and the average number of photons per pixels is typically very low (<10\({}^{-2}\)). In this case, more sophisticated methods of converting the measured ADU values to photons might be necessary. That simple thresholding--the method we use here--might fail to unambiguously identify a photon. This can be seen from the histogram of Figure 3a, where the noise and the photon peaks partially overlap. Thus, it is not possible to univocally attribute the number of photons for a certain fraction of events. For detectors with smaller pixels than the AGIPD, one photon might create charges in several adjacent pixels. Then, dropletization algorithms [36] can be used to recombine the ADU values to photons and assign the correct number of photons to each pixel. In case of AGIPD, the large pixel size limits the spatial cross-talking between pixels and dropletization becomes equivalent to photonization. In the future, a special measuring mode (high-CDS) will be available for MID users, which is supposed to increase the separation of the photon peaks and the accuracy of the photonization. Another solution would be to increase the energy of the photons which would also increase the distance between the photon peaks. However, this approach comes with a drawback: a higher energy implies more possibilities to excite fluorescence in some elements of the beamline or the sample and the intensity of the speckle patterns will be weaker for the same fluxes. Furthermore, the speckle size will be smaller, which will reduce the contrast and the signal-to-noise ratio. In general, for the outcome of a very low intensity experiment, any method that can make the AGIPD work with a precise single photon resolution, either an even more refined calibration pipeline or different experimental conditions, should be pursued. In this paper, we showed the key aspects of sequential XPCS experiments that make use of the unique pulse structure of the European XFEL. We outlined the unique technical details of this class of experiments, and the general steps that should be considered for a successful data analysis. We tested different calibration pipelines on a prototypical sample composed of diluted nanoparticles dispersed in water. Among the various steps that can be taken to process the data, we identified two steps that provide the largest effect for the determination of dynamical quantities: (i) photonization of the speckle pattern that expresses the values in the recorded frames as integer numbers and (ii) correction of the TTC by the cross-correlation matrices obtained from neighbouring trains. Lastly, we outlined the limitations of the current pipelines which might have severe consequences on other photon correlation techniques and we proposed some possible solutions. Conceptualization, F.D., M.R. and F.L. ; Software, F.D., M.R. and A.J. ; Formal Analysis, F.D. and M.R. ; Investigation, F.D., M.R., I.L., J.M., M.S., A.M., F.P. and F.L. ; Resources, I.L. ; Supervision, G.G., F.P. and F.L. ; Writing--Original Draft Preparation, F.D., M.R. and F.L. ; Writing--Review and Editing, all authors. All authors have read and agreed to the published version of the manuscript.