event is so coarse, i.e., it is significantly under-sampled, and this automatically causes the centroid to become systematically less accurate as the event centre lies closer to a pixel boundary. A centroiding ‘box’ of one pixel allows no sub-pixel centroiding; a box of three pixels in one dimension improves upon that significantly, but is still relatively under-sampled, and a sample of 5 only improves slightly because very little energy is deposited in the outer pixels, and noise of various types becomes significant.

A second reason is that a linear centroiding scheme is being used to calculate the centre of a generally Gaussian signal. This also causes a systematic error in the calculation of the centroid. Gaussian centroiding schemes were tested in the initial phase of the Electronics Unit design, and were found to be extremely accurate at centroiding the expected event profiles. However, these schemes require logarithmic calculations and were too slow for the FPGA hardware which was limited in how many cycles were available for each centroid calculation, in order to keep up with the required high data rates.

The third, but possibly not final reason, is that an individual CMOS pixel is not uniformly sensitive across its area 