is somewhat less, but still apparent. If the average FWHM of the events were only 0.1 pixels or more wider, the 5x5 algorithm performs much better than the 3x3, as shown in paper 1. However, the smaller events sizes result from the deliberate choice of small window gaps, to yield the best spatial resolution. At low MCP gain, both algorithms perform equally poorly.

The resolution of the photocathode (see following sections), and the resolution of the UVIT telescope are of order 0.4 readout pixels, so the flight science images will be centroided to 0.25 pixels. If we centroid the flat fields to 0.06 pixels or better, we can resolve the MCP pores, as shown in Fig 2. This is of use in that the pore grid can be used as a measure of the centroid error and the correction algorithms.

Centroid error corrections
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The systematic centroiding bias, or Fixed Pattern Noise, can be corrected for significantly, while leaving the scientific information intact. This is simplest to do for the 3x3 Square centroiding algorithm because a mathematical solution can be programmed which closely mimics the real-world behaviour of the algorithm. In the absence of noise, the 3x3 Square algorithm would 