- some small areas of a pixel are more sensitive than other areas, and this is also a general systematic effect.

Also, if the source of photons is non-isotropic, meaning that the distribution of photons on the detector face is not “flat”, such as an extended source with flux gradients, or a point source convolved with the relevant resolution function, this will also introduce topological effects in the centroid histogram. However, these effects are due to the source, and generally this is the scientific information one desires, and it is also not systemic. The goal of course is to be able to correct for the systemic effects, while leaving the source-topological effects in place. For this reason a flat field source, as provided by the integrating sphere and the resulting images, is used to determine the systemic centroiding effects.

These systematic centroid errors are called Fixed Pattern Noise, or FPN. It would be somewhat more accurate to call it fixed pattern bias. Figure 1 displays the effects of the Fixed Pattern Noise. At nominal MCP gain, the 3x3 algorithm produces a distinct “screen-door” appearance, even though the source was illuminating the detector face evenly. For the 5x5 algorithm the effect 