size of the photon event on the CMOS readout. For our flight detectors, with the 3x3 Square algorithm, there are approximately 1-2% of centroids which fall outside of the central pixel boundary, and therefore these will not be adjusted to the correct position and will thus constitute noise. For the 5x5 Square algorithm, the fraction is approximately 5-7%. However, it should be pointed out that the reason these centroids were outside of the pixel boundary in the first place was because of a noise skew, and so the correction scheme doesn’t actually introduce more noise, but rather simply conserves it.

Figures 2 and 3 show parts of flat-field images with 3x3 centroiding before and after correction. The final science images will have the 1/4 pixel resolution of Figure 2. A final point to note is that in flight, there will be spacecraft jitter of amplitude a pixel or more over the time of an observation. This will be monitored by guide-star centroids from the VIS channel and the photon centroids corrected in building up the science image. The effect of this will be to smooth out the FPN, so that it may be a matter of post-observation observer choice whether 