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My understanding is as follows:
Stacking of calibrated undithered subs (ie no registration - which what you should have if your tracking is sub-pixel) does not improve S/N by sqrt(number of subs). The improvement is less because additional noise is introduced by the calibration frames.
The reason why many calibration subs should be used to make the calibration frames is to minimise the fpn of these frames. But with undithered subs the stacking multiplies the fpn contribution to the overall noise by the number of stacked subs.
However, if dithering is performed the build-up does not occur, and the calibration noise contribution is closer to that from a single calibration frame. Dithering puts the image onto different pixels each sub - that is critical to its value in reducing fpn.
Mark
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