Excellent @keesscherer,
You captured quite a few of those cosmic rays ;-)
Regarding the statistics and rejection of them:
The outlier rejection works a little bit differently. An outlier rejection filter while integrating several frames, is a filter the determines the central value (average/median for example) of a pixel stack. So all pixels of all frames that will make up 1 pixel in the integration are analysed. So the surrounding pixels don’t have any influence on the rejection for a pixel in the stack.
The principle is the same, like you mention. From that pixelstack, we calculate the central value (in APP this is the median, since this is way more robust than the average). And we calculate the standard deviation for the rejection boundaries relative to this central value.
For Winsorized clipping, a winsorized central value and standard deviation is calculated at each rejection iteration.