Stacking improves resolution (i.e., detail) only if the images are not coincident, and you can use schemes like Drizzle integration).
Otherwise, the normal purpose of stacking is really used to improve Signal-to-Noise Ratio (SNR). But even here, you can't stack identical images to improve SNR, since the noise in the images all have precisely the same patterns.
Each time you add multiple images, the signal part keeps accumulating (the signal is in the same pnace and has the same avergae value), but the noise parts are randowm and starts averaging themselves out, thus leading to SNR improvement -- i.e., a cleaner image, where less bright parts of your target will start to pop out above the lowered noise.
But with two identical images, the "noise" part will accumulate just like a signal, since the two images are identical. So there cannot be any SNR improvement.
Chen
