Astronomical images are not easily viewed on standard image processing / showing software. You need to perform an initial stretch of the data. All the capture software makes a "virtual stretch" while they are capturing images, so you can see what you are shooting at. But they store the original (linear, non stretched) data.

The best approach will be to use some viewer specific to handle astronomical image. For example, you can use SiriL (siril.org/download/). It's open source and it can help you to align / stack / colour correct and save as stretched TIFF or PNG image

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