Frames selection

In my case periodic error is the major source of movements in frames, but wind and seeing also count. I usually use between 40% and 60% of the frames, exceptionally 80%.
Some programs like DeepSkyStacker, MaxIm DL or RegiStax, evaluate the quality of each frame with some parameters such as image contrast, star diameter (FWHD) or roundness, the number of stars detected, or a combination of these parameters and enable you to sort the pictures in it and filter function to stack boxes.
I find that the evaluation of roundness and diameter are the most useful as a guide to select images from deep space objects, but best results are obtained by looking all the frames one by one and discarding those that have deformed stars.
Below you can see a good frame and a bad one with stars deformed that should be discarded.

If you don't do a proper frame selection you can obtain a blurred picture with with deformed stars as the following one.

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Monitor calibration

Monitor calibration
Adjust your monitor to see all grey boxes