Auto alignement astrophotography

Hi!

My Problem:

I have a problem with the auto alignment of photoshop layers in astrophotos. I am working with about twenty photos of the night sky, all taken from the same position, that I want to median stack to reduce noise. Because of the earths movement, the stars pixel position is different in each image, The foreground does not move.

The usual way to deal with it is to mask the foreground, auto align the layers, convert them to a smart object after having removed the layer masks and median stack them. In this aligned and stacked image the foreground seems to move and then you compose your image of one aligned stack for the sky and one not aligned stack for the foreground.

However, for many sets of photos this doesn’t work.

I think that it is due to the fact that the total surface of star containing pixels is very small to the dark sky surface that contains much noise. Photoshop does not know that it is looking at stars and also tries to align the background noise.

My question:

To improve the ratio of star containing pixels to dark sky pixels and therefore hopefully the alignment results I am looking for a function to mask pixels with a brightness value below a certain threshold. This should mask the dark (noisy) sky and just let some “holes” in the image where stars are.

Does this exist?

Thanks!