How to extract the alpha channel from an image?

Question

Adobe Photoshop

On the Internet, only the reverse task is easily googled - to make / overlay an alpha channel. All this is well known to me.
And vice versa? There is a 32-bit PNG with a transparency channel. I want to get this same channel as an 8-bit greyscale image. It seems to be a simple task, but it’s not clear where to poke.
I would like it in FS, without programming, all sorts of imagemagik and the like.

Answer 1

Open the file, create a new layer on top of the original one, fill it with black (Shift+F5, "Use: Black"). Next, with the CTRL pressed, click on the thumbnail of the layer with the original image. A selection will be created based on the opaque pixels of the layer. Make sure the layer filled with black is selected and make the fill already white (Shift+F5, "Use: White"). As a result, you will get a layer with a b / w transparency map.