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To put it simply, I don't want to just switch black to alpha. I want to make it so that the more black a pixel is, the more alpha it has.
I've got this nebula I made following a tutorial:

The image shown doesn't do it justice; there's a lot of very minor flushes of color around the nebula that I need to keep. As you can see, it's also black.

I want to make it so that the more black an image has, the more alpha it has. How would I do this in Photoshop (CS6)?

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Assuming that you do not want "the darker, the more transparent" (=normal layer mask function) but the more black, the more transparent should that black be to make visible the details behind. But the dark sky still must stay dark

Unfortunately one layer RGB image has no info what's behind the black. You must add brightness. It fights against the black. The easy way is to take Image > Adjustments > Curves

enter image description here

The histogram pattern shows how far there's room to stretch without causing clipping.

This adds contrast and color saturation, too. I don't see them bad. The color saturation can be taken back by Image > Adjustments > Hue/Saturation. No saturation increase happens if you work in Lab color system and do the curves in the luminosity channel only.

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