This is part one of a two-part explanation of the procedure involved in getting stars, in a nighttime shot, to appear brighter, but without making everything else look brighter. This is accomplished by using a luminosity mask, but, in order to get the best results from the luminosity masking activity, I believe the photo should be sharpened, properly, before attempting any luminosity masking. In other words, before getting into Photoshop. The original thread that generated a request for this explanation is here:
Star light, star bright -- Where the hell are the Northern Lights?I'll tell the tale of sharpening through the eyes of Lightroom, though the same things can be done in ACR.
First, edit the photo the way you want it to look, but without sharpening or noise reduction. (Figure 1)
Zoom in on the photo, to get a closeup look at the noise in the night sky. (Figure 2)
Notice the digital noise. Bump up “Luminance” to get rid of that. For this shot, I set it at 35, but your requirements may be different. (Figure 3)
Now, set the Sharpening – Amount setting. Because of what we are about to do next, you might be able to set the Amount higher than you think. I put mine up to 70. (Figure 4)
You might consider 70 too high, but we’re going to do something about that. If you were to press the Alt key (on a PC), while left-clicking the mouse on the Masking slider, you would see a totally white image. This key/mouse combination is showing you, in white, what parts of the photo will have sharpening applied. Because your mask is at zero, everything will be sharpened, but that is too much. While holding down the Alt key, move the Masking slider to the right, and more of a mask will be applied. Keep going until the sky (but not the stars) are masked away. I set my Masking to 40. This is what it looked like, when I pressed the Alt key while clicking on the Masking slider (while being zoomed in). (Figure 5)
I released the Alt key and the mouse, and zoomed out, and this is what I have. (Figure 6)
That sets us up for the next part. This part is useful, all on its own, if you’ve never learned how to use that Masking slider.