Photoshop's Layer Mask is a powerful tool to have in your astrophotography arsenal. With a Layer Mask you can bring details or highlights to specific areas of your image. Once you learn the basics of using a Layer Mask you can apply this tool many different ways.

This tutorial will cover:
How to create a layer mask.
How to use the "Unsharpen Mask" (Very useful too!)
How to use "Gaussian Blur" to Blend".

Layer Mask to Sharpen.

Example *image before:

In this example I think there are some selective areas that will benefit from sharpening, and bring the sharpen area to the front. This will give the image a "3D" affect.

Before we begin I high lighted the area we are going to sharpen. Our goal is to sharpen only a small portion of the image while leaving the rest of the image intact.

First step.
Make a new layer copy of your original image in Photoshop. If you don't see this screen press F7 to toggle it on/off.

Click, Hold, Drag and Drop on top of “Create New Layer Icon”.

Now we have a “Background Copy” we can play around with, and keep the original image as “Background” on the layers.

Step Two.
After we have created our new "Background Copy" lets apply the "Unsharpen Mask". Yep! The your first thought "Unsharpen"?? I want to Sharpen the image! I don't want to unsharpen it! Unsharpen means sharpen in Photoshop...

Click the "Background Copy" so its highlighted, then from the menu in Photoshop Click FILTER-->SHARPEN-->UNSHARPEN MASK... Next you will see a screen like this:

Play around with the Amount and Radius until you have an exaggerated sharpening affect. We will smooth this out later with a Gaussian Blur. Adjust to taste.

Now you should see an image that looks like this:

You may like the image like it is now! But to illustrate the Layer Mask we have to take another step.

Step Three.
With the "Background Copy" still highlighted, from the menu click "LAYER-->LAYER MASK-->HIDE ALL"
Now you will see a "Black Box" next the Image with a little chain link. This is your new Layer Mask!

Creating the "Layer Mask" has hidden all of the "Unsharpen Mask" we did earlier. Now we can control how much we wish to come through the "Layer Mask". To SHOW the "Unsharpen Mask" we must paint White to Reveal, and paint Black to Conceal. The black box means everything is BLOCKED or Concealed. If we brush the image White it will reveal or show what is under the mask.

First Click the brush tool:

Note on this tool bar the WHITE and BLACK color boxes. Remember when we use the brush WHITE REVEALS and BLACK CONCEALS!

We must select the proper brush. Here select an "AirBrush Soft Round". Select a size that matches the area, if you are doing very fine details select a smaller sized brush, and use a larger brush size for larger areas.

Brush Control HINTS:
While using the brush the [ or ] brackets will make the brush larger or smaller.
Also, while using the brush the "X" key switches quickly back and forth from white to black, making it very easy to add or remove the effect (sharpening).

Now we can start painting WHITE to REVEAL our "UnSharpen Mask". Paint the area you wish to sharpen. As you start painting you will see the black box with WHITE lines, and the area you paint will show the sharpening.

You will notice a hard break between the sharpened part, and the rest of the image. We will take care of this and blend it with a Gaussian blur.

Step Four.
Next lets Select from the Menu FILTER-->BLUR-->GUASSIAN BLUR

This illustrates the WHITE area we brushed out. And as you can see there is a hard difference between the WHITE and BLACK. We must BLUR this so the brushed area is blended. This will make the transition smooth.

When we apply the BLUR you can see how it makes the smooth transition. Use the "Preview" button to see the results on your image...Slide the Radius Blur to taste.

"Gaussian Blur" means "Smooth Blend".

Final.
We started with this:

Ended with This:

This was a subtle change, but knowing how to use the "Layer Mask" helps pinpoint areas of your image you need to adjust without changing other areas of your image.

*Image source B.Seltzer