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Layer magic for your astrophotos: unleash the full potential of your astrophotos with Photoshop layer masks.

TODAY'S ASTROPHOTOGRAPHERS know that crearnng a masterpiece requires nours at the telescope and even longer at the computer. The image processing alone typically involves dozens of steps, producing many files that are saved or discarded, before the final image emerges. What some may not know, however, is that it's possible to preserve all the intermittent steps in one file that can be later changed at any point the process. All you need to do is work with Layers, one of the most powerful tools in the Adobe Photoshop arsenal.

What makes Photoshop Layers so useful is its "three-dimensional" structure. Layers work from the top to the bottom. Imagine stacking sheets of clear acetate with images painted on portions of each, and then looking through the stack to see the result. You'd see a picture made up of the background and the painted portions of each layer.


Photoshop works exactly the same way, only digitally. You can create sharpening layers, Curves (contrast-modifying) layers, or any other image-adjustment layers, and then create masks that apply these changes selectively to any parts of the image you want. It doesn't matter what type of photograph you're processing. Images of star clusters, galaxies, nebulae, the Sun, the Moon, and planets can all be enhanced using similar steps. Even your family photos can benefit from processing with layer masks.

How to Create Layers and Masks

Begin by opening your image in Photoshop and activating the command via the drop-down menu Window > Layers. This opens the Layers window, with your image listed as the Background. All your image adjustments will appear in subsequent layers.

Let's say you'd like to boost the brightness of a galaxy but not the surrounding sky. Choose Layer > New Adjustment Layer > Curves. A window opens titled New Layer with a few options from which to choose. Stick with the defaults that Photoshop has chosen, but you can change the name of the layer at any time by clicking on its name (it defaults to Curves 1). Click OK, and the Curves window will appear. Adjust the curve to make the galaxy look the way you want, while ignoring how it affects areas you don't want changed; we'll get to them in a minute. When you're happy with your adjustment, press OK.

Your Layers window now shows two layers: Background and an adjustment layer titled Curves 1 (unless you changed it) with a blank white thumbnail image to its right. This is where you'll create a layer mask that hides and reveals selected regions of your background layer. Adjustments to this layer can be made two ways. By moving the layer's Opacity slider, you can change the strength of the layer. The default is 100%, so lowering this value will lessen the layer's effect on your image. The second method is to create a layer mask by painting it with white, black, or shades of gray. This lets you selectively apply the Curves adjustment to various parts of the image.


Wherever you paint a layer mask with black, it becomes transparent, and you see through it to the layers below--and, in the case of an adjustment layer, the adjustment is not applied to that portion of the image. Wherever you paint the layer mask with white, the layer becomes opaque and you see the layer completely (or apply the full strengh of an adjustment layer). As you might expect, painting a layer mask with gray makes the layer semitransparent, and it "blends" with the underlying layers. The degree of blending depends upon the shade of gray. Darker grays hide more of the layer; lighter grays reveal more. It's helpful to remember the phrase "black conceals and white reveals."

In the case of your galaxy image, you don't want the Curves adjustment to change the background sky, so paint black onto the areas of the layer mask that you don't want changed. Click on the blank thumbnail to the right of your Curves 1 layer, and then select the brush from the Tools palette. Adjust the brush's size to accommodate the area you're going to paint, and choose a feathered (blurry-edged) brush so that the mask's edges blend gradually with the revealed areas. Now click on the small "Default Foreground and Background Colors" icon in the bottom left of the Tools palette. This will set your foreground color to black, and the background to white. Gray can be made buy decreasing the opacity of the brush tool.


Now paint out the areas you don't want your curves layer to affect. If you forget to click on the mask thumbnail before painting, a big black stripe will appear in your image as you move the brush (which is one good reason why Photoshop has an Undo function). With the mask thumbnail clicked, you'll see the result of your painting by how it allows your Curves adjustment to affect the image.

To preview the mask itself, hold down the Alt key while clicking the layer-mask thumbnail. This will show you any areas you missed with the brush. If you've painted unintended areas, you can make the foreground color white by clicking the "Switch Foreground and Background Colors" arrow, and simply paint over the offending region. Once you've created this layer mask, you can go back and modify it at any time.

Now you can add a new layer for each additional tweak you want make to the image. I typically end up with about a dozen layers, including ones for unsharp masking, high-pass filtering, and color saturation. Any of these layers can be refined at a later date, or turned off completely by clicking the "eye" button to the left of the corresponding thumbnail in the Layers window.

CAB Layers

To create a filtering layer, you'll need to make a new layer that combines all the work you've already done to the image. This is called a CAB (combine-all-below) layer. Do this by holding down the Shift, Ctrl, and Alt keys, and pressing N followed by E. Everything you've done to the image up to this point will be stacked on top as a single new layer, while retaining your previous layers underneath. I find it very helpful to rename this layer "CAB" followed by whatever filtering I'm about to perform.

For example, here's how to selectively apply unsharp masking (Filter > Sharpen > Unsharp Mask) to your image. Start by adding a layer mask (Layer > Layer Mask) and choosing either Reveal All or Hide All. Reveal All creates a white mask that you can then paint with black to protect areas you don't want the unsharp masking to affect. Hide All creates a black mask that you can paint with white to reveal the regions you wish to enhance. I prefer to use a mask that requires the least work; in this case, a black mask will only need a few brush strokes of white and gray to reveal the areas that need sharpening.

Masking with Channels

You can create a powerful layer mask by using a color channel from your photo as a mask. For example, we can reduce the background noise of our galaxy image with a Gaussian Blur (Filter > Blur > Gaussian Blur) set to 3 pixels. Start with a new CAB layer and apply the filter. Now open the Channels window (Window > Channels) and use the green channel as a mask by holding the Ctrl key and clicking on the green-channel thumbnail. You'll see the familiar dashed selection outline appear around all the bright areas of the image in the green channel.

Now click on your new CAB layer and choose the menu Layer > Layer Mask > Hide Selection. Everything within the selected areas is now unaffected by the blur filter (in this case the brighter details of the galaxy and the brightest stars). Any channel, or any combination of them, can be used the same way. Simply hold down the Shift and Ctrl keys while clicking on each color-channel thumbnail. You can further modify this new mask with the paintbrush tool.

Layer masks are great tools for your arsenal of image-processing choices. By performing all of your processing steps with layers, you can avoid accumulating dozens of files that are intermediate versions of the same image. Your processing steps are recorded in the file, and each step can be refined at any time in the future.



All recent versions of Photoshop as far back as Photoshop 5.5 have layer-mask capabilities, but only version CS or higher can utilize them on 16-bit images. Photoshop Elements, which comes bundled free with many new digital cameras and scanners, also includes some layer-mask options. Many of the keyboard shortcuts mentioned here are written for PC users, so be sure to look up the equivalent ones if you're using a Mac.

Image-processing guru R. Scott Ireland is the author of Photo shop Astronomy (Willmann-Bell, 2005).
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Title Annotation:Photoshop Layer Masks
Author:Ireland, R. Scott
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2008
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