Narrowband imaging is becoming a popular way to take images that show incredible detail.  Imaging through narrowband filters also allow images to be captured in light polluted skies.  In fact, the example used here was taken in suburban Tucson under a first-quarter moon -- not ideal imaging conditions by any means.

While narrowband-filtered images may be combined to create a color image in MaxIm DL or another CCD image processing program, layer clipping masks in Photoshop allow more control over the compositing of the images.  Since narrowband images are false-color images, it is beneficial to have more control over the final colors of the combined images to create a more aesthetically pleasing result.  This method is called color mapping, as each image is mapped to a specific color; for example, H-Alpha might be assigned, or mapped, to red.

The images below were shot with the standard assortment of narrowband filters (Hydrogen-alpha, Oxygen-III, and Sulfur-II), but the method is applicable to any combination of filters.  It can even be used for combining shots through more than three filters.  See the section on More Narrowband Color Techniques for additional mapped-color methods.

Layering the Images

Begin by opening the raw frames, in this case an H-alpha image, an OIII image, and an SII image.  Be sure the images are properly aligned, using one of the appropriate methods outlined in the image processing sections.  In this case the images were aligned using the Auto Star Align routine in MaxIm DL, then saved as TIFFs to be opened in Photoshop.

Copy and paste the OIII image onto the H-alpha image.  Do the same with the SII image.  This will result in a three-layer image.  Double-click on the H-alpha layer (named Background), and rename it H-alpha.  This makes it an active layer, which is also necessary for this method.  Rename the other layers so you can keep track of which filtered image is where.

Above:  Layered image.

Above:  Raw H-alpha frame.

Adjustment Layers

The next step is to create an adjustment layer that will colorize each layer.  These adjustment layers can always be altered later -- one of the big advantages of the clipping mask technique -- so for now we just need to create the basic red, green, and blue colors.

The first decision is which layer will be which color.  The standard arrangement is for H-alpha to be red, OIII to be green, and SII to be blue.  Other methods are possible as well.  See the Narrowband Imaging section for more ideas.

Make sure the H-alpha layer is the active layer by clicking on it in the Layers window.  Select Layer > New Adjustment Layer > Hue/Saturation, or click on the New Adjustment Layer icon at the bottom of the Layers window and select Hue/Saturation.

Above:  New adjustment layer icon.

The Hue/Saturation window will appear.  To make the H-alpha layer red, check the Colorize box, then set the Hue slider to 0, Saturation to 100, and Lightness to -50.

Above:  H-alpha adjustment layer settings.

Above:  The Hue/Saturation adjustment layer applied to the H-alpha layer.

At this point, you will still see only the top layer, the SII layer, which will still appear black and white.  To see the red H-alpha layer, you will have to turn off the visibility of the other layers above it.  But there is no need to do this at this point.  For now, leave all layers visible.

Create another Hue/Saturation adjustment layer for the OIII layer.  Use the same settings, expect the Hue slider should be set for 120.  This will create a green color for the OIII layer.

Finally, create another adjustment layer for the SII layer.  Repeat the same settings but with Hue set to 240 to create a blue SII layer.

Now the top SII layer should appear blue.

Above:  Each layer has an adjustment layer above it.

Creating Clipping Masks

The next step is to apply each adjustment layer to only its associated image layer.  Without this step, the adjustment layer applies to every layer below it, meaning currently the image is completely blue, due to the top adjustment layer.  By associating each adjustment layer with its own image layer (a technique called a clipping mask), we can make the H-alpha layer red, the OIII green, and the SII blue.

To create a layer clipping mask, hold the cursor over the line between the adjustment layer and the image layer below it.  Hold the Alt key down and you will see the cursor change from a hand to an arrow next to two overlapping circles.  When this icon appears, click and an arrow will appear next to the adjustment layer showing that it is now directly associated with the image layer below it.

Do this for each layer, and if you like, rename each layer mask so you can keep track of the colors.

Above:  Layer clipping masks applied and adjustment layers renamed to their respective colors.

At this point the image still looks like the blue SII layer, since it is above all the other layers and is opaque.  The next step will address this.

Blending Mode

The last step in colorizing the image is to change the blending mode.  By changing the blending mode of the image layers, the final image will take on the colors of the respective layers, the layers essentially acting as RGB channels would.  The difference is you will be able to control all aspects of the layers, which cannot be done with channels.

Select the SII layer and change the blending mode from Normal to Screen.

Repeat this for the OIII layer.  The image is now colorized, if still in need of some adjustment.

Above:  Raw frames colorized.

Adjustments

Adjustments can now be made by altering the Hue/Saturation adjustment layers.  Hue, Saturation, and Lightness settings can be altered to achieve the desired colors.  More adjustment layers, such as curves or levels, can be applied to continue enhancing the image.

Above:  Curves adjustment layers applied.

Above:  Image after hue/sat and curves adjustments.

Final Image

A little cleaning up with the Healing Brush (to remove blooming spikes and artifacts), and some overall standard adjustments (Curves, Saturation, Color Balance, and Levels), and we have a final image.

Above:  Final narrowband image of the Orion Nebula.

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