This is an easy and quick technique for enhancing faint detail using color channels in Photoshop.  It works best on objects which are one color, such as diffuse red nebulae.  An object like the Trifid Nebula which is both red and blue can be enhanced using this method but must be done in two steps using layer masks or a similar technique.  Blue objects like the Pleiades also work well (see example at the bottom of the page).

Above:  Unprocessed single exposure of the Rosette Nebula.  Image by James McGaha.

Extracting the Color Channel

This example will use the Red channel, but a blue object like the Pleiades would use the same technique but use the Blue channel instead.

Begin by selecting the Channel tab in the Layers menu.  This will display the RGB, Red, Green, and Blue channels.  Click on the Red channel to make this the active channel.

Above:  Selecting the Red channel

The image will now show only the Red channel, which appears black and white.

Above:  Red channel

Next, select the entire image (Ctrl-A) and copy the Red channel (Ctrl-C).  Now click on the RGB channel to make all channels active again.

Click on the Layers tab.  This will allow you to now paste the Red channel as a new layer.  If you do not change back to the Layers tab, you will paste a new channel instead of a layer.

Paste the Red channel as a new layer (Ctrl-V).

Above:  The Red channel pasted as a new layer

Coloring the Channel

The next step is to colorize the black-and-white Red channel.  To do this, open the Hue/Saturation window (Ctrl-U).  Check the box next to Colorize.  Move the Hue slider to 0 to select a red color.  (For a blue object, choose a Hue value of around 220.)  You will have to adjust the Saturation slider to an appropriate amount based on the particular object.  For this example, 60% was used.

Above:  Hue/Saturation settings

Above:  Red channel after being colorized

Blending the Layers

Change the layer blend mode to Lighten.  For some objects, Screen mode works well, but it can blow out the highlights in some images, so experiment to see what works best.  Changing the Opacity might be helpful as well, again depending on the object.  For this example, the Opacity was lowered to 70%.

Above:  Layer blend mode set to Lighten

Above:  Image after setting layer blend mode to Lighten

Final Processing

Now the image can be flattened (Ctrl-E).  Normal aesthetic image adjustment can now be done.  For this image, Levels and Curves adjustments were made, and the Starizona Zone Mask Filter plug-in was used.

Above:  Final processed image of the Rosette Nebula.  Single 15-minute exposure with Canon EOS 20Da.  Image by James McGaha.

Above:  Color channel enhancement technique applied to Pleiades image using the Blue channel.  Single 15-minute exposure with Canon EOS 20Da.  Image by James McGaha.

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