Hydrogen-alpha images can show amazing detail in emission nebulas. However, they are only black and white images. Adding color by combining with an RGB image can yield spectacular results. Creating an LRGB image using H-alpha data (called an HaRGB) is very similar to a standard LRGB composite, but there are some differences that require some extra steps to achieve the best results.
To see why some extra techniques are needed, take a look at the example below.
Why Standard HaRGB Doesn’t Work
Above: H-alpha and RGB images of the California Nebula
Simply combining these images using standard LRGB techniques leads to the image below.
Above: Standard HaRGB composite
While the image above shows all the detail of the H-alpha image, the color saturation is severely lacking. (It is not uncommon to see images that look like this, so if you’ve ever seen a pink nebula, now you know why.)
Also, looking at a close-up of this image shows another problem. The stars in an H-alpha image tend to be smaller in diameter than those taken through RGB filters. This leads to colored haloes appearing around the stars in an HaRGB composite.
Above: Mismatched star sizes in a standard HaRGB composite
A Better HaRGB Method
To create a better HaRGB image, we need to accomplish two tasks:
- – Enhance the color saturation of the final HaRGB
- – Eliminate the colored halos
There are two methods that give good results. Each yields a slightly different final image, so either method can be used depending on personal preference. Both methods involve extracting the Red channel from the RGB image to enhance the color saturation of the final image. The methods differ in the way they deal with the mismatched star sizes.
Extracting the Red Channel
The first few steps are the same for both HaRGB methods.
Bring up the Channels window (Windows > Channels) in the RGB image. Click on the Red channel to make it the only active channel.
Above: Selecting the Red channel in the Channels window
This makes the Red channel visible in the image window. Press Ctrl-A to select the entire image, then Ctrl-C to copy.
Above: The extracted Red channel
Creating an HaR Image
The next step is to create a new image that is a composite of the H-alpha and Red images. Select the H-alpha image and press Ctrl-V to paste the Red channel image as a new layer in top of the H-alpha image. Change the Opacity setting to 20-30%.
If necessary, select the Move tool (keyboard shortcut V) to shift the top layer with respect to the bottom one until they are properly aligned.
This creates a blend of the Red and H-alpha images. The detail of the H-alpha image is retained, but the larger and more numerous stars from the Red image become visible as well.
Above: HaR composite
Save this new HaR image as a separate file. This will be used both for the Luminance image and also to create a new RGB image.
Creating a New RGB
Press Ctrl-A and Ctrl-C to select the entire HaR image and copy it. Go back to the RGB image and make sure the Red channel is still the active channel. Press Ctrl-V to paste the HaR image into the Red channel.
Click on the eye icon next to RGB in the Channel window. This will allow you to align the new image if necessary. The entire RGB is visible but only the Red channel is active. Once the images are aligned, click on the RGB channel to activate the entire image again.
Above: Red channel is the active channel (highlighted blue), but all channels are visible (each has an eye icon next to it). This allows movement to align the Red channel if necessary.
This method uses the HaR image to enlarge the star sizes in the Luminance channel to better match the larger stars of the RGB.
Creating the LRGB Composite
Copy the HaR image and paste it as a new layer on top of the new RGB image. Change the Opacity to 30-40%. If necessary, shift the image into place using the Move tool.
Once aligned, change the Blending Mode in the Layers window to Luminosity.
This creates the new HaRRGB composite.
Above: HaRRGB composite
This method also uses the new RGB image, but uses the original H-alpha for the Luminance channel. The stars are eliminated from the RGB image and only the H-alpha stars are used to create the final image. This results in fewer overall stars, but the stars are much smaller and the nebula tends to be much more detailed since the full H-alpha image is used for the Luminance channel, rather than the watered-down HaR image.
Begin with the new RGB image. Select Filter > Noise > Dust & Scratches.
Set the Radius to a value that causes most (but not all) of the stars to disappear. For the large (2200×2500) image shown here, a value of 5 was used.
This results in a slightly blurred image with few stars, but remember the fine detail and stars in the final image will come from the Luminance image anyway. The RGB only “paints” the image with color.
Paste the original H-alpha image onto the new RGB. Align the image if necessary. Change the layer blending mode to Luminosity and set the Opacity to 75%.
Below is the HaRGB composite. It starts out a bit pink, but this will be easily corrected without the problems of the normal HaRGB composite method.
Above: HaRGB composite
Fine Tuning the Image
There are many processing techniques that can be used after the composite has been made, but there is one technique that works especially well before flattening the layers in the HaRRGB image: Curves.
Select the bottom RGB layer and open the Curves window (Ctrl-M). Raise the center of the curve to enhance the color saturation and faint detail in the image.
Then select the top HaR layer and again open the Curves window. This time, lower the curve.
These changes work very well to enhance the HaRRGB image and bring out the true colors.
Above: Method 1 on top, Method 2 on the bottom. Click each for larger images.