Photoshop can give you more control than a program such as MaxIm DL over how you combine LRGB images.  You can use layers to combine a Luminance image with an RGB color image. Ideally, the two image will already be aligned using a registering program like MaxIm DL.

Begin by opening both the Luminance and RGB images in Photoshop. The Luminance image should be stretched already in MaxIm DL (or similar CCD processing software) to show as much detail as possible. The RGB image, on the other hand, should be stretch to show the full dynamic range of the image. In the case of this Horsehead Nebula image, the faint nebula detail is almost invisible because the image has been stretched to prevent the bright stars from being washed out. We will be able to bring out the full range of color later.

Highlight the Luminance image and press Ctrl-A (or go to Select > All) to select the image. Then highlight the RGB image and press Ctrl-V (or go to Edit > Paste) to paste the Luminance image as a new layer on top of the RGB image.

Change the layer blend mode to Luminosity by selecting from the pulldown menu at the top of the Layers palatte.

Turn off the visibility of the top layer by clicking on the eye icon next to it in the Layers palatte. Then highlight the bottom (RGB) layer as shown below.

Go to Image > Adjustments > Curves (or press Ctrl-M) to open the Curves window. Stretch the center of the curve upward, then stretch the right side of the curve downward, as shown below. This brightens the main portion of the image while keeping the brightest parts from washing out.

Repeat this curves procedure again, to enhance the picture once more. The image will appear as below.

Go to Image > Adjustments > Shadow/Highlight. In the Shadows/Highlights window, click on More Options to expand the window. Bring the Shadows slider over to 50% and the Highlights slider down to 0%, as shown below. Bring the Color Correction slider up to 100%.

The RGB image will look a bit hideous at the moment, but hang in there.

Go back to the Layers palatte and turn the visibility of the top (Luminance) layer back on. Keep the bottom (RGB) layer highlighted so you are still operating on that layer.

The image should now show a lot of detail and some pale color.

While the bottom layer is still highlighted, open the Levels window by going to Image > Adjustments > Levels (or press Ctrl-L). Bring the black-point slider (left slider) up to the toe of the histogram curve as shown below. It is important that you do NOT move the slider past the left edge of the histogram curve, as you will lose data if you do.

Now highlight the top layer and open the Levels window again. Repeat the same procedure by bringing the black-point slider up to (but not beyond) the toe of the histogram curve.

Now the image should show more prominent colors.

Go to Layer > Flatten Image to combine the two layers. You should see only one layer in the Layers palatte.

To enhance the colors further, go to Image > Adjustments > Hue/Saturation. Bring the Saturation slider up 20-30 points. Try to avoid over-saturating any of the brighter portions of the image.

Finally, you can apply the usual aesthetic image adjustments, as described here, to finish processing the image. The final Horsehead Nebula LRGB image is below.

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