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.

Preparing the Image Files

You should already have an RGB color image (usually created in another program such as MaxIm DL) as well as a Luminance image.  The Luminance file can be any number of different images:  it is usually an image taken through a clear filter, but for a nebula it is sometimes an image taken through a red filter, or for a galaxy it might be an image through a blue filter.  For the ultimate in image detail for an emission nebula, a Hydrogen-Alpha filtered image is often used as a luminance layer.  This is a more advanced technique and is covered in the Advanced Processing Techniques section.

Open both image files in Photoshop.  The images of galaxy M82 below are an RGB composite made in MaxIm DL (see section on RGB images) and a black and white image which is a composite of three red-filtered exposures (median combined) to bring out detail in the gas clouds erupting from the center of the galaxy.  Sometimes this is termed an RRGB instead of an LRGB to denote the use of the red-filtered Luminance image, but the process is identical.

Above:  RGB image of M82

Above:  Red-filtered image of M82 to be used as luminance layer

You may find it helpful to align the two images with each other using a program such as MaxIm DL before transferring them to Photoshop, especially if they were taken on different nights.  If all the images were obtained together, you may only need to shift the image to align it which is easily done in Photoshop.  Any fancier alignment techniques such as scaling and rotating are best done in another program.

Compositing Images

Make sure the Layers window is open.  If not, select Window > Show Layers from the top menu (keyboard shortcut F7).

Be sure the luminance image is the active image and select the entire image by pressing Ctrl-A.  Select Edit > Copy or press Ctrl-C.

Select the RGB image as the active image and paste the image by selecting Image > Paste or by pressing Ctrl-P.

Tip:  You can simplify this process somewhat by opening the images, selecting the Move tool (keyboard shortcut V), and dragging the luminance image onto the RGB.  You will have to align the images afterward.  If the images have already been aligned in another program, use the first method to keep from having to align them again.

This will put the luminance image into a new layer on top of the RGB image.  This is shown in the Layers window although currently only the black and white image is shown.

If the images need to be aligned, use the following method.  In the Layers window, change the Opacity to about 50%.

Select the Magnify tool and zoom in until the offset between the two layers is easily visible.

Above:  Offset images of M82.  The L layer needs to move down and to the left to match the color layer.

To align the overlaid image with the RGB image, select the Move Tool from the toolbar (keyboard shortcut V).

Using the arrow keys on the keyboard, shift the top image until it aligns with the RGB image.  Return the Opacity in the Layers window to 100%.

In the Layers window use the pull-down menu to change the blending mode to Luminosity.

This will combine the L channel with the RGB channels to create the final LRGB.

Above:  The final LRGB composite of M82.  Compare to the original RGB at the top of the page.

At this point you can adjust and enhance each layer independently which allows you more control over the final appearance.  (See the Aesthetic Image Adjustment section for more details.)

To finish the image, select Layers > Flatten Image, or choose Flatten Image from the pop-up menu in the Layers window.

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