LRGB Image Acquisition and Processing

John A McCubbin

Principles

1. Image depth (pixel saturation) is easier to reach in a monochrome image
2. Image sharpness is generally better in a monochrome image
3. A quality monochrome image must be obtained as all of the sharpness, brightness, and contrast of the LRGB image is obtained from the monochrome image
4. The quality of the RGB image needs to revolve around color saturation, not necessarily sharpness
5. LRGB combining is best done in Adobe Photoshop v4.0 or later

Background

Before you read this article you should be very familiar with image stacking and alignment. If some principle seems vague as you read this, you should review the Image Stacking and Blending, Why and How article.

LRGB image acquisition and processing came about from a desire to present "prettier" deep sky CCD images. It is definitely "less scientific" in that LRGB processing makes all stars appear white, since in the monochrome image they are almost all supersaturated. Photometric data is lost. The trade off is that images come out more aesthetically pleasing. In my opinion, it's a good trade off. I want CCD images that interest the viewer and make nice web pages! If I want to do photometry, I'll do it in monochrome later, but for now I want images that are pleasing to the eye.

LRGB processing requires a very high quality monochrome image. It needs to have adequate image depth and sharpness. When you LRGB combine, the brightness, contrast, and sharpness of the LRGB image comes from the monochrome image you layer into it. Only the color information comes from the RGB image layer. In fact, the only requirement of the RGB image is that it contain enough color information to adequately represent the object. It doesn't have to be a sharp image at all. High quality RGB images have always been difficult because filters in front of a CCD camera have always decreased the chip sensitivity (especially for blue). Noise is a problem due to the poor image depth of the color frames. As you will see later, noise can be all but eliminated by purposely blurring the image. This doesn't affect the final image because only the color information comes from the RGB image.

Image Acquisition

How to acquire a good monochrome image is covered in the article, Image Stacking and Blending, How and Why. Suffice it to say, I spend most of my time in LRGB acqusition getting my "monochrome" component. For a dim galaxy, I may acquire and stack 6-10 ten minute images. I want a lot of image depth to work with when it comes time to manipulate the monochrome component. I generally take two each through a red, green, and blue filter. If the object is very dim, I will actually change camera resolution and image with the camera binned 3x3, so that the colors can be acquired at a greater image depth (see M33, I used that technique). The image can be resized later to "overlay" the monochrome image. Each image must be carefully dark subtracted and flat framed. I recommend averaging at least 8 flat frames for the monochrome image, whereas a single flat frame is sufficient for the color images. Once the monochrome and three color images are acquired and calibrated, they are ready for combination.

LRGB combining

I RGB combine in MaxIm DL. It allows image alignment to a sub-pixel level and can be done visually for confirmation. Monochrome images are combined in the same way. I then end up with two master files. A "luminance" image (the monochrome image) and the RGB image (color combined image). I name the files for clarity, i.e. M27lum.tif is the luminance image. Any fancy stretching you do should be done in the 16-bit mode prior to converting the image to allow maximum flexibility. (After all that's why you use a 16 bit camera in the first place!)

Step 1: Open Photoshop and then open your monochrome and RGB images. Choose the RGB image by clicking its title bar (I click title bars instead of images because invariabley I've activated some tool that will alter the image). If the Layers Palette isn't visible make it so by using the Windows > Show Layers menu selection. The Layers Palette should look like the figure below. The RGB image should be the "Background".

Step 2: You will notice that at the bottom of of the Layers Palette there is a icon that looks like a piece of paper with the lower left corner folded up. This is the "New Layer" button. Click that and a new layer should be added above the "Background" layer. It will be blank, and nothing should appear different in your RGB image. It will be named Layer 1.

Step 3: You should now choose the monochrome image by clicking it's title bar. You now must "select" the whole image for copy (you're going to paste it into Layer 1 of the RGB image). To do this use the Select > All menu choices. The dotted line selection border should be around the entire image. Now to "copy" it use the Edit > Copy menu choices. The monochrome image is now in the hinterlands of the Windows clipboard for later retrieval.

Step 4: You should now make the RGB image active by clicking on its title bar. Make sure that Layer 1 is active in the Layers Palette. If it is active the paintbrush should be visible in the checkbox next to the thumbnail image and the title bar should be blue. You now paste the monochrome image into that layer with the Edit > Paste meun choices. You should immediately notice that your RGB image now only shows the monochrome layer. That is because the layer is currently displayed as a normal image. Note that the drop down list box in the upper left hand of the Layers Palette says Normal.

Step 5: You must now align the images. You do this by first reducing the opacity of the monochrome image. To accomplish this, click on the right arrow of the opacity field. You will be presented with a slider bar. Reduce the opacity to around 40 percent. Magnify the (now crude) LRGB image with the magnifier tool to 400% normal size (or smaller if you desire). Select the Edit > Free Transform menu options. You then may nudge your monochrome image with the keyboard cursor keys and align the stars of the monochrome image with those of the color image with immediate visual update. When you are finished, press the Enter button on your keyboard to update and finish the alignment. Return the opacity slider to 100% opacity for Layer 1. Your image should now look like a monochrome image.

Step 6: Here's the magic. Now that everything is aligned, you are ready to convert the monochrome to a luminosity layer. Simply select the dropdown list just to the left of the opacity field and select luminosity. You should see a beautiful color image magically appear before your eyes. Viola! An LRGB image. The layers palette should look like the one below.

You now have a complete LRGB combined image ready for final processing.

Post Combination Processing

Once combined, you may still process color and monochrome information with all the power of Photoshop and get immediate visual feedback in the image. You have to be very careful to make sure the layer you want to manipulate is selected. For instance, if you want to maniuplate the colors of the image, select the background (previously the RGB image) for manipulation. If you want to adjust overall brightness, contrast, or gently unsharp mask the image, then select the monochrome layer, Layer 1 in the example above.

Colors can be brought out by adjusting the levels of color in the Image > Adjust > Levels menu. This brings up a dialoge box that allows you to manipulate each color individually with respect to brightness, contrast, and gamma (via the slider button). Again you receive immediate visual feedback on your trial and error adjustments. Final color balance can be adjusted via the Image > Adjust > Color Balance menu.

Noise and gamma ray hits are frequently a problem in color frames. You will notice bright areas of pure color on the images you take. These are gamma ray hits that appear as solid color because they supersaturate the pixel only in the color frame in which they hit. Noise is apparent as graniness in the color. A fast way to eliminate most, but not all, of these sources of image degrading noise, is to blur the color layer. Because the sharpness comes from the monochrome layer, the sharpness of the LRGB image is almost totally unaffected, only the noise eliminated from the color layers. Colors appear more even. You may use the Filter > Blur > Blur, the Filter > Blur > Blur More, or the Filter > Blur > Gaussian Blur menu options depending on how much you wish to blur the color layers. Experiment with this to get the best result for your particular image.

At this point, experimentation is the key, so save often and take notes! If something works well you'll want to remember it later.

Does it work?

You decide...

Personally, I like the image behind door number three ;-)

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I consider your heavens, the work of your fingers, the moon and the stars, which you have set in place ... Psalms 8:3