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Color space

Astroimager Adam Block describes how to work in Lab color space to enhance the blues, yellows, and magentas of your images.
RELATED TOPICS: ASTROIMAGING
AdamBlock_2013
During workshops I deliver at the Mount Lemmon SkyCenter, few processing techniques elicit verbal “Wows!” from participants. No matter how important standard deviation sigma rejection techniques may be, they just do not inspire quite like the technique of enhancing color you’ll see in this article. We will work in the “Lab” color space to bring out the blues, yellows, and magentas that are common in celestial images.

In this color model, L represents the luminance (brightness) of a value. The A and B channels model the way our visual system works.
Image 1
#1. The "Lab Color" mode is in the "Image" menu. Note that the current mode is RGB.
All images: Adam Block
Although we see red, green, and blue light with our eyes’ cone cells, our brains also process differences in the amounts of color we see because there is some overlap in the colors detected by each cone channel. This is called the “opponent process” color theory, and it explains a number of perplexing things related to vision.

Certain colors cannot be mixed. Greenish-reds and yellow-blues don’t exist. These are color opposites in this theory. You can put this to practice by staring at something red (on your computer screen) for a few moments and then looking at the object’s afterimage against a white background — it will appear green!
Image 2
#2. First click on the B channel, and then click the eyeball of the "Lab" channel to display the image.
The A channel, then, is a color space that includes red and green information, and the B channel includes blue and yellow. In Photoshop, changing the contrast of the A or B channel allows us to operate on two colors simultaneously. This can lead to some pleasing effects. For most astroimages, the B channel is the most important, which is easy to remember since the B channel contains blue information.

To begin, change the color mode of an RGB image by looking in the “Image” menu and changing the “Mode” to “Lab Color” (see Image #1). Next, select the “Channels” tab, and make the B channel and the “Lab” channel visible (see Image #2). Now open “Curves,” and change the “Contrast” of the B channel without changing the overall color balance.
Image 3
#3. Using "Curves," click on the center of the line. Then with a second point, pull up or down on a part of the curve, and it will flex through the first point.
To do this, create an inflection point by clicking on the center of the diagonal line (the input and output values will be equal), and then create your curve (see Image #3). If your curve has the same shape as mine, you will see a transformation (see upper right image). What was once dull blue will become vibrant azure, and muddy reds will become more luminous yellows.

Adjusting the B channel is especially effective for spiral galaxies and certain types of reflection nebulae. You can adjust the A channel in a similar way to enhance reds and magentas of an image. This is good to know for enhancing emission nebulae.
NGC 6286
This frame of NGC 6286 shows the difference between before (left) and after applying a contrast adjustment to the B channel. The author pushed the effect a bit more than normal in order to make sure it showed well in print. You’ll find a high-resolution version of NGC 6286 at http://skycenter.arizona.edu/gallery/galaxies/ngc6286.
But be careful! The opposing color in this channel is green, and introducing green into an image can make it look awful (with the exception of some planetary nebulae).

After making your adjustments, convert the image back to RGB color space. If you forget to convert to RGB, you will notice that many Photoshop tools will be unavailable because they do not work in “Lab” mode. In my next column, we will investigate rejection techniques and see if there is any fun in them.
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