January 2013: Doing a little more work can turn an ordinary astroimage into a knockout.
By Tony Hallas |
Published: November 26, 2012 | Last updated on May 18, 2023
Let’s say you just imaged some obscure nebula in narrowband under the Full Moon. For our purposes, we’ll work on NGC 7822 using a set of 5-nanometer (nm) Hydrogen-alpha (Hα), 5nm Sulfur-II (SII), and 3nm Oxygen-III (OIII) filters. These three choices constitute what imagers call the Hubble palette because the famous space telescope’s images often use the same filters.
Photo 1: This image of NGC 7822 looks fine with the standard Hubble palette, although some might consider it a little bland. // All photos: Tony Hallas
You’ve already stretched the data, converted it into visible colors (Hα mapped to green, SII to red, and OIII to blue), and combined them using layers and clipping masks in Photoshop’s “Lighten Mode.” This allows you to adjust the strength of each individual contributor for maximum effect. At this point — before adding RGB star colors — you probably have something that looks like Photo 1.
You might be content with this result, but it likely looks a little bland. Wouldn’t it be nice if we could somehow make the colors more lively and, most importantly, get them to appear more distinctly to show a better representation of the narrowband signatures? As it turns out, we can. All it takes is two basic Photoshop techniques: converting to “Lab Color” to saturate the colors and using “Selective Color” to emphasize their differences.
Photo 2: Running the image through Photoshop‘s “Lab Color” to increase color saturation helps make the colors more vivid, but one step remains to separate them more.
First, open your 16-bit TIFF photo and go to “Image” > “Mode” > “Lab Color.” Photoshop then separates your image into its luminance and color components. Use an “S” curve in “Curves” (see my February 2012 column) to increase the color saturation of your image. Then, convert the image back to RGB. Note that the amount of saturation will be equal to the intensity of the “S” curve that you generated in “Curves.” The result should look something like Photo 2.
Now, here is where you get to use some poetic license. In this case, we want to use the Hubble palette but at the same time create as much difference as possible between the colors. Open your post-“Lab Color” photo and select “Image” > “Adjustments” > “Selective Color.” After taking a good look at the basic colors of the image, you can emphasize and “cultivate” each one. Here’s what I did:
Photo 3: By using “Selective Color,” you can tweak and “cultivate” each of your photo’s basic colors, resulting in a much more dramatic image.
CYAN = cyan +40, magenta –40, yellow –40, black –10. YELLOW = cyan –40, magenta +0, yellow +40. BLUE = cyan +20, magenta –30, yellow +40, black +10. GREEN = magenta –40, yellow +40, black –10. MAGENTA = cyan +20, magenta –20, yellow +20, black +10.
These settings affected the targeted colors, making them stronger and removing any spurious colors I didn’t want. The final result is Photo 3.
Remember, narrowband imaging is what you make of it. I hope this helps you free your imagination.
