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Proper luminance

Astroimager Adam Block explains how to decide how bright to make your luminance image.
RELATED TOPICS: ASTROIMAGING
AdamBlock_2013
No other processing step is so fraught with pitfalls as the decision of how bright to make a luminance image before adding color. One thing is certain: If you impulsively brighten it to make it look good on your monitor and then attempt to add color, the end result is not pretty. Properly preparing a luminance image takes planning and the restraint that small incremental steps require.

The main issue is that the electronics in today’s digital cameras record many thousands of discrete brightness levels. A 16-bit camera records 65,536 brightness levels. However, software maps this range of values into the 256 (8-bit) grayscales that computer monitors display. From the original data, a screen displays a white point, black point, and the translated grayscales in between.

No single correct choice for this screen stretch (or scaling) exists, but when you save this image, you’re committed — there’s no going back. The key is to make certain all of the important features in the luminance are mid-gray in value so that the color will blend in easily.
Color saturation and luminance
1. The bottom bar of color shows how the grayscale values blended with color vary in saturation. To the right side of the bar, above 200, the color fades.
All images: Adam Block
These images show why. Image #1 shows a grayscale line, representing the luminance image, beneath a line of solid color. The bottom line is the result using the “Luminosity” blending mode in Photoshop.

Note that as you go from left to right along the blended line, the color eventually fades and at the far right is nearly white. This is true for any color, which means that an overly bright luminance image will not have good color contrast.

If the grayscale line represents values from 0 to 255, at what value does the color start to significantly fade? Conservatively, the last fifth of the bar looks quite faded. So, grayscales that are approximately 200 or greater are in the “danger zone” of being challenging to colorize.
RGB values
2. Near the core (#1), RGB values of 204 will be OK for colorizing this bright area. Other areas (#2) should have values less than 200. The author chose 115 for this image. A background value (#3) of 15–20 is good; here it is higher (he chose 29) due to the galaxy’s halo. Bright stars (#4) clip at 255, the value here. Stars are not as important to color as galaxy features.
In Image #2, I labeled four elements (areas) of a brightened luminance image I captured of the galaxy NGC 3675. Note that some features have values greater than 200; however, I don’t judge them as “important.”

Neither bright stars nor the stellar nucleus of the galaxy should be the metric for determining how bright to make the image. Instead, you will want to monitor the galaxy’s features in bright areas like the core, spiral arms, and nebulae.

I brighten my luminance image in CCDStack because it reports the bitmap grayscale values, and I can use the real-time adjustment while I measure the image. You also can accomplish this feat in any image-processing software as well as in Photoshop using the eyedropper tool with “Curves.” Doing so will determine the white point for your image.
NGC 3675
The author’s resulting color picture of NGC 3675 incorporates the initial luminance adjustments he describes. To view the full image online, go to the author’s website at http://skycenter.arizona.edu/gallery/Galaxies/NGC3675.
Do not forget to set a black point, and do not make the sky black. I generally make the sky gray with values around 15. This will leave enough overhead to permanently set the black point near the very end of your image manipulations.

Finally, if you brightened the image outside Photoshop in another program, you need to save your image with these new permanent screen stretch values. Image-processing programs have different language for this, including “Auto-Stretch” and “Create Scaled Image.”

Save the image as a TIFF file, and you can then open it in Photoshop with the confidence that once you brighten your color layer, the luminance image will respond accordingly. Taken together, they will color the scene realistically and just as you imagined.
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