An astroimage that has white stars is fine, but we all know that stars come in many colors. If we can create the same picture with a background of beautifully colored stars, it becomes magical — and it takes our imaging to a whole new level of technical excellence.
The problem with stars is that some of them are much brighter than the deep-sky target we want to record. So, when we stretch our image for the object, the stars blow out and become pure white. What we need to do is to stretch the stars in a way that preserves their color. Here’s how I do it.
Image 1: To stretch the image data, the author creates a color curve that curves within the shadowed areas but remains flat in the ranges of midtones and highlights. // All images: Tony Hallas
1. I begin with the unstretched 16-bit RGB TIFF file. I need to stretch this data in a way that the shadows come up but the highlights remain compressed, so I create a color curve line like you see in Image 1. To do this, open “Curves.” What you will see is a flat line. The image is “resting,” but you are going to disturb it. Any time you raise the curve, it lightens the image, and where you raise the line dictates what part of the image you are brightening. In this case, we are trying to brighten the shadows (the area in the lower left on the curve) and leave the midtones and highlights untouched. To leave them resting, keep their parts of the curve a straight line.
2. I repeat the previous step as many times as I think it is necessary, using “Levels” every now and then to reset the black point. I also increase the saturation 20 points as often as the image can absorb it without the stars pixellating. In the end, I am looking for an image where the stars have colorful bodies and I can see the background.
Image 2: Combining layers in Photoshop‘s “Soft Light” mode lets you push the colors of stars while at the same time darkening the background.
3. It’s all right to leave your image a little on the light side; our next step will be to make the stars even more colorful and to darken the background. That will make it easier to transfer the star color to the blown-out stars in our normally stretched deep-sky image. Push F7 to open your “Layers” palette, and make a duplicate layer. Combine in the “Soft Light” mode (Image 2). Immediately, your background should have darkened and the stars should have become more colorful. Do this one more time by duplicating the duplicate layer, and the background should be almost black with colorful stars. Then flatten the layers.
4. We now want to spill some of the colorful edge of the star into the lighter, less colorful nucleus. For the smaller stars, go to “Blur,” then “Surface Blur,” and put in a radius of 1 or 2 and a threshold around 125. Click the “Preview” button on and off to see how this is smoothing out the stars and mixing the colors into their centers. For larger stars, select an area around the stars and use a radius of 5 to 10.
Now we have some colorful stars to add to the washed-out stars in the normally stretched image. Because these stars all came from the same master file, registration is not an issue.
Image 3: Pasting the colorful stars (middle) atop the washed-out ones (left) does two things. It preserves the brightness of your original non-stellar target, and it enhances the colors of the surrounding stars.
5. The final step (Image 3) is to combine the colorful stars into the washed out image. Open both images and use “Select,” then “Color Range” at a strength of 30 to 40 to select all the colorful stars in the second image. Do this by clicking on the “+” eyedropper after your first selection. Then hit Control–C to copy these, close the image (leaving the first image open), and hit Control–V.
The colorful stars will now paste on top of the washed-out stars as a layer. Align them. You now can adjust the density, saturation, and transparency of the layer to fit. You also can slightly blur the stars to make a smoother transition between them. For a slightly different effect, try changing the “Combine” mode from “Normal” to “Color.” Sometimes this creates a better look. Either way, you now will have the best of both worlds — a deep-sky image with colorful stars.