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RGB Star Overlay

I have developed this technique for adding correct star color to narrowband tri-color images. This works best for nebulae that are basically red when imaged in RGB light. The general concept is to turn all the stars white with PhotoShop's color noise reduction routine, then reduce the "white" brightness and overlay the RGB stars.

Although there are several was to overlay stars, I have found this to work reasonably well.  Here's the details. 

Figure 1 shows typical false star colors in a narrowband color mapped image. In this case, there are may incorrect cyan stars. Often, the image will be dominated with magenta stars. This process will work for all cases.

In Figure 2, the stars are now generally all white. PhotoShop's Reduce Color Noise was used to gain this effect. On the Reduce Noise Basic Screen, set the filters "strength" to zero, "reduce color noise" to 100% and "sharpen details" to zero and press OK. The noise reduction routine seems to identify the stars as color noise and will set the stars to white. You may have to re-execute the filter a couple of times to get them really white. Press "Ctrl+F" to run the routine again.

In Figure 3, the intensity of the white stars has been reduce by using PhotoShop's Image|Adjustments|Selective Color routine. Select "White" in the color drop down box. Then reduce the brightness of the white by moving the "Black" slider to the right, probably all the way, and click OK.

Figure 4 is an RGB image to be used as the source for the star colors.

Figure 5 is the RGB image with the lightness of the red data reduced. This can be done with the Image|Adjustments|Hue/Saturation or the Image|Adjustments|Selective Color function. You may also want to reduce the lightness of any other colors that are not correct for the stars.

The final step is to paste the stars, Figure 5, on top of the narrowband tri-color image, in this example, Figure 3. In the Layers Window, make Layer 1 active and select Lighten for the blend mode, with the Opacity at 100% and only the colored stars will be added to the background image. Flatten the image and you are finished. See Figure 6 for the final result.


Fig. 1


Fig. 2


Fig. 3

Fig. 4

Fig. 5

Fig. 6