The 'Star Separation Technique': When to Use It Correctly, and Common Fallacies

In discussions of astro image processing, the “star separation technique” is a term that comes up often but is just as often misunderstood. I went out of my way to look into it, wanting to figure out what it actually is and how to use it correctly.

What the Star Separation Technique Really Is

Getting to the root of it, the so-called “star separation technique” is really just a technique in Photoshop that uses a “starless image” to blend and thereby enhance color. Many of those beautifully colored nebulae we see from experts and fellow hobbyists were, in fact, processed with a similar method. In essence it is a means of enhancing color, not some magical, exclusive secret art.

Common Misuses

The problem is that quite a few hobbyists use this method as a “cure-all for avoiding damaged stars,” which actually misunderstands what it does:

• It cannot save stars that are already bloated. If the stars became too bloated during shooting or the early stages of image processing, this method cannot bring them back.
• Using it too early sacrifices nebula detail. If you separate out the stars at the very start of the workflow, a lot of nebula detail may vanish along with the stars before it has even been processed out.
• Blending with Luminosity is problematic. I have also seen some online tutorials that use luminosity for the blending; this usage easily causes the stars to lose their color, and is not ideal.

The Correct Time to Use It

As I understand it, the correct time to use this method is: when post-processing is nearly complete and before star reduction has been done, and without using luminosity to blend, so as to avoid the stars losing their color.

Steer Clear of Galaxies

One last thing I especially want to warn about: this method does not apply to galaxies. Used on a galaxy, there is a very high chance it will damage the galaxy. Even for targets like dwarf galaxies (companion galaxies), it is in fact just as unsuitable. In other words, the star separation technique is a technique in service of nebula color; when you run into a galaxy, please think twice before acting.