January 2012: Here's how to deal with light pollution on your images.
By Tony Hallas |
Published: November 28, 2011 | Last updated on May 18, 2023
Bright stars near celestial targets are the nemesis of optics and CCD cameras alike because they create flare. Flare has two basic components — the light “smear” you see as a glow around the star, and the central part of the star that typically looks pure white. If you stretch your image properly (see my November and December 2010 columns), however, and do not clip the highlights, you’ll find a lot of detail buried under that flare that you can recover. Here’s how I do it.
The Lost in Space Galaxy (NGC 6503), which glows at magnitude 10.2, suffers in this image because of the flare caused by the magnitude 8.5 star SAO 8937, which lies 4′ to the east. All images: Tony Hallas
1. To begin the process, select Photoshop’s “Elliptical Marquee” tool, hold down the “Shift” key, and make a circular selection about half the diameter of the light smear. Move it to center on the star.
2. Once you have the selection in place, “feather” it 30 to 50 pixels. (I discussed feathering in the January 2011 issue.) How much depends on the size of the star and the image scale. Try several settings. When you apply the feathering, you still want to see the feature I call the “marching ants” outside of the white core of the star.
One of the steps that will let you tame the bright star is to make a selection around it and “feather” that area using Photoshop. This image shows the region after feathering.
If you feather too aggressively, the ants, which indicate the middle of your feathered selection, will disappear. This means the selection has ballooned past the outer boundary of the star. So, when you darken the flare, you will be darkening the background as well. If you do this a few times, you will see an ugly, dark, diffuse circle around your star, meaning your darkening bled past the star’s outer flare.
3. Next, check that you’re in 16-bit mode. Open up “Curves” and pull straight down on the center of the curve. Note how the light smear darkens and the star appears to shrink. Ideally, the feathered selection will be such that it includes all of the outer parts of the smear. If it doesn’t, go back and make the selection larger.
After multiple selections, featherings, and using the “Curves” function to darken the area, you will have reduced the star’s glare noticeably.
4. Repeat this using different selection sizes. Be sure to always shrink the flare and be careful not to over-darken the sky just outside the circle. You control this by choosing where you make the selection and how much you feather it.
5. Once you have subdued the light smear to your satisfaction, the star will appear as a hot core. Now it’s time to shrink the core. You’ve been in 16-bit mode to use Curves. Convert to 8-bit mode to use the “Distortion Filter.” Make a circular selection (like in step 1) just outside the star’s core and feather it about 10 pixels.
As your final step, use a negative setting for the “Spherize” function (under the “Distort” menu in “Filters”) to reduce the star’s core size.
6. You’re getting close. Now go to “Filters” and select “Distort,” then “Spherize.” Typically, you would use the Spherize filter to turn something into a sphere and, in the process, make it bigger. But you also can use this filter in the negative mode to make the sphere smaller. With your star selected, move the slider to the left until you have a value around –60. Note how the core of the star got smaller. You should experiment with the selection, feathering, and amount of negative spherizing to find what works best for your star. Do not overdo this last step, no matter how tempting — if you do, your star will look suspicious.
The author’s final corrected image shows fine detail in NGC 6503 even in the regions of the galaxy near the star.
Using this method, you can fix the two negative elements of a bright star — the outer flare, or light smear, and the hot, bright core. The result will be more attractive images that show more detail.
