How to remove Star Trails

Tips and tricks, and maybe even a few samples.
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Posts: 37
Joined: November 10th, 2009, 9:16 am

How to remove Star Trails

Post by mcaramb »

Without a sidereal tracking scope, long exposures will of course cause star trails to appear. Even with a tracking scope, if you track something moving faster than the sky (ie: Comets) you'll get these same streaks of starlight. So, I've come up with a pretty nifty way, using just Photoshop, to REMOVE star trails back to the original star's shape, intensity and location, including the ones superimposed in the comet halo.

EXAMPLE: Let's work on Ben's wonderful Hartley2 Image he sent a few weeks ago. He got a great shot of Hartley2, but the startrails made it hard to discern what part of the sky it was in, and they obscured much of the comet's halo.

HERE'S WHAT IT LOOKS LIKE: Before and After Processing
hartley2enhanced.jpg (63.09 KiB) Viewed 12971 times



1. Open Image in Photoshop

2. Double-click the default Background layer and name it "Starfield"

3. Duplicate the "Starfield" Layer and call it "Comet Only". Put it on top.

4. In the "Comet Only" Layer, make a rough selection around the comet and delete out the area outside of the selection

5. Carefully CLONE out the remaining star trails superimposed in front of the comet using the CLONING tool. Be very care not to disturb parts of the image that are NOT the startrails you are removing. This part requires a bit of artistic skill.

6. Duplicate the "Comet Only" layer and name it "Comet Mask", put it between the "Comet Only" and "Starfield" Layers.

7. Turn off the top-most "Comet Only" Layer, but do not delete it (You'll need it later)

8. Change the "Comet Mask" layer mode to "Difference". Like magic, the comet will disappear, leaving behind only the startrails!!

9. Select the "Starfield" and "Comet Mask" Layers and merge them. Now you should just have "Comet Only" and "Starfield"


1. Rotate the entire canvas of layers so that the trails are completely horizontal (This means that only straight-line startrails will work with the following procedure... sorry!)

2. Add a layer called "Black" at the bottom of your layer stack, and fill it with a black color sampled from the "Starfield" layer.

3. In the "Starfield" layer, select the most typical length star trail with the rectangular marquee tool. Only go end-to-end when selecting the startrail. Do not add space to either side unless it is an equal amount on both sides. Only select ONE startrail at a time.

4. Select Edit->Free Transform.

5. Make sure the LOCK is not checked between Width & Height in the Transform attribute window. Adjust just the Width Percentage until the startrail returns to a nice point. Remember this Width Percentage setting. Apply.

6. Now comes the time consuming bit. You need to go to each an every startrail INDIVIDUALLY and apply the same procedure as outlined in steps 3-5. Do not attempt to select a bunch of startrails at once. This would indeed reduce them to dots, but their positions in relation to one another WOULD NOT be maintained. I am still looking for ways to automate this, but it involves resizing just negative space, leaving the stars alone, which I haven't found a way to do yet.

Because you need to repeat this with some forty-odd stars, it GREATLY speeds things up if you record the transform procedure into an ACTION, then just apply the action to each selection. This way you don't need to keep going to the attribute box, or drag by hand. As each startrail will have EXACTLY the same scale percentage as the first star you did, so you can hardwire the scale percentage into the action and apply it to each star.

If you encounter a star trail which is longer than the average, it means that more than one star is in the trail. I just start on the left of the trail and select the common width and apply the action, then move to the next bit until the whole thing is gone. Yeah, it's a guess at best, but it doesn't happen often.

7. Merge the "Black" and "Starfield" Layers, leaving just "Starfield" and "Comet Only"

8. OK, now that all the startrails are points, use "Levels" to bring their intensity up a little. I do this using the LEVEL's RIGHT slider by dragging it to the left. Use the LEVEL's MIDDLE slider towards the right to bring down mid-ground noise and other artifacts, leaving just the star points.

9. Apply a .5 guassian blur to make the stars more rounded than squarish and it softens them a bit.


1. Turn the "Comet Only" layer back on.

2. Change it's mode to "Lighten Only"

3. Duplicate This layer, Calling the new one "Comet Color". Put it on top and select it.

4. Change "Comet Color" layer mode to "Color Only"

5. Give it a 2 guassian blur. This will smooth out some graininess.

6. Set it's opacity to 65%

7. Hold down OPTION and select the "Comet Color" layer so the selection will grab only the solid areas of the layer.

8. Fill the selection with the color of your choice, chosing "Color Only" in the FILL options box.

9. I think I added another layer with a yellow radial blend to give the center of the comet some color as well... all this colorization stuff is totally arbitrary.

10. Flatten Image and rotate back to original position and re-crop. VOILA! All done!

There is, of course, much margin for error in this highly subjective procedure, especially if two stars were smeared together along the same line, forming one large line. In such case, you simply have to guess where one star starts and the other ends inside the line.

Also, nothing can recover information "hiding" behind a startrail in the comet's halo. When those stars are reduced back to points, the area removed needs to be filled using the cloning tool. Also, adding arbitrary color to a B&W image of course is a venture of a completely artistic nature.

All in all though, I'm very pleased with the results. It's not an automatic procedure; it's a pretty hands-on, manual operation, but it's not as time consuming as it seems. To give you an idea, Ben's image took me about 15-20 minutes to process.


Christopher K.
Posts: 5988
Joined: October 12th, 2009, 3:28 pm
Location: Baton Rouge, LA

Re: How to remove Star Trails

Post by Christopher K. »

The above instructions are of course for those who have already a basic to intermediate understanding of astrophotography.

Chris Desselles, the current Vice President of the Baton Rouge Astronomical Society, will give two talks on astrophotography in February at HRPO. The intermediate talk will be on Monday the 11th at 7pm, during that month's BRAS meeting. The beginners' talk will be on Friday the 15th at 7:30pm. Both talks are open to the general public, have no admission fee and are aimed at a general adult audience.
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