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What to do about satellite trails

March 2012: Long, bright lines through a high-quality image aren't fun. Here's how to eliminate them.
RELATED TOPICS: ASTROIMAGING
tony_hallas
Satellite trails have always been a big problem for astroimagers. The reason is that any “outlier suppression” (a general term that means eliminating stuff that doesn’t belong in the picture) strong enough to eradicate a bright satellite trail might also remove parts of the image you do want. I try to keep outlier suppression to the bare minimum for this reason and in current images mainly use it to attack cosmic-ray hits.

Because I’m careful how I use outlier suppression, there will always be a few satellite trails crossing my image. In the past, I had to laboriously clone them out by hand. Not any more.

The developers of Photoshop CS5 added a new and important feature called “Content Recognition.” (Note: Photoshop CS5 is the only version to have this.) In a nutshell, Content Recognition allows some existing functions to look around them and import this data detail into the utility. What we are specifically interested in is “Content-Aware Fill.” Here’s how to do it.

From your menu of tools on the left, select the third one down, which is the “Lasso” tool. Right-click on it (or Control-click if you have a Mac), and you will see the “Polygonal Lasso” tool. Left-click on this tool to choose it.
Satellite-trail
A bright satellite trail goes through this segment of galaxy cluster Abell 2151. Note that it bisects a star (above center) and a small galaxy that lies to the star's right. Both images Tony Hallas
2  Go to one end of your satellite trail and, just beyond and to one side of it, left-click the tool. A little circle will appear next to the tool showing that you established an anchor point. Once you make the anchor point, drag the mouse along the satellite trail to create a selection line.

3  Keep dragging the mouse until you get to the other end of the satellite trail. Remember, you want to go just beyond and to the same side of the trail as where you started, not on the trail itself. When you’re at the end, left-click the mouse and make another anchor point.

4  Cross over the trail slightly and make a third anchor point just on the other side of the satellite trail. Move the mouse back along the trail on the other side, paralleling the first long line.

When you’ve gone as far as the starting point, anchor (left-click) and move back to your starting point. When you get there, the little circle will show up again. Left-click in the circle. Your lines will now become “marching ants” because you have just created a selection around the satellite trail.
Abell-2151
Abell 2151 looks much better in the author's final image. After making a selection around the satellite trail and deleting that selection, he used Photoshop CS5's "Content-Aware Fill" menu selection to replace the trail with background data. This removed the obstruction from both the star and the galaxy.
5  Press Control-H to hide the ants, go to “Edit,” and then “Fill.” In the Fill dialogue box, choose “Content-Aware.” Click “OK” and watch the satellite trail vanish while Photoshop fills the area with surrounding detail.

6
  Here’s a refinement: If the satellite trail went through a star (as in the top photo), Photoshop will remove the star when it does the fill. So, once you establish the selection (at the end of step 5), go to the “Marquee” tool (the one just above the Lasso tool), right-click, and choose the “Elliptical Marquee Tool.” Go to the position of any star in the selection, and while you hold down the ALT key, make a “deselection” just around the star. Do this for all affected stars.

Now apply the Content-Aware Fill, and it will work its magic on everything except those stars. Basically, you are cutting them out of the selection. With a little practice, you will be able to deselect just enough so that the star remains but the satellite trail right next to it disappears.

Another method involves duplicating the trailed image and using it as a layer on top of the image you deleted the trail in. Go to the “Layer” menu, select “Layer Mask” and then “Hide All.” Then paint with white where you want to replace the star. Flatten the layers when you are satisfied.

It’s going to be a long time before all those satellites come crashing back to Earth. Until then, use Photoshop’s Content-Aware Fill to do the next best thing.
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