Sky/ground blending in star trails images to avoid trails of planes/satellites near the horizon

The most common way to process a star trails image is to select the maximum image for the ground part, since it is the easiest way to obtain a more natural result.

However, the big disadvantage of using the maximum image for the ground is that it can be more difficult to get the plane/satellite trails out of the horizon area, since the maximum image DOES contain trails of planes and satellites.

Below you can see an example of this:

Maximum image in which a large number of trails of planes and satellites can be observed. Some of them are very close to the ground area and, therefore, may be a problem to avoid that they do not appear in the final image.

Final star trails image, using the maximum image for the ground. Because the maximum image has some trails of planes and satellites close to the ground area, in the final image some of these trails appear in the transition zone between the ground and the sky (which is not desirable).

Enlargement of the previous image in which you can see better the trails of planes and satellites that end up appearing in the final image (being in the transition zone between the sky and the ground and having selected the maximum image for the ground).

Adjust sky/ground blending so that planes and satellites do not appear in the final image

Two things can be done to prevent the planes/satellites from appearing in the final image:

1. In case the horizon of the image is quite flat and there are no large objects protruding above the horizon (especially trees), it is usually easiest to select the sigma-clipping image for the ground, since no trails of planes, satellites or meteors ever appear in this image.
2. In any other case, it is usually most effective to decrease the radius and/or opacity of the sky transition so that unwanted trails do not end up appearing in the final image.

So, in case you have to select the maximum image for the ground (because sigma-clipping does not give a natural result), the process to obtain an image without trails of planes, satellites or meteors near the horizon is as follows:

1. Define a mask for objects protruding above the horizon (if any) and decrease the radius and opacity of the sky transition until no trails of satellites, planes or meteors are visible.
2. If any unwanted trails are still visible anywhere on the horizon, also apply a reduction of the radius and/or opacity of the general sky transition (the one that affects the entire image) until these trails are no longer visible.

Below is a practical example of this:

To get rid of the trails of planes and satellites that appear in the final image in the area close to the ground, the first thing to do in this case is to define the mask for objects that protrude above the horizon (red arrow).

By selecting the specific blending adjustments for objects protruding above the horizon (red arrow) and decreasing the radius (20) and opacity (70) of the sky transition a little, no more trails of planes and satellites appear in the final image.

Enlargement of the previous image so that you can see in more detail the result obtained. If you look closely, on the left side of the tree you can still see a small trail of a plane that has not been removed (being completely attached to the tree), although it is quite unnoticed. The rest of unwanted trails have disappeared completely.

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