Sky/ground blending in star trails images

Sky/ground blending in star trails images

The sky/ground blending in star trails images is done in the same way as in aligned star images, except that in star trails you have to be especially aware of which image is used for the ground part.

Thus, if you drop down the image selection list (the one on the left in the bottom toolbar) you will see that there are at least 2 options (or 3 if you have also imported a master image) to compose the star trails image:



  1. Star trails (sky) + Sigma-clipping (ground): The sigma-clipping image is used for the ground part, which is the image that is always used in a star-aligned image. The advantage of the sigma-clipping image is that it has no trails of planes or satellites, but the disadvantage is that it also has no trails of stars, so it can be a bit more complicated to adjust the sky/ground blending well.
  1. Star trails (sky) + Maximum (ground): In this case the maximum image is used for the ground part (the one traditionally used for stacking star trail images). The advantage is that the maximum image has star trails (so it is very easy to do the sky/ground blending), but the big disadvantage of this image is that it also includes the trails of satellites and planes, so these unwanted trails may end up appearing partially on the horizon.
  1. Star trails (sky) + Master image (ground): In this case an image that has been imported specifically for the ground (master image) is used. Usually this image has been specifically illuminated and probably also captured with different parameters (to minimize noise on the ground). Therefore, it may be a bit more complicated to adjust the sky/ground blending properly, but this option allows you to directly obtain an image in which both the sky and the ground look the way you wanted them to.

Therefore, depending on the particular circumstances of each case, it may be more convenient to use one or the other (maximum, sigma-clipping or master image) as the ground image.


ADVICE: It is advisable to make a quick test by selecting all the available options for the ground (in order to get a first idea of how the final image looks in each case) and then choose the one that looks best.


Maximum image for the ground in star trails images

The maximum image is the one that is usually chosen for the ground part of a star trails, since it is the easiest and most natural blending with the sky.

And very specially, the maximum image is the one that usually offers (by far) the best results when there is vegetation on the horizon, as you can see in the following example:

Star trails image in which the sigma-clipping image has been chosen for the ground. As can be seen, in this case the sky/ground blending is a bit weird in the tree area, since the star trails are interrupted when the tree is reached, something that is not natural because the tree is not a totally opaque element (between the branches you can see part of the sky).

However, in that same star trails image, if the maximum image is selected for the ground, a much more natural blending is achieved, since star trails can also be seen through the tree.


Therefore, by default, to compose a star trails image it is always convenient to try first with the maximum image for the ground, since it is the one that usually generates the best and easiest sky/ground blending.



Sigma-clipping image for the ground in star trails images

As mentioned in the previous section, in star trails it is usually more interesting to use the maximum image for the ground part, since it tends to generate a more natural and easier to adjust sky/ground transition.

However, there may be situations where it is more interesting to use the sigma-clipping image for the ground, for example:
  1. With fairly flat horizons and where there are no elements that protrude much above the horizon.
  1. When the maximum image has many trails of planes or satellites in the horizon area.
  1. When the maximum image has become too bright.

Below you can see an example in which it was finally decided to choose the sigma-clipping image for the ground:

Star trails image using the maximum image for the ground. In this case the lights of the ships passing through the sea were registered in the maximum image, generating that intense horizontal trail in the horizon area.

The same star trails image above, but simply selecting the sigma-clipping image for the ground. This alone makes the final image much "cleaner", as there are no unwanted light trails in the horizon area (in this case due to the lights of the passing ships).


ADVICE: In the case of processing a star trails image in which the horizon is the sea, it is advisable that the sky mask does not reach the horizon (to avoid that the lights of the ships, being over the sky area, end up appearing in the star trails image). Therefore, when the horizon is the sea, it is always advisable to leave a small margin in the sky mask so that it does not reach the horizon.


And this is another example where the sigma-clipping image for the ground gave a better final result (due in this case to an effect caused by the moon):

Star trails image using the maximum image for the ground. Due to the moon during part of the night, a large "burnt" area appears in the water due to the intense reflection of the moon.

Same star trails image above, but using the sigma-clipping image for the ground. In this case the ground part appears perfect (without any reflection or anything "burnt") because in the stacked sigma-clipping image anything that is not static does not appear.


Well, as you have seen, there may be situations in which it is interesting to select the sigma-clipping image for the ground in a star trails (although it is true that the first option should always be to try with the maximum image).


IMPORTANT: In case there is a reflective surface on the ground (e.g. lake water), if you choose the sigma-clipping image for the ground, no star trail will appear on the water. Therefore, if you want to keep the star reflections you should use the maximum image for the ground.





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