Flying object alignment modes

Flying objects can be rendered on the final image in two different positions:

1. In the position in the sky where they appeared.

2. In the image position in which they were captured.

In the case of using an equatorial mount (star tracker) during the capture both representations are identical, since the star tracker mount moves the camera continuously (compensating the Earth's rotation) so that the sky is always in the same position in all captured images.

However, the most common in landscape astrophotography is to capture with the camera directly on the tripod (without star tracker) so the sky in the images is changing over time. In other words, the sky in the base image is in a different position than in any other image captured some time before or after.

Thus, in Nebulb there are two buttons so that the user can easily decide if he/she wants the meteors aligned according to their position in the sky or according to the position in which they appear in the images.

Alignment of flying objects according to their position in the sky

When capturing without an equatorial mount (star tracker) it can be said colloquially that the sky "moves" in the images as the night goes by and, therefore, the radiant also "moves". Well, precisely to get images with all the meteors aligned according to the radiant (even if an equatorial mount has not been used) Nebulb has the alignment mode of the flying objects according to the sky.

That is, by selecting this option the flying objects are rendered at the exact position in the sky where they appeared. Below you have an example in which this can be understood much better:

Image in which all the meteors have been rendered aligned according to their position in the sky. A meteor next to the Pleiades has been marked in yellow in order to better explain how the alignment works according to the sky.

Region (at 100% zoom) where the meteor (yellow arrow) that appeared next to the Pleiades (red circle) is located.

We manually select the meteor to check in which position of the image it was captured. To review the original image in which any meteor is: (1) click the hand button, (2) select the desired meteor with the mouse and (3) click the image review button (or press "Ctrl + R").

Original RAW image in which the selected meteor was captured. The meteor in question is shown in green, the Pleiades in red and the position of the meteor in the final image in yellow.

Enlargement of the previous image so that the captured meteor can be seen in more detail. As can be appreciated, the meteor was recorded in a different position (green arrow) than the one shown in the final image (yellow arrow), but here the important thing is to verify that the position in the sky where the meteor appeared is maintained with total precision. In the original image it is next to the Pleiades (red circle) and in the final image it is exactly in the same position in the sky (next to the Pleiades as well), what happens is that the sky in the final image has "moved" and is not in the same position it was when the meteor was captured.

In short, the alignment mode of the flying objects according to the sky is as if you were capturing with a software equatorial mount, since Nebulb compensates the movement of the sky to render all flying objects in the exact position in the sky in which they appeared.

For this reason, selecting the alignment according to the sky directly shows that all meteors in a meteor shower emerge from the same point in the sky: the radiant.

Alignment of the flying objects according to their position in the image

The other Nebulb alignment mode allows to render the flying objects directly in the position in which they are recorded in the image, i.e. without any alignment of any kind.

By selecting this option all flying objects are rendered according to the position of the image in which they were recorded (not according to the sky). Consequently, the same meteors that previously appeared sorted according to radiant now appear totally disordered, since, Nebulb has not compensated for the movement of the sky.

Graphical representation of all the captured meteors. In this case, no order can be seen, since the sky has moved a lot during the whole capture time and the meteors have been rendered directly (without compensating the sky movement).

Final image with all the meteors aligned according to their position in the image in which they were captured.

In principle, the alignment according to the image is not usually the most interesting option to render the meteors on the final image. In fact, it is much more common to leave the meteors aligned according to the sky so that, in this way, the irradiation effect can be observed since they all come from the same point in the sky (the radiant).

However, there are occasions when using the image alignment may be convenient, for example:

1. In the review of doubtful meteors (those in which there are doubts whether they are meteors or may actually be some satellite). In this other article you can find more information about how to do this review: https://help.nebulb.com/portal/en/kb/articles/visual-review-of-flying-objects

1. In the rendering of satellites and planes, since these objects do not move according to the sky (they do not follow the movement of the stars).

Rendering of the trajectories followed by the planes and satellites detected. It can be seen that, as the alignment according to the sky is activated, many of these trajectories are over the ground area, which does not make sense because the satellites/planes do not follow the movement of the stars.

In this case the trajectories of the satellites and planes are shown as they really were (having activated the alignment according to the image), since these flying objects do not move due to the Earth's rotation (as the stars and the radiant point do).

Why are there flying objects shown above the ground?

As the night goes by it is normal that there are parts of the sky that are no longer visible, either because they go out of the frame, because they fall below the horizon or because they get behind some element on the ground. This is the reason why (since the alignment is selected according to the sky) there are many flying objects that Nebulb renders virtually on the ground.

This is something that initially may seem strange, but it is totally correct and, in fact, it is a visual aid to be aware of the number of meteors that will not appear in the final image because they are below the horizon or behind some element on the ground.

Look at the selected meteor in the following image and you will be able to see more clearly why it is rendered above the ground:

Image in which a meteor (marked in yellow) has been selected and rendered on the ground (as the alignment according to the sky is selected). This is because the exact position of the sky over which this meteor appeared is below the horizon for the image shown by Nebulb. That is, this part of the sky had not yet risen above the horizon at the time of the night when the image that Nebulb is displaying was captured, and so this meteor is rendered virtually below the horizon (because its position in the sky was exactly there at that time of the night).

Nevertheless, when selecting the alignment according to the image (red arrow) no meteor remains in the ground area, since, in this case, the flying objects are rendered in the exact position where they were recorded in the image (not in the sky). The same meteor that was previously below the horizon is now rendered above the sky (yellow arrow), since it was captured at that position in the image.

IMPORTANT 1: The issue of flying objects virtually rendered on the ground part (when the alignment according to the sky is selected) is only a visual aspect that has no effect when merging the real image of the flying objects on the background image. In fact, this is a visual aid so that the user can be aware of the number of meteors that have been captured, but will not appear in the final image because, for the selected time of night, they are below the horizon.

IMPORTANT 2: When merging flying objects on the background image Nebulb always takes into account the sky mask (so as to never render the real image of any flying object ahead of the ground).

IMPORTANT 3: If at any time you notice that the real meteor image is in front of any object on the ground (such as a tree), the reason is that the sky mask is not well defined and that part of the ground is included as part of the sky. If this happens to you, simply refine the sky mask in that part (so that it does not include any ground element) and press the refresh masks button. You will automatically see that the meteor image is no longer rendered on the ground.

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