Automatic classification of flying objects according to the radiant

Automatic classification of flying objects according to the radiant

In photographs of meteor showers it is usually desired to include only the meteors coming from the main meteor shower (the one photographed) and, in addition, that all these meteors are aligned according to the radiant (alignment according to the sky). In this way, it is possible to obtain images with a great visual impact, since the meteors in the image transmit a powerful irradiation effect from one point (the radiant).

Well, this is easy to say and seems simple, but it can be a great effort, since:
  1. First you have to identify the meteors that belong to the main meteor shower (and there are many other meteors and satellites that make this task very difficult).
  2. Secondly, all the identified meteors must be aligned with respect to the exact point in the sky where they appeared, which is also complicated if an equatorial mount (star tracker) is not used.
However, in Nebulb all this is done automatically (all the software does it), so it is very easy to obtain this kind of images:

Photograph of the 2023 Perseids showing only meteors from this meteor shower and all are aligned according to the radiant (they are in the exact position in the sky where they appeared).



Automatic alignment according to the radiant

By default, and without you having to do anything or select any option, Nebulb tries to determine during the stacking the main radiant, i.e. the point in the sky from which most meteors originate. To do this, Nebulb analyzes the direction of all potential meteors and classifies as "meteors" only those that converge on the detected radiant and the rest are classified as "strangers" objects (which will be meteors from secondary meteor showers and also some satellites).

That is, without you having to do anything, at the end of the stacking Nebulb will show you the image with all the potential meteors that follow the main radiant.

Result obtained by Nebulb directly after finishing the stacking, in which you can see the graphical rendering (in "line" mode) of the potential meteors that follow the main detected radiant (in this case the Perseids). There are some potential meteors whose direction seems to deviate a little from the general trend and should be checked visually (in case it could be a satellite), but except for this, there is nothing more to do (Nebulb has automatically done all the work of detection, identification and alignment of the meteors).


So, from this initial automatic classification, you only have to visually check if there are any doubtful meteors (which could be really a satellite) and classify them as such if so. In this other article you can find more information about how to do the visual check of doubtful meteors: https://help.nebulb.com/portal/en/kb/articles/visual-review-of-flying-objects



Assisted alignment according to the radiant

In the case that very few meteors are detected (either because the total capture time has been short or because the meteor shower is not very intense) it may happen that Nebulb is not able to determine the main radiant and, in that case, all those flying objects that meet certain requirements will be classified as meteors (here you can find more information about these requirements: https://help.nebulb.com/portal/en/kb/articles/flying-object-types#Meteors).

Therefore, since the position of the radiant cannot be used (because it is unknown), among those potential meteors there can be also isolated satellites or meteors coming from a secondary meteor shower.

However, in Nebulb it is very easy to solve this situation and get an image in which only the meteors coming from the main meteor shower appear.

To do this in Nebulb you have to follow these steps:

1. Visually identify the area of the image where the meteor shower radiant should be.

To do this, knowing how the camera was positioned during the capture, it is easiest to look at the stars in the image to identify the constellation in which the radiant of the meteor shower is located (the name of which is usually related to that constellation).

In this case, before starting the capture, the camera was framed so that the radiant of the Leonids (2023) was centered in the image. In addition, after reviewing the image, it was possible to identify the constellation Leo and the area where the radiant of the Leonids should be ( shown in this image with a red circle).

2. Activate the "line projection" mode to identify the meteors passing through the area where the radiant is located.

By activating the "line projection" mode, the direction of all meteors can be observed.

3. Select several of the meteors (at least 2) that pass by the radiant (the one visually identified in point 1).

Clicking the hand selection button (red arrow) enables the selection of flying objects with the mouse. In this way, 3 meteors (rendered in white) whose directions converge to a point in the radiant zone (red circle) are selected.

4. Adjust the deviation tolerance with respect to the radiant.


Keep in mind that the radiant is not a super-precise point, but has a certain spread, i.e. the direction of the meteors does not converge at an exact point, but rather in a region of the sky (which is very specific, but has a certain amplitude). By means of this slider it is possible to define the tolerance that Nebulb must use to decide which meteors to keep and which to exclude (depending on how much they deviate from the midpoint of the radiant area).


IMPORTANT: Nebulb never eliminates any meteor, so the meteors that are excluded (because they do not follow the radiant defined by the user-selected meteors) go to the category of "stranger" objects. And if desired, there is no problem to manually re-categorize any of them as meteors (manual classification of flying objects).


By moving this slider to the left, Nebulb is stricter with the classification of meteors, i.e. it allows less deviation of the meteors from the midpoint of the radiant. Thus, by lowering the tolerance level, fewer meteors are kept, but the remaining meteors follow the radiant much more accurately (thus generating a greater sense of harmony).

On the other hand, by moving the tolerance control to the right, Nebulb widens the range of allowable deviation of the meteors from the midpoint of the radiant (it is less strict with the classification). Thus more meteors are included, but there may be some that deviate more than the rest from the midpoint of the radiant.

That said, Nebulb defaults to an average tolerance (center position of the slider) which is usually optimal for most cases, since it allows to keep a large number of meteors, excluding only those that deviate too much.


IMPORTANT: In case a too low tolerance is set and too many meteors are excluded, you can return at any time to the classification of flying objects that Nebulb had initially made and try again with a higher tolerance. For this purpose there is an option in the menu: "Flying Objects -> Restore default sorting".

5. Click on the classification button according to the radiant.


When pressing this button Nebulb displays a confirmation message indicating that only those flying objects that follow the radiant point defined by the selected meteors will be automatically classified as "meteors".



And by confirming the previous message, only the meteors that pass through the radiant appear, while the rest have been moved to the category of "stranger" objects.

Of all the potential meteors, only those passing through the defined radiant have been retained, with the rest moving to the category of "stranger" objects.

Final image obtained from the Leonids of 2023 in which, although it was very little intense, it was possible to obtain an image with more than a dozen meteors and all of them perfectly aligned according to the radiant.



Alignment when the radiant is outside the frame

The interesting thing about meteor showers (besides the fact that they offer the possibility of capturing unique and unrepeatable images) is that they allow to obtain very different images depending on where the radiant is located.

One of the most common cases is usually to include the radiant inside the image, since, in this way, a very powerful effect of irradiation from a point in the sky is achieved.

However, it can also be very interesting to leave the radiant outside the image, since, in that way, the effect generated by the meteors is very different (trails of light converging at an external point of the image).

After all, the radiant of a meteor shower is a compositional element that can be used to make images express emotions.

Example of an image in which the radiant is outside the frame (the meteors converge to a point outside the image).


But what happens when the radiant is outside the frame?

In any situation Nebulb always tries to automatically determine the position of the radiant (even if it is out of the image). However, as in the previous section, there may be situations in which due to a low rate of meteors it is not possible to automatically classify the meteors of a meteor shower.

And in those cases, it is necessary to do exactly as described in the previous section, but taking into account that the point of irradiation (the radiant) is outside the image and, therefore, the point to which the meteors converge is also outside the image.

Below you can see an example of this:

Image obtained from a session of the 2022 Perseids in which the radiant was not included in the frame (in fact the camera was pointed in the opposite direction). In this image we can already observe many meteors that seem to follow the same direction (although their convergence point is outside the frame).

By activating the "line projection" it can be seen more clearly that there are many meteors that have a convergent direction (towards the bottom of the image). In this way, several of them are selected in order to apply an automatic classification according to their radiant.

And by pressing the button for automatic classification of the meteors according to the defined radiant (red arrow), you get an image in which only the meteors that have the same origin and therefore come from the same meteor shower (Perseids) are kept.

Final image obtained in which, in this case, the radiant of the meteor shower photographed was behind the camera. It was framed in the opposite direction to the radiant, thus achieving a meteor "rain" effect from the sky.



Use radiant alignment to check ghost flying objects (or any other type)

Another useful feature of the automatic radiant classification is the ability to check doubtful flying objects (such as "ghost" objects).



By definition, "ghost" flying objects are those that have a very small length and, therefore, Nebulb is not clear whether they are really meteors or have been generated by a spot light (such as a flare) or an artifact (derived from noise) that appears in a particular image.

In order to check in a very fast and easy way these doubtful flying objects it is very useful to use the radiant classification, since, thanks to this feature, it is possible to select only those flying objects that follow the radiant defined by the meteors.

Thus, in order to check the "ghost" flying objects and determine which of them are really meteors, the following is necessary:

1. First of all, the meteors must be aligned according to the radiant.

Image with the rendering of the meteors classified according to the radiant.

2. Visualize only the "ghost" flying objects.

Rendering in "line" mode of the "ghost" flying objects.

3. Activate the hand selection and press the automatic classification button according to the radiant.

By pressing the hand selection button (1) and then directly pressing the radiant classification button (2) Nebulb selects only those flying objects that follow the radiant defined by the meteors.

4. Select the "rectangle" rendering mode together with the "real image" rendering mode.

Selection of the "rectangle" and "real image" rendering modes in order to perform the visual check of the "ghost" flying objects.

5. Visually review the selected flying objects to check if any may be doubtful.

And in case any meteor looks doubtful to the naked eye, you can also verify the original image in which it was captured (in this other article you have more information on how to do this: https://help.nebulb.com/portal/en/kb/articles/visual-review-of-flying-objects).



6. Classify "ghost" flying objects that have passed the above review as meteors.

By pressing the hand sorting button as "meteors" (red arrow) all "ghost" flying objects that were selected move to the "meteors" category.
Finally, when viewing the meteors, it can be seen that the "ghost" flying objects that have just been classified now appear in this category.



IMPORTANT: All this can be done in the same way for reviewing flying objects of any kind, i.e. it is also valid for reviewing "stranger" flying objects or "satellites/planes".




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