Exclude twilight images from star trails stacking
It is very common in a meteor shower session to try to extend as much as possible the time during which you are capturing in order to catch as many meteors as possible. That is why many times we start capturing at the beginning of the astronomical twilight (at dusk) or we finish capturing at the end of the astronomical twilight (at dawn), when the sky is quite bright (not dark) but it is still possible to see some meteors.
However, this, which can be interesting to "catch" some more meteors, is disastrous if you want to use all the images captured during the session to obtain also a star trails, since the images captured during twilight have much more light than the rest (they are overexposed) which can generate a very white (even burnt) star trails image.
Below is an example of the star trails image obtained by directly stacking all the images captured for the 2023 Perseid meteor shower:
To avoid this problem, a different stacking can be done for the star trails, using only the fully nighttime images (not those captured during twilight). In this way, the star trails image obtained will be correct (it will not be as overexposed as the previous one), but this solution implies having to perform a second specific stacking only for the star trails image.
To avoid the latter, Nebulb incorporates the option "Exclude twilight images", by means of which it is possible to obtain a perfect star trails even if you have imported images that are overexposed because they were captured during twilight.
By selecting this option Nebulb calculates the variation of the light intensity of all the images, thanks to which it is able to automatically discard (only for the stacking of star trails) those images that are too overexposed.
Here is the result obtained by stacking the same images as in the previous example (Perseids 2023), but in this case with the "Exclude twilight images" option activated:
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