Smooth and fill star trails
Another of Nebulb's unique features is the possibility to effortlessly obtain star trails images in which the star trails are super smooth and have no imperfections or gaps.
Thus, just by selecting the "Smooth and fill trails" option, Nebulb automatically performs specific processing during stacking so that the star trails are completely continuous and smooth.
Below is an example in which you can perfectly appreciate the difference in the quality of the star trails between a traditional maximum stack and the one specifically made by Nebulb:
Also, to the right of the "Smooth and fill trails" option is a drop-down list where you can select the level of smoothing that Nebulb should apply to the star trails during stacking.
It should be noted that, obviously, the smoothing of star trails implies that more tasks have to be performed during stacking and, therefore, the stacking time becomes longer. Thus, in order that the stacking time does not increase more than necessary there is the possibility to indicate the time interval to be used by Nebulb to smooth and fill in the gaps of the star trails.
Generally speaking, this time interval represents the maximum time gap that the star trails can have for Nebulb to fill it smoothly. For example, if the "15 min" option is selected, Nebulb will obtain an image with perfect star trails even if (for whatever reason) the capture was interrupted for 15 minutes or a cloud covered the stars for 15 minutes.
Thus, this functionality, in addition to generating much smoother star trails, eliminates all the gaps in the trail of any star (both the most intense and the faintest). Moreover, this feature is effective regardless of whether the gaps are constant (due to the time interval between each shot) or completely irregular (e.g., those generated by clouds).
Having said all this, the available smoothing intervals are as follows:
- Minimum: In this case Nebulb automatically calculates the smoothing interval based on the exposure time and the shooting time between photos. This option provides the lowest level of smoothing, but it is the fastest option.
- 5 min: In this case Nebulb uses a 5-minute interval to smooth the trails, i.e., small gaps and discontinuities in the trails are filled in. It is an option that generates a greater smoothness than the previous one and the stacking time does not increase significantly.
- 15 min (by default): This is the option that offers the best balance between the result of smoothing and gap filling, with respect to the increase in stacking time. With this option you get a great smoothing of the star trails (as you have seen in the example images above), compensating the increase in stacking time by this great improvement in the quality of the trails.
- 45 min: This option generates the maximum smoothing and gap filling possible, since, in this case Nebulb is able to fill gaps in the star trails of up to 45 minutes. This option, although by far the most resource-intensive and time-consuming, is ideal when there are a lot of clouds obscuring the stars.
It is not desirable, but sometimes in the middle of a session it can happen that many clouds appear and, therefore, a large number of gaps are generated in the star trails (which can become quite long depending on the level of clouds and the speed at which they move). It is precisely in these difficult cases when it is very useful to be able to select such an extreme smoothing, since, thanks to it, it is possible to recover images that otherwise would be impossible, as in the case of the following example:
One of the original RAW images showing the level of clouds that were present
Image of star trails obtained by the traditional stacking method (based on the maximum)
Star trails image obtained by Nebulb specific stacking and using 45 minute smoothing and gap filling
IMPORTANT 1: The star trails obtained will never be longer than those corresponding to the total capture time (between the first image and the last one). This a priori may sound a bit strange, but the thing is that Nebulb could computationally extend the star trails as much as he wants, since he knows perfectly well the movement that stars follow in the sky, so he could use the smoothing process to generate much longer star trails than the ones captured in reality. However, out of absolute respect for photographers (who spend many hours capturing photos at night to get star trails images with very long trails) Nebulb has a software limitation whereby it is never allowed to create images with trails longer than those actually captured. The length of the star trails always corresponds to the time taken to capture the images (no more, no less).
IMPORTANT 2: To achieve the best possible smoothing and gap-filling, it is important that the stars are quite point-like or, at most, form small trails. That is, the longer the trails of the stars in the captured images, the less accurate Nebulb will be able to align the stars and, therefore, the less accurate the software will be at smoothing and filling in the star trails. For this reason, it is usually a good recommendation to use capture times of about 30 seconds (1 minute maximum), since it is a good compromise between getting images with quite pointed stars, without having to capture an excessive number of images.
IMPORTANT 3: For smoothing and gap filling to be applied, multiple images (at least two) must be imported. In other words, the smoothing and filling of star trails cannot be applied to individual images, nor can it be applied to images that have already been composed (such as those obtained using the Live Composite mode of Olympus cameras).
It is important to take hot-pixels into account
In case the original images have a lot of hot-pixels it is usually convenient to activate the luminance noise reduction, otherwise all those hot-pixels may appear later in the final image as small trails (due to the smoothing process).
Below you can see an example where, because there are many hot-pixels in the photos, they appear as trails in the final image. However, just by activating luminance noise reduction, all these hot-pixels are removed before stacking, so that only traces of stars appear in the final image.
In the following article you can find more information on when it may be convenient to apply noise reduction before stacking: https://help.nebulb.com/portal/en/kb/articles/when-to-apply-noise-reduction-before-stacking-preprocessing
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