Refining the stacking
At the top of the "Stack" module are all the buttons that control the execution of stacking in Nebulb and, in particular, the button below is used to refine the stacking.
This button is by default disabled and grayed out (until the stacking is finished). Then, as soon as the stacking (or the processing of an individual image) is finished, this button is displayed in orange:
What is the purpose of stacking refinement?
The stacking refinement is useful to be able to modify (after finishing the normal stacking) the time of the night for the sky to be aligned, which is especially useful when processing meteor showers (so that the meteors can be aligned according to the time of the night that offers the most visual impact).
Below is a typical example of when it is necessary to apply stacking refinement:
1. Image obtained at the end of stacking
By default, Nebulb always selects as base image (on which the rest of the images are aligned during stacking) the one that is chronologically in the center of the total capture interval. Thus, when the stacking is finished what you get is that base image completely free of noise (thanks to the stacking) and, in the case of a meteor shower, with all the meteors aligned according to the sky at the time of capture of the base image. Below is an example of the image I obtained after a stacking for the Geminids:
The image may be interesting as is, but it is true that the radiant of the meteor shower is out of the frame, since, at the time the base image was captured the radiant was out of the image.
2. Image I want to get
In this case the idea was to obtain an image in which you could see the irradiation effect of all the meteors from the radiant, for which I wanted the radiant to be inside the image and centered.
To see if this is possible, in Nebulb you can move the bottom time slider (1) to visually check if at some point of the night the meteors were as you wanted. If so, simply press the button (2) to the right of the time slider and the sky (along with the meteors) will be in the exact position they were at the time of the night you have selected.
IMPORTANT: Nebulb obtains the position of the sky for each moment of the night from the captured images, thanks to which Nebulb can then leave the sky as it was in any of the captured images (not only for the base image).
ATTENTION: It is not possible to leave the radiant of a meteor shower at any position in the image, since that would allow to place the radiant in positions where it is not really there. Thus, Nebulb only allows leaving the meteors aligned according to the exact position in the sky for any time of the night when photos were being captured. The idea is that Nebulb does not invent anything.
Below you can see the image obtained after selecting the time of the night for which the radiant was centered and inside the image:
The previous image was the one I wanted to get, except for a small detail: this image has noise, as you can see in the following detail of a 100% enlarged area:
The reason for this is that, due to a limitation of computer resources (memory and hard disk), Nebulb cannot apply the noise reduction for the whole capture interval, since it can be many hours long and during all this time the sky moves a lot (in the case of not using an star tracker).
Therefore, what Nebulb does is to optimize the stacking at a time interval around the base image. So, in the event that the base image is subsequently modified, it is necessary to do the stacking refinement for that new time of the night.
3. Result after stacking refinement
Once the stacking refinement (which is nothing more than a small stacking around the newly selected base image) has been executed, the same image as before is obtained, but now already free of noise (just like the one initially obtained at the end of the stacking).
Below you can see a region of the image enlarged to 100% in which it is perfectly visible that the noise level in this image is much lower than in the one obtained before refining the stacking:
How to execute stacking refinement
In case the user modifies the time of the night for which the sky is aligned (in the bottom time slider) the stacking refinement button appears enabled and grayed out, as you can see in the image below:
By clicking this button Nebulb performs a micro-stacking centered on the time of the night selected by the user. In this way, thanks to the stacking refinement you can always obtain the best possible result (in terms of noise and light pollution reduction) for any time of the night.
And in order to perform a stacking refinement (once the stacking is finished) it is only necessary to perform 3 simple steps:
- Select the time of the night for which you want to leave the sky aligned (using the bottom time slider).
- Press the button that modifies the selected base image. By pressing this button the sky will be displayed as it was at the time of the night selected in the previous step.
- Press the stacking refinement button. When doing so Nebulb performs a micro-stacking, centered on the new base image and with the time interval selected in "Stacking refinement interval".
Once the stacking refinement is completed, this button appears again in orange, indicating that the displayed image is the one on which the rest of the images have been aligned to reduce noise during stacking.
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