How to stack images to greatly reduce light pollution
Here are the steps to follow to greatly reduce light pollution when stacking images in Nebulb.
1. Import images
The first thing to do is always to import the images to Nebulb (if possible in RAW format the better).
For more information you can read this other article: How to import images
2. Define the sky mask
Secondly, the sky mask must be defined (so that Nebulb knows where the stars are).
For more information you can read these other articles:
3. Set the stacking configuration
The next step is to configure the stacking, that is, to let Nebulb know what to do with the images you have imported.
Depending on the type of image you want to obtain (Milky Way, meteor shower, star trail...) the tasks to be performed during stacking are different.
In order to configure the stacking you must use the "Stack" section (it is at the top of the tools right panel).
For more information you can read this other article: How to configure and execute a stacking
4. Save the project
In order to continue, you must save the project, since Nebulb does not allow you to execute the stacking if the project is not saved (to avoid losing information because it has not been saved).
To save a project you must open the "File" menu and there you will find the options to do it.
If it is a new project, only the "Save project as..." option will be enabled, and if it is a project that you have already saved previously, the "Save project" option will also be enabled.
When saving the project a file with the extension ".nbp" and the name you have indicated will be created in the hard disk. In that file Nebulb will save all the information related to that project, including all the information generated during stacking and also all the settings made by the user. The idea is that the NBP files are self-contained, that is, they include everything necessary for the user to view and/or adjust the image resulting from stacking whenever he/she wants.
5. Execute stacking
Once the project is saved, the next step is to execute the stacking, for which you simply click on the following button:
When you do so, a confirmation message will be displayed (which you must accept to continue) and, finally, Nebulb will ask you to select the temporary working folder (which is nothing more than a folder where Nebulb can write and read the temporary files it generates during stacking).
For more information you can read this other article: How to configure and execute a stacking
6. On completion of stacking
Once the stacking is finished, the result image is displayed and the right panel tools are enabled depending on the configuration previously set for the stacking (if, as in the case of this example, flying object detection has not been checked, then the "Flying Objects" tools will remain disabled).
Actually (as you can see highlighted with a red arrow in the previous image) what Nebulb shows at the end of the stacking is a fusion between two images: "Aligned stars (sky) + Sigma-clipping (ground)".
This is because Nebulb performs different types of stacking depending on the image area (sky / ground) and the stacking configuration (aligned stars / star trails).
In the case of aligned stars, what Nebulb does for the sky is to detect and align the stars in all images before performing the stacking to reduce noise (you could say it is like a kind of software star tracker).
However, for the ground part a stacking based on statistical techniques is directly applied to reduce noise, but without any prior alignment.
Therefore, at the end of the stacking, at least 2 different images are obtained (which can be more depending on the stacking configuration) and what Nebulb then does with them is to merge them (it uses the sky of one of them and the ground of the other).
And to select the image that is represented on the screen you have the drop-down list on the left of the lower toolbar.
Later, in the section "Sky/ground blending", you will see how it is possible to adjust in Nebulb the transition between these two different parts of the image.
IMPORTANT: Although all the information generated during stacking is still stored in the temporary working folder and will not be lost even if you close Nebulb, it is recommended that after the stacking is finished you save the project again. By doing so all information in the temporary working folder will be stored in the Nebulb project file (*.nbp) and all temporary files generated during stacking will be automatically deleted.
7. Apply light pollution reduction
The first thing that may strike you after the stacking process is that the image that appears is practically the same as the original, but without noise. And this is precisely one of the great advantages of stacking: it greatly reduces noise and keeps all the good information, so that it will now be possible to reduce light pollution considerably.
Original image | Image after stacking |
So that you can reduce (a lot) the light pollution of the sky in your night images Nebulb has the "Light Pollution" tool (in the right tool panel).
All you have to do to achieve this is to increase the value of the "Reduction" slider until the objects in the night sky (the Milky Way in this case) look the way you like them (don't worry for the moment if the sky is too dark or the transition between the sky and the ground looks a bit weird, you'll adjust it later).
As you can see in the example image above, only by means of the light pollution reduction slider control (set in this case to a value of 4.23 steps of reduction out of a maximum of 5 steps) the galactic center of the Milky Way is perfectly visible, although in the original image it was practically imperceptible.
Likewise, in the lower part of the sky there is also some faint cloudiness that was also present, but that, due to the intense light pollution, was not visible in the original image.
In other words, Nebulb's light pollution reduction tool reduces (as much as the user wishes) the light in the sky to show the rest of the information that is present there.
IMPORTANT: Nebulb does not use any artificial intelligence technique for light pollution reduction. That is, what appears in the image when applying light pollution reduction is because it is there, since Nebulb does not invent anything (it simply reduces the light and lets what is there appear).
Finally, in case the sky in the image is too dark, you can also use the "Gain" control to make the sky lighter (or darker), as you can see below:
However, although the "Gain" control allows you to modify the intensity level of the sky, in many occasions it is more practical and comfortable (once light pollution reduction has been applied) to adjust the levels of the different areas of the image (sky and ground) using the Nebulb development tools. That is, you may prefer not to use the "Gain" control because in the next point you will see a more complete way to make general adjustments on the image.
8. Apply image development settings
At this point it is time to apply the general development adjustments to the image.
When doing so it is important to consider whether you are going to finish processing the Nebulb image in other software (such as Photoshop or Luminar, to give two examples) or not.
In case you want to finish the image in Nebulb (you are not going to process it later with any other software) you can apply all the development adjustments you want to get the image the way you like it.
However, in case you are going to continue processing the image in other software, I recommend that you do a much more neutral development (without pushing the image to the limit) so that you can then give it the final touch in the other software you use. The idea would be to prepare the image in Nebulb in such a way that it will be easier for you to post-process it in the other software.
That said, to be able to apply the general development settings in Nebulb there is the "Develop" tool.
The controls you can find in this section are similar to the normal ones of any image editing software, except for the 3 buttons at the top.
With these three buttons you can make the development settings apply to the whole image (left button), only to the ground part (middle button) or only to the sky part (right button).
Since Nebulb internally works with two different images (one for the sky and one for the ground) and then merges them into one (the one shown on the display), it is possible to apply different development settings to each part of the image.
To do this you simply have to press the button corresponding to the area you want to modify (the whole image, just the ground or just the sky) and apply the desired adjustments.
Adjustments for the entire image | Adjustments only for the ground | Adjustments only for the sky |
Below you can see the difference between the original image and the result after stacking, light pollution reduction and development in Nebulb.
Original image | Image processed in Nebulb |
ATTENTION: The way the images are developed is always based on a totally personal taste, so take these examples simply as a reference of what you can achieve with Nebulb.
9. Sky/ground blending
Before finalizing the image in Nebulb, there is one last step: adjusting the fusion between the sky and the ground.
As I mentioned before, Nebulb works internally with one image for the sky and another for the ground, on which you can apply different settings.
Because of the latter (different settings for the sky and the ground) it is necessary to apply a fusion technique that allows to combine these two different parts into a single image (which is what you want to get in the end).
To do this in Nebulb you have the "Blend" module.
As you can see in the image above, the blend module is divided into three sections:
- Sky transition: transition zone that goes from the horizon to the sky (without including any of the ground).
- Ground transition: transition zone going from the horizon to the foreground of the image (without including any of the sky)
- Exposure balance: over- or under-exposure that can be applied to the sky transition zone to balance its intensity with that of the ground and thus make the sky/ground transition look natural (no brightness imbalance between the two zones).
Below is a graphical representation of these 3 zones:
- Blue zone: Sky transition.
- Red zone: Ground transition.
- Yellow zone: Region in which the exposure balance has effect.
Therefore, now we only need to know what each of the controls in the "Blend" module does.
- Radius: Sets the length of the corresponding transition zone. The larger the radius, the longer the transition zone.
- Hardness: Sets how progressive the transition effect is. The lower the value, the softer the transition (generally it looks better with lower values).
- Opacity: Sets the transparency of the transition. With 100% opacity the transition is applied completely over the image, while with 0% opacity the transition is not applied at all.
And regarding the "Exposure Balance" setting, what it does is to modify the exposure of the transition zone of the sky so that you can match it with the exposure of the ground on the horizon, so that the sky/ground transition is completely natural (there is a balance of brightness in both zones).
Having said all this, the steps I recommend to adjust the sky/ground transition are as follows:
- Modify the radius first to see which one generates a more natural transition for you.
- Try playing a bit with the "Exposure Balance" to leave it at the point you like the most.
- Finally, you can also try adjusting the hardness and/or opacity a bit (although this is often not necessary).
ATTENTION: The adjustment of the sky/ground blending is an iterative process (test and check, test and check...) and depends completely on each photograph (there is no recipe that works for all cases) and, above all, it depends on how the part of the ground is in relation to the sky (especially when there is an object that protrudes predominantly above the horizon). So, don't worry if at the beginning you find it a bit difficult to adjust this.
Below you can see the final result I have obtained after adjusting the sky/ground blending (according to my personal taste):
Image before adjusting the sky/ground blending | Final image after adjusting the sky/ground blending |
And these are the values I have used for the sky/ground blending in this example:
IMPORTANT: If you want to know more about how to adjust the sky/ground blending in Nebulb for more complex situations than the ones in this example, you can check the specific section in the documentation on "Sky/ground blending".
10. Export the final image
Finally, you only have to export the final image, for which you can use the "Export Images" module at the bottom of the left tools panel.
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