When and how to define the exclusion mask
In Nebulb there are three types of masks:
- Sky mask: used to let Nebulb know the part of the image that is sky and, therefore, where it should look for stars to align them, compensating for the movement of the sky before stacking (due to Earth rotation).
- Exclusion mask: it is used to define areas of the sky that Nebulb should exclude from the light pollution estimation, since they are objects that are in the sky but should not be taken into account as such. Clear examples of sky objects to be included in the exclusion mask are the moon and clouds (they are in the sky, but should not be taken into account for light pollution estimation).
- Mask for objects above the horizon: this mask is only used after stacking to facilitate the blending between the sky and the ground in the final image, especially when there is some object (like a tower or a tree, for example) that protrude very predominantly above the horizon.
And to be able to define all these masks there are a set of utilities in the top toolbar.
That said, below you have all the information specifically related to the exclusion mask.
1. When to define the exclusion mask
Whenever there is any object in the sky that is not the sky itself, it will be interesting to add it to the exclusion mask, so that Nebulb does not take it into account when estimating light pollution.
And among the typical objects to be added to the exclusion mask are the following:
1.1. Moon
Whenever the moon appears in the image, it is highly desirable to add it to the exclusion mask.
IMPORTANT: It is advisable to leave plenty of margin when masking the moon and not to stick the mask just to what is the moon. Always make a circular mask, centered on the moon and large enough to include the large glow generated by the moon around it.
1.2. Clouds
Clouds are also another element that it is highly recommended to mask so that Nebulb knows that this area of the image should not be used to estimate light pollution.
Unlike on the moon, when masking clouds it is not necessary to apply too much margin around them. A mask that roughly fits the clouds is sufficient.
1.3. Nebulae
Finally, if nebulae are clearly visible in the image, it is also advisable to add them to the exclusion mask so that Nebulb does not include them in the light pollution estimation.
The masking of the nebulae is done in a completely approximate way (no precision is necessary) and, in addition, it is always advisable to make them small rather than large. It is not usually necessary to cover the whole nebula, otherwise there would be too large areas in which Nebulb would not be able to estimate the light pollution.
IMPORTANT: Do not worry about masking the nebulae (which I know may seem a bit confusing at first) because in practice you can modify this mask as many times as you want (making it bigger or smaller) and immediately see how the image looks in each case (so you can leave the mask that looks best).
2. How to define the exclusion mask
The first thing to do is to activate the exclusion mask editing by simply pressing the planet button on the top toolbar.
When you do this you will see that both the normal brush and the eraser brushes are activated. The rest of the tools are disabled (magic wand and paint bucket) because they are not necessary to define the exclusion mask.
The next step is to select the normal brush to be able to define the exclusion zones on the image, as you have seen in the previous examples.
IMPORTANT: You can change the brush size using the mouse wheel.
In case you want to erase anything from the mask, you can do it using the erase brush.
And if you prefer to erase the mask completely (to start from scratch) you can do it simply by pressing the broom button.
3. Does the exclusion mask have to be defined before or after stacking?
Unlike the sky mask (which must be defined before stacking), the exclusion mask can be defined either before or after stacking, or it is even possible to define it in one way before stacking and modify it afterwards.
In other words, it is not mandatory that the exclusion mask be defined before stacking, although it is advisable to do so when clouds or the moon appear in the image.
However, it is always possible (and, in fact, normal) to define the exclusion mask after the stacking is finished. This is especially true for masking the nebulae of the Milky Way, since these nebulae are often not well seen until light pollution reduction is applied after stacking.
Well, to define the exclusion mask after stacking you have to do exactly the same as you have seen in the previous section, with the addition that each time you modify this mask you must finally press the refresh masks button (so that the new mask is applied).
When you press the refresh masks button, Nebulb recalculates again the light pollution estimation taking into account the masks that are defined at that moment, so that when the calculation is finished the updated image is displayed on the screen according to the new masks.
Therefore, using the refresh masks utility you can make as many tests as you want with the masks and immediately check the result offered by each of them.
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