Kiera Jefferson
Light pollution can be an issue when taking photographs of the night sky, especially in areas near cities. There are several software packages available to remove light pollution from images, including Adobe Photoshop, GIMP, Siril, PixInsight, Astro Pixel Processor, and StarTools. This article will focus on how to get rid of light pollution using GIMP, a free image editing software.
How to get rid of light pollution in GIMP
| Characteristics | Values |
|---|---|
| GIMP version | 2.10 |
| Filters | G'MIC' filters |
| Layer blending mode | Difference |
| Opacity | 50% |
| Duplicate layer | Duplicate Layer in the Layer drop-down menu |
| Layers window | In the Windows drop-down menu, click on Dockable Dialogs, then Layers |
| Curves function | Used to lift up the fainter stars in the image |
| Gradient tool | Used to construct a gradient and subtract that from the image |
| Gaussian blur | Used to blur the image |
| Clone tool | Used to get rid of any residual nebula or galaxy |
| Color picker tool | Sample average over a large area, set to foreground color |
| Multiply | Use the upper right slider in the curves tool and pull it straight down to decrease intensities |
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What You'll Learn
Using the 'Dust and Scratches' filter
The 'Dust and Scratches' filter in Adobe Photoshop can be used to remove light pollution from astronomical images. To achieve a similar effect in GIMP, you can download and install the 'G'MIC' filters from 'https://gmic.eu/gimp.shtml'. These filters will then appear at the bottom of the filters drop-down menu.
Once you have downloaded the filters, open the light-polluted image in GIMP and duplicate the layer. This will give you two layers to work on. You can duplicate the layer by going to the 'Layer' drop-down menu and selecting 'Duplicate Layer'. To view the layers, go to the 'Windows' drop-down menu, click on 'Dockable Dialogs', and then select 'Layers'.
Now, with the two layers visible, you can apply the 'G'MIC' filters. The specific filter settings may vary depending on the image and the level of light pollution. However, as a starting point, you can try using the 'Dust and Scratches' filter with a radius of around 30 pixels. This should help remove most of the stars or bright regions caused by light pollution.
After applying the filter, you may want to use the clone tool to further remove any remaining bright regions or light pollution artefacts. Select similar areas with similar brightness levels to clone over the bright spots. Additionally, you can apply a Gaussian Blur of around 40-50 pixels to smooth out the image and further reduce the appearance of light pollution.
Finally, to combine the layers and complete the light pollution removal process, set the layer blending mode to 'Difference' and then flatten the layers. This can be done in two passes for better results. First, set the 'opacity' slider to 50% and flatten the layers, then repeat the process with the opacity set to 100%.
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Duplicating the layer
To duplicate the layer in GIMP, first, download and install the set of ‘G’MIC’ filters from ‘https://gmic.eu/gimp.shtml’. These will then appear at the bottom of the filters drop-down menu. Next, load the light-polluted image into GIMP. Duplicate the layer by selecting ‘Duplicate Layer’ within the ‘Layer’ drop-down menu. This will give you two layers, which can be seen when the Layers window is opened: in the ‘Windows’ drop-down menu, click on ‘Dockable Dialogs’, then click on Layers.
Once you have duplicated the layer, you can use the clone tool to get rid of any residual nebula or galaxy while staying within the lines of the gradient map you've made. Set the gradient map layer to subtract and lower the transparency until you have only evened out/subtracted the background without disturbing any nebulae or galaxies. You can tweak the blur or the noise settings as needed and you can set the layer to subtract earlier on if it helps you use the clone tool or visualize what's happening better.
If a nebulous region is at a high elevation, it is reasonable to assume that the light pollution might be uniform across the field. In this case, you can blur the duplicated layer slightly and sample its colour away from any stars before painting this over the whole image.
Another technique is to use the "color picker tool" with the settings "sample average" over a large area (a good portion of the image area) and "set to foreground color". Click on the image over an area that is relatively empty of galaxies/nebulae/stars (mostly background noise). This allows you to detect the average colour of just the light pollution in the image, which in a deep-sky image should be pretty uniform. Then, create a new layer filled with this foreground colour. Set the layer mode to "difference", and then slide the opacity until it best cancels out the light pollution. This is usually around 50%.
Finally, the layer blending mode is set to ‘Difference’ and the two layers flattened. This has removed the light pollution from the image, which can be stretched to bring up the fainter stars. It sometimes works best if this is done in two passes: first, remove only 50% of the light pollution by setting the ‘opacity’ slider to 50% before flattening the two layers and then repeating the process with the opacity set to 100%.
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Setting the layer blending mode to 'Difference'
GIMP is a powerful software that offers a range of blending modes to alter the appearance of an image. One such blending mode is the "Difference" mode, which can be used to effectively remove light pollution from astronomical images.
To begin the process of removing light pollution, the light-polluted image should be loaded into GIMP. The first step is to duplicate the layer, creating two identical layers. This can be done by selecting 'Layers' from the 'Windows' drop-down menu, then choosing 'Duplicate Layer' from the 'Layer' menu.
Once the layers have been duplicated, the next step is to set the layer blending mode to 'Difference'. This can be done by selecting the 'Mode' selector tool, which is GIMP's equivalent of Blend Modes in Photoshop. Clicking on the Mode selector will reveal a list of available blend modes, including the 'Difference' mode.
The 'Difference' blending mode emphasizes the differences between layers, creating dramatic effects. When applied to removing light pollution, this mode will help to isolate and remove the light pollution elements from the image.
After setting the blending mode to 'Difference', the two layers can be flattened to merge the changes. This process removes the light pollution from the image. For optimal results, it is recommended to perform this process in two passes. In the first pass, set the 'opacity' slider to 50% before flattening the layers, and then repeat the process with the opacity set to 100% for the second pass.
By following these steps and utilizing the 'Difference' blending mode in GIMP, users can effectively remove light pollution from their astronomical images, revealing the true beauty of the night sky.
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Using the clone tool
To get rid of light pollution in GIMP, you can use the Clone Tool. This tool removes blemishes or problem areas on digital photos. It clones an image or area and “paints over” the blemishes. To access the Clone Tool, click on "Tools" from the menu, select "Paint Tools", and click "Clone". You can also click on the Clone Tool icon or simply press "C" on your keyboard.
When using the Clone Tool, selecting your clone source is vital. Choose a clone source with similar edges to the image you’re covering. Look for a point that runs a perpendicular line to the gradient angle so that your cloned image will seamlessly blend with the edges of the clone source. It’s also important to pay close attention to the color and light gradients in your photo. If you're cloning a pattern, GIMP will copy the pattern by "tiling" it and repeatedly copying it from one side to the other.
To smooth your image further, apply a Gaussian Blur of around 40-50 pixels. Then, in the "Layers" box, select the "Difference" blending mode before going into the "Image" menu and clicking on "Flatten layers". This will give you the original image with the light pollution removed. It sometimes works best to do this in two passes: first, remove only 50% of the light pollution by setting the "opacity" slider to 50% before flattening the two layers, then repeat the process with the opacity set to 100%.
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Adjusting colour levels
Another method involves using the colour picker tool to select the average colour of the light pollution in the image. First, make sure the settings are on "sample average" over a large area, and "set to foreground colour". Then, click on an area of the image that is relatively empty of stars, galaxies, or nebulae. This will allow you to detect the average colour of the light pollution.
Additionally, you can adjust the colour levels by using the curves tool. To decrease intensities, use the upper right slider in the curves tool and pull it straight down (a multiply). For example, to decrease the intensity by 10%, move the point from 255 to 230 (255*0.9 = 230) and then subtract that revised background file.
It is important to note that GIMP has limited support for working outside of RGB colour spaces, which can affect the accuracy of colour representation in astrophotography images. Therefore, it is recommended to perform most of your work in a luminance channel to avoid issues with the eyedropped colour.
Furthermore, applying a white balance before starting can help prevent galaxies from appearing with blue outer fringes. This issue is caused by starting with an incorrect white balance and applying gamma curves multiple times using the middle slider of the levels tool. By adjusting the white balance and gamma curves, you can improve the colour accuracy of your images.
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Frequently asked questions
First, download and install the 'G'MIC' filters from 'https://gmic.eu/gimp.shtml'. Open the light-polluted image in GIMP and duplicate the layer. Set the layer blending mode to 'Difference' and flatten the two layers. You can also try using the ''Dust and Scratches' filter, or blurring the image and subtracting that from the original.
You can use the clone tool to get rid of any residual nebula or galaxy while staying within the lines of the gradient map you've made. Set the gradient map layer to subtract and lower the transparency until you've only evened out the background.
Yes, you can use other software such as Siril, PixInsight, Astro Pixel Processor, or StarTools.
