Kiera Jefferson
Light pollution is a common issue in astrophotography, resulting in an ugly red-brown colour in images. This is due to long exposures revealing the true colour of the nighttime sky, which is usually caused by light pollution in urban and suburban observing locations. There are various methods to remove light pollution in post-processing, including the use of image-editing software such as Adobe Photoshop, GIMP, and PixInsight. One technique involves using the Hue/Saturation tool to adjust the colours causing the cast in the image. Another method is to duplicate the background layer, blur the new layer to remove star detail, and then subtract it from the primary image. Additionally, the 'Dust and Scratches' filter in Photoshop can be used to remove light pollution, and similar results can be achieved with GIMP's downloadable filters.
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What You'll Learn
- Utilise image-editing software such as GIMP, Photoshop, or PixInsight
- Use the Gaussian Blur technique to remove light pollution
- Avoid aligning histogram peaks to prevent colour destruction
- Employ the Roger Clark method for post-processing astrophotos
- Use the Hue/Saturation tool to tweak colours causing casts
Utilise image-editing software such as GIMP, Photoshop, or PixInsight
GIMP, Photoshop, and PixInsight are all image-editing software that can be used to remove light pollution from images.
GIMP
GIMP is a free image-editing software that can be used to remove light pollution from astronomical images. To remove light pollution in GIMP, you can download and install the 'G'MIC' filters from 'https://gmic.eu/gimp.shtml'. Once installed, open the light-polluted image in GIMP and duplicate the layer. Next, use the clone tool to edit out as many stars as possible, especially the bright ones. Then, apply a Gaussian blur of around 40-50 pixels. Finally, set the layer blending mode to 'Difference' and flatten the two layers. This process can be done in two passes, first removing 50% of the light pollution and then repeating with the opacity set to 100%.
Photoshop
Adobe Photoshop is another image-editing software that can be used to remove light pollution. A technique to remove light pollution in Photoshop involves duplicating the background layer and renaming it to something like "light pollution filter." Next, use the clone tool to remove as many stars as possible, especially the bright ones. Then, apply a Gaussian blur of at least 50 pixels to create an average background. After that, apply the negative filter to get a light blue gradient. This technique can help reduce the orange glow effect and bring out more details when combined with image stacking.
PixInsight
PixInsight is a powerful tool for astrophotographers that can be used to remove light pollution, colour calibrate images, and reduce noise, among other things. However, it has a steep learning curve. To remove light pollution in PixInsight, you can use the DynamicBackgroundExtraction (DBE) tool. First, increase the "Default sample radius" to 150 and add sample points around the image. Then, change the "correction" to "subtraction" in the "Target Image Correction" settings and click "execute." Next, preview the background gradient and apply an autostretch to the new image. To neutralise the background, equalise the red, green, and blue components to get a neutral grey sky background. Finally, colour calibrate the image to ensure accurate colours.
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Use the Gaussian Blur technique to remove light pollution
Removing light pollution from astrophotography images can be challenging due to the presence of multiple gradients, such as the Milky Way brightness, light pollution from nearby cities, and airglow. One effective technique to address this issue is the Gaussian Blur method, which can be applied using software such as Photoshop, GIMP, or ImageMagick. Here is a detailed guide on how to use the Gaussian Blur technique to remove light pollution in post-processing:
Step 1: Open the Image in Suitable Software
Start by opening your astrophotography image in a photo editing software that supports layers and Gaussian blur, such as Photoshop or GIMP. This technique may also be achieved using ImageMagick through the Unix shell.
Step 2: Duplicate the Layer
Once the image is open, duplicate the layer. This step is crucial as you will apply the Gaussian blur to the duplicate layer while keeping the original layer intact.
Step 3: Apply Gaussian Blur
With the duplicate layer selected, go to the blur options and choose Gaussian blur. Adjust the radius of the blur to a value that completely blurs the objects you want to retain, such as stars, nebulas, or galaxies. Ensure that the blur does not affect the light pollution or other unwanted elements like clouds or fog. A radius of 70-150 pixels is often sufficient to wash out bright stars. Experiment with different blur sizes to achieve the desired effect.
Step 4: Set the Layer Mode to Subtract
After applying the Gaussian blur, set the layer mode to "Subtract." This step is essential as it will subtract the blurred layer from the original image, effectively removing the light pollution while preserving the desired objects.
Step 5: Merge the Layers and Adjust
Finally, merge the blurred layer with the original image. At this stage, you may need to adjust the brightness, contrast, or light curves to fine-tune the image. Play around with the settings to achieve the desired result. Additionally, you can explore other techniques, such as characterizing the light pollution and using subtraction or difference modes in your image editor for further refinement.
By following these steps, you can effectively utilize the Gaussian Blur technique to remove light pollution from your astrophotography images. Remember to experiment with different blur sizes and adjust the image settings to suit your specific needs.
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Avoid aligning histogram peaks to prevent colour destruction
When removing light pollution from an image, it is important to avoid aligning the histogram peaks to prevent colour destruction. This is because each colour has a different black point, and aligning the peaks will result in an incorrect black point for some colours. This can cause a variable colour balance with scene intensity and make the image too warm.
One way to avoid this issue is to use a reference photo or grey card to set the white balance before editing the image. This involves shooting a grey card in the same lighting as the subject and using the WB picker tool to copy the white balance values to the other images. Additionally, it is important to consider the scene's lighting conditions and the colours present in the image. If one colour is dominant in the shadows, mid-tones, or highlights, aligning the peaks may not work and can result in an unnatural image.
Another technique to remove light pollution is to blur the image and subtract that from the original. This can be done using image editing software such as GIMP or Photoshop, where you can adjust light curves and use the "colour picker tool" to select a foreground colour. Additionally, UHC filters can be used when nebulas are involved to reduce light pollution.
It is worth noting that removing light pollution can be challenging, especially when there are multiple gradients in the image, such as the Milky Way brightness, light pollution from a city, and an airglow gradient. In such cases, a simple subtraction may not be effective, and more advanced techniques or software may be required. However, it is always important to consider the impact of any editing steps on colour accuracy and to avoid destructive steps that can negatively affect the final image.
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Employ the Roger Clark method for post-processing astrophotos
Roger Clark's method for post-processing astrophotos involves a few key steps to effectively remove light pollution and produce accurate, natural colours. Firstly, it is important to work with raw data and a good raw converter with 16-bits/channel. Clarkvision.com, Roger Clark's website, provides a comprehensive guide to the process, including the use of the curves tool to subtract light pollution.
The first step is to set the white balance to daylight. This is followed by adjusting the colours by selectively removing light from the colour channels. The red channel typically has the most light pollution, followed by green and then blue. This is done by moving the lower left point in the curves tool to the right, as demonstrated in Figure 4b on Clarkvision.com. Moving this point by different amounts in each colour channel will subtract varying amounts of light pollution.
The next step is to increase the contrast by adding points on the curves function line, as shown in Figure 6b on the website. This process can be done using any image editor with a curves tool and 16-bits/channel support, such as Photoshop or GIMP. Additionally, the rnc-color-stretch algorithm, a free open-source program, can be utilised to achieve a final or near-final stretch result.
It is worth noting that some people prefer to select the brightest parts of the image, feather the selection, and then invert the selection to create a mask before applying the curves stretch. However, this technique may result in the creation of halos around the selection boundaries. Practising on personal images and experimenting with different methods can help refine the process and achieve the desired results.
By following these steps, astrophotographers can effectively remove light pollution and produce images with accurate colours, enhancing the beauty of the night sky.
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Use the Hue/Saturation tool to tweak colours causing casts
To remove light pollution in post-processing, you can use the Hue/Saturation tool to tweak colours causing casts. This tool is available in image editing software such as Photoshop and GIMP.
The Hue/Saturation tool allows you to adjust the hue (colour), saturation (purity), and lightness of the entire image or specific colours within it. To use this tool effectively to remove colour casts caused by light pollution, follow these steps:
- Open your image in the image editing software.
- Go to the "Layer" menu and select "New Adjustment Layer" or "Hue/Saturation" to create a new Hue/Saturation adjustment layer.
- In the control panel, you will see a drop-down menu that says "Master". If you want to adjust all colours in the image, leave this selected. If you only want to adjust a specific colour, select that colour from the drop-down menu.
- Use the Hue slider to shift all the colours in your image clockwise or counterclockwise around the colour wheel. This can help neutralise unwanted colour casts.
- Use the Saturation slider to increase or decrease the richness of the colours. Reducing saturation can help minimise the impact of colour casts.
- Optionally, use the Lightness slider to lighten or darken the colours. Be careful when using this slider on the entire image, as it can reduce the tonal range.
- Make adjustments until you achieve the desired result. You can always reverse any changes using the reset function or the backward arrow in the control panel.
By following these steps and making adjustments to the hue, saturation, and lightness of colours, you can effectively use the Hue/Saturation tool to tweak colours causing unwanted colour casts due to light pollution.
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Frequently asked questions
One simple way to remove light pollution is to blur the image and subtract that from the original. Another method is to use the Hue/Saturation tool in your image-editing software to tweak the colour causing the cast in your image.
You can use the downloadable 'G'MIC' filters in GIMP to remove light pollution. Duplicate the light-polluted image layer, set the layer blending mode to 'Difference', and then flatten the two layers.
The best way to avoid light pollution is to get out of light-polluted areas. Alternatively, you can use UHC filters when nebulas are involved and the Roger Clark post-processing method.
