How To Subtract Light Pollution From Your Night Sky

Light pollution is the excessive or inappropriate use of outdoor artificial light, which is affecting human health, wildlife behaviour, and our ability to observe stars and other celestial objects. It is a global issue, with only the most remote regions on Earth remaining in total darkness.

There are several ways to reduce light pollution, including the use of warm-coloured bulbs, dimmers, motion sensors, and timers. One method to remove light pollution from astrophotography is the Blur and Subtract technique, which involves taking a photo with the correct focus, and then taking the same photo but blurring it as much as possible. The blurred image is then subtracted from the original image, resulting in a darker background with the original stars.

Using the Blur and Subtract technique

Step 1: Take your desired photo (Photo 1)

Set up your camera and compose your shot. Ensure the focus is set to infinity. Take the photo.

Step 2: Take the same photo with a blurred focus (Photo 2)

Without changing the composition, adjust your camera settings to manually blur the image as much as possible. This can be done by focusing on a nearby object or using a manual focus setting. Take the second photo.

Step 3: Subtract Photo 2 from Photo 1

Using photo processing software or image manipulation tools, open both images. Subtract the blurred image (Photo 2) from the original image (Photo 1). This can be done in Pixlr by opening the images as layers and selecting the "Difference" mode. In ImageMagick, you can use the "convert" command with the "-compose minus" option to perform the subtraction.

The resulting image will have reduced light pollution, with a darker background and the original stars visible. You may need to make further adjustments, such as removing diffused bright stars or stray light using healing tools.

This technique is particularly effective for astro-photography in urban areas with light pollution. It can also be applied to daylight photos and images with terrestrial objects, creating interesting surrealistic effects.

Reducing light pollution with LED and compact fluorescent lighting

Light pollution is a human-made alteration of outdoor light levels from those occurring naturally. It has harmful effects on wildlife and ecosystems, energy and climate change, and human health.

LEDs and compact fluorescents (CFLs) can help reduce energy use and protect the environment, but only warm-coloured bulbs should be used. "Warm" toned or filtered LEDs (CCT 3000 K or lower; S/P ratio 1.2 or lower) should be used to minimise blue emission.

• Use shades or covers that force the light downward, producing less spillover.
• Minimise light use in general by using fewer lights, creating a smaller targeted illumination area, and reducing the duration of their use.
• Reduce decorative lighting usage.
• Use lighting controls such as dimmers, motion sensors, and timers to reduce the intensity of the light and control when lights turn on and off.
• Direct the light away from neighbours and neighbouring properties by using shields, also known as light shrouds or glare shields.
• Avoid using lights in the blue spectrum, as blue light has a larger geographic reach and creates more glare.

Using editing software to remove sky glow

Light pollution can be a nightmare for astrophotographers, but there are ways to remove it using editing software. The process will differ depending on the software used, but the underlying principle is the same: by subtracting the light pollution, you can restore the image to its natural colours.

Using Adobe Photoshop

One method for removing light pollution in Photoshop involves duplicating the base image to provide two identical layers. Then, apply the 'Dust and Scratches' filter with a radius of around 30 pixels. This will cause most stars to disappear, leaving only the brighter ones. Next, use the clone tool to remove any remaining bright spots, such as stars, by sampling nearby regions with similar levels of light pollution. To smooth the image further, you can apply a Gaussian Blur of around 40-50 pixels.

Finally, set the layer blending mode to 'Difference' and flatten the two layers. This will remove the light pollution from the image, and you can then stretch the image to bring out fainter stars. It may be helpful to do this in two passes, first removing 50% of the light pollution before flattening the layers, and then repeating the process with the opacity set to 100%.

Using GIMP

GIMP is a free alternative to Photoshop that can also be used to remove light pollution. First, load the light-polluted image and duplicate the layer. Then, click on the 'Filters' tab and scroll down to the 'Repair Image' sub-menu, where you will find a filter called 'Remove Hot Pixels'. This will remove most of the stars from the image.

Next, use the clone tool to remove any remaining bright spots, such as stars, by sampling nearby regions with similar levels of light pollution. As with the Photoshop method, you can smooth the image further by applying a Gaussian Blur of around 40-50 pixels. Set the 'Difference' blending mode in the 'Layers' box and flatten the layers. This will remove the light pollution, and you can then export the image to the appropriate folder.

Using Adobe Lightroom

Start by setting the white balance to daylight and increasing the saturation. Enable lens corrections to correct for vignetting, which will make the next step easier. Zoom in on the image and adjust the noise reduction settings to balance cleaning up the image and retaining faint stars.

Crop the image to show only the sky, with no foreground or clouds. This is temporary and will help you get relevant information from the histogram. Set the contrast to 100% and push the exposure until the lower corner of the histogram is touching the right.

Now for the step that removes the light pollution: select the coloured point curves (red, green, and blue) and drag the black point to the left of the histogram for each colour. This works because space is dark, and the darkest pixels are lit purely by light pollution. By dragging the black point, you are subtracting that light. Any remaining light should be from celestial objects.

Now you can undo the cropping of the image and use the gradient tool to correct for any remaining light pollution or haze. Radial gradients can be used to selectively darken or brighten portions of the image, and you can also make tiny changes to the colour.

Using PixInsight

Dynamic Background Extraction (DBE) is a useful tool in PixInsight for removing light pollution. Push the tolerance up to 1.5, 2, or even 3 and experiment to find the best result. Usually, subtraction is the best method, but division can also work.

General Tips

• It is important to note that editing software cannot work miracles. Detail smeared beyond recognition by light pollution will not be rescued, but you may be surprised by how much you can improve an image.
• Remember that the night sky is not black. Even at the darkest observing site in the world, you can still distinguish between the sky and the horizon.
• If you are photographing nebulae, be aware that light pollution removal techniques may not work as well. They can leave dark halos or remove parts of the nebulae along with the light pollution.
• The best way to get good astrophotos is to get out of light-polluted areas!

The negative impact of light pollution on human health

Light pollution is the human-made alteration of outdoor light levels from those occurring naturally. Research suggests that it can negatively impact human health, increasing the risks of obesity, depression, sleep disorders, diabetes, breast cancer, and more.

Circadian Rhythm and Melatonin

Like most life on Earth, humans adhere to a circadian rhythm — a sleep-wake pattern that is governed by the natural day/night cycle. This cycle is largely influenced by photoreceptors in the eye that are more sensitive to blue wavelengths of light. It is important for humans to experience outdoor lighting during the day and minimize the amount of light entering their eyes at night to maintain a good circadian cycle. Unfortunately, artificial light at night can disrupt that cycle.

Night-time exposure to artificial light also suppresses melatonin production, which is needed to keep us healthy. Melatonin has antioxidant properties, induces sleep, boosts the immune system, lowers cholesterol, and helps the functioning of the thyroid, pancreas, ovaries, testes, and adrenal glands.

Not All Artificial Light is Created Equally

Exposure to any light at night can suppress melatonin, but the photoreceptors in the eyes that drive this suppression are more sensitive to blue light. This is why it is recommended to reduce blue light content at night to help improve sleep. Unfortunately, most LEDs used for outdoor lighting, as well as computer screens, TVs, and other electronic displays, create abundant blue light.

For this reason, a 2016 American Medical Association report expressed concern about exposure to blue light from outdoor lighting and recommended shielding all light fixtures and only using lighting with a correlated color temperature (CCT) of 3000 K and below.

Glare from Outdoor Lighting is a Safety Hazard

Glare from poorly shielded outdoor lighting also harms human health because it decreases vision by reducing contrast. This limits our ability to see potential dangers at night. Aging eyes are especially affected.

You can do your part to reduce glare by using shielded fixtures and dimming the light levels to only what is required. Often, outdoor light fixtures produce more light than is necessary, and dimming them to more appropriate levels reduces glare and saves energy.

The impact of light pollution on wildlife and ecosystems

Light pollution is the human-made alteration of outdoor light levels from those occurring naturally. It has been found to have harmful effects on wildlife and ecosystems.

Impact on wildlife

Light pollution disrupts the daily cycle of light and dark that plants and animals rely on to govern life-sustaining behaviours such as reproduction, nourishment, sleep, and protection from predators.

• Nocturnal animals: Light pollution radically alters the nighttime environment for nocturnal animals, turning night into day. It also makes them more vulnerable to predators.
• Amphibians: Artificial lights can disrupt the nocturnal breeding rituals of frogs and toads.
• Sea turtles: Artificial lights can disorientate adult and baby sea turtles, causing them to move inland instead of finding safety in the ocean.
• Birds: Artificial lights can cause birds to wander off course and collide with buildings and towers. Migratory birds may also abandon preferable roosting sites to avoid lights.
• Insects: Insects are drawn to light, but artificial lights can create a fatal attraction. Insects may deplete their energy reserves, increase exposure to predators, and miss courtship cues from mates.

Impact on ecosystems

Light pollution also has a broader impact on ecosystems.

• Wetland habitats: Glare from artificial lights can impact wetland habitats that are home to amphibians.
• Plant growth: Lighting disrupts photosynthesis, the process by which plants grow.
• Food webs: Some predators exploit the attraction of insects to light to their advantage, affecting food webs in unanticipated ways.

Frequently asked questions