Bronte Wells
Light pollution is a growing environmental concern, with its effects reaching far and wide. It has been linked to a range of health issues and has a detrimental impact on the environment and night skies. For photographers, light pollution can ruin what would otherwise be a perfect image. PixInsight is a software program that offers a solution to this problem, with tools that can reduce or even remove light pollution from images. The software achieves this by examining a starless image and removing any background unevenness, adjusting pixel values to create a flat sky of uniform brightness and colour. This article will explore the various methods and tools offered by PixInsight to combat light pollution in images, including the use of filters, exposure control, and masking.
| Characteristics | Values |
|---|---|
| How to remove light pollution | Use PixInsight's gradient reduction tools, such as DynamicBackgroundExtraction (DBE) and AutomaticBackgroundExtraction (ABE) |
| DBE | Allows direct user interaction with the image |
| ABE | Produces fantastic results without any manual steps involved |
| How it works | Sample the sky background at various places without stars or DSO, create a map, and adjust pixel values so they're all the same |
| Subtraction vs Division | Subtraction is intended to remove gradual effects like light pollution, while division is intended to remove vignetting and atmospheric diffraction/absorption |
| Other tools | Filters, exposure control, and masking |
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What You'll Learn
Using PixInsight's Automatic Background Extractor
Light pollution from cities, the moon, and low-level ambient sky glow can affect astrophotography. PixInsight's Automatic Background Extractor (ABE) is a powerful tool to remove light pollution gradients and tints from astro-images.
To use the ABE tool, first, open your image stack in PixInsight. Go to Process -> Background Modelling -> Automatic Background Extractor. The default values are generally good, but be sure to select “Subtraction” instead of “None” for the Target Image Correction parameter. This will produce two images: the original and the background-extracted stack.
The ABE tool works by examining a starless image and removing the background's unevenness. It adjusts pixel values to make the sky appear flatter, with the same brightness and colour throughout. However, it cannot distinguish between signals from light pollution and celestial objects like nebulae, so it makes educated guesses.
ABE is a good starting point for removing light pollution, but it might not be suitable for final image processing. If you are not satisfied with the initial extraction, you can run it again and experiment with the function degree parameter under the interpolation and output submenu. Increasing this value will result in more aggressive background extraction.
For more precise control, you can also try the manual Dynamic Background Extraction (DBE) tool. This allows you to place reference samples on true background areas, avoiding celestial objects. DBE requires a minimum of three samples and works best with linear images that have not been stretched. You can adjust various parameters like Sample Radius, Samples per Row, Smoothing Factor, and Shadows Relaxation to fine-tune the extraction process.
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Using Dynamic Background Extraction (DBE)
Dynamic Background Extraction (DBE) is one of the two primary tools PixInsight offers for removing unwanted gradients from images. It is a manual process that involves defining background points and creating a background model.
To begin using DBE, open the tool and click on the image you wish to extract a gradient from. The image should be linear, and it is helpful to use a screen stretch to see the image and the gradients within. A four-panel grid will appear, and you can start placing background reference samples. DBE requires a minimum of three samples, but more may be needed for optimal results.
Once the samples are placed, you can adjust the model parameters. Increase the sample radius to 25-50 to make it easier to manipulate, and set the samples per row to 10-15. You want most of the sample points to be applied, so you have less editing to do.
After running DBE, you will see a gradient across the background model, which indicates the unwanted gradient being removed. You may need to run DBE multiple times to achieve a smooth and flat background. It is important to note that this process should be applied to all images, and the approach may vary depending on the type of image being processed.
Additionally, there may be times when the standard custom method does not produce satisfactory results. In such cases, you can increase the number of samples per row to account for abrupt changes in the image. However, ensure that the samples do not overlap with stars or other important features. You can use an inverted image to identify the sample points more easily.
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Removing sky glow
Light pollution is a common issue in photography, especially astrophotography, due to its interference with image quality. PixInsight is a software program that can effectively reduce or even remove light pollution from photos, including sky glow, using various tools and techniques.
One of the primary tools offered by PixInsight for sky glow removal is DynamicBackgroundExtraction (DBE). This tool allows direct user interaction with the image, providing maximum flexibility in identifying and removing unwanted gradients. Users can experiment with different tolerance levels in DBE, typically ranging from 1.5 to 3, to achieve the desired result.
Another tool available in PixInsight is AutomaticBackgroundExtraction (ABE), which includes both automatic and manual versions. The automatic version, also known as the Automatic Background Extractor, often produces excellent results without requiring manual steps. To access this tool, navigate to Process -> Background Modelization -> Automatic Background Extractor in PixInsight. It is important to select “Subtraction” instead of "None" under the Target Image Correction -> Correction parameter.
While ABE is generally effective, it may not always provide perfect results. In such cases, it is recommended to run the process a second time or experiment with the function degree parameter under the interpolation and output submenu. The higher the number, the more ABE extracts.
Additionally, PixInsight offers other tools such as filters, exposure control, and masking to combat light pollution. These tools enable photographers to minimise sky glow and capture the natural beauty of the night sky. It is worth noting that background extraction should be one of the initial steps when post-processing astronomy images.
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Utilising filters, exposure control, and masking
Pixinsight is a software program designed for professional astrophotography. It offers a range of tools to reduce and remove light pollution, including filters, exposure control, and masking.
Filters are used to physically block light pollution from reaching the camera sensor. Light pollution filters are designed to target specific light wavelengths, such as those emitted by sodium, halide, and mercury lamps. However, with the increasing prevalence of LED lighting, which emits a broad spectrum of light, the effectiveness of these filters is reduced as they end up blocking desirable signals as well. Broadband LP filters, for instance, have been reported to reduce the overall light spectrum by half or more, resulting in a loss of galaxy signals. Dual narrow-band filters, on the other hand, are more effective in light-polluted environments for emission nebulae.
Exposure control plays a crucial role in managing light pollution. Longer exposures tend to exacerbate the issue, leading to images with a pinkish or whitish background. By carefully controlling the exposure duration, photographers can minimize the impact of light pollution on their images.
Masking is a powerful feature in Pixinsight that allows for precise control over image processing. Masks enable users to selectively target and protect specific areas of an image during processing. With masks, it is possible to modify brightness and colour, remove gradients, and improve contrast. Pixinsight offers a user-friendly interface for creating, modifying, and utilizing masks, making it a valuable tool for astrophotographers.
While filters can be effective in certain situations, such as with emission nebulae, many photographers prefer to rely solely on Pixinsight's processing capabilities to remove light pollution. The software's gradient reduction tools are particularly noteworthy, as they intelligently identify and reduce the impact of light pollution while preserving the desired data. This approach, combined with background extraction techniques, often yields superior results compared to solely relying on filters.
In conclusion, Pixinsight's tools for managing light pollution, including filters, exposure control, and masking, provide photographers with a comprehensive set of options to reduce and remove light pollution from their astrophotography images. By carefully utilizing these techniques, photographers can capture stunning images of the night sky while mitigating the unwanted effects of light pollution.
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Understanding light pollution gradients
Reduced Contrast:
Light pollution gradients often result in reduced contrast within astro-images. The bright light from urban areas can overwhelm the dimmer parts of the photograph, making faint stars and distant nebulae difficult to see or entirely invisible. This challenge highlights the importance of mitigating light pollution to ensure that the finer details of the night sky are not lost.
Color Shifts:
Unwanted light sources can introduce undesirable color shifts in images. The dominant colors in light pollution, typically orange or yellow hues, can taint the colors of celestial objects. Even a small amount of color introduced by light pollution can have a significant impact. Removing these color gradients is crucial to restoring the natural hues of stars and deep-sky objects.
Image Noise:
To compensate for light pollution, photographers may increase the camera's sensitivity (ISO setting). While this can help in combating light pollution, it also introduces image noise, further degrading the overall image quality. It is important to strike a balance between addressing light pollution and managing image noise to obtain the desired results.
Limited Exposure Time:
In areas with severe light pollution, achieving long exposures without overexposing the image becomes challenging. This limitation restricts the ability to capture fine details and can hinder the overall astrophotography experience. Overcoming this issue is essential for capturing the night sky in all its glory.
Solutions with PixInsight:
PixInsight offers powerful tools to address light pollution gradients in astrophotography. The software's Dynamic Background Extraction (DBE) tool is highly effective in identifying and removing unwanted gradients. It provides maximum flexibility by allowing direct user interaction with the image. Additionally, the Automatic Background Extraction (ABE) tool offers an alternative approach that is often easier and more effective for certain image types.
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Frequently asked questions
Light pollution is a phenomenon where there is too much light in a place, usually in cities and suburbs. It has been known to cause a range of health and environmental issues. For photographers, it interferes with image quality.
PixInsight has a range of tools to reduce and remove light pollution, including sky glow and light trespass. These tools include filters, exposure control, and masking. You can also use the Automatic Background Extractor tool, which is found under Process -> Background Modelization -> Automatic Background Extractor.
ABE is automatic and usually produces fantastic results without any manual steps. DBE, on the other hand, is manual and allows the user to directly interact with the image, giving them maximum flexibility in identifying any gradients.
