Bronte Wells
Light pollution can be a common issue for astrophotographers, especially in light-polluted areas. DeepSkyStacker is a free application that can be used to stack astrophotography images. However, it is important to note that DeepSkyStacker does not recognize light pollution and processes it as any other source of light. Therefore, light pollution removal techniques must be applied during pre- or post-processing. Pre-processing adjustments, such as reducing lights and brightness, can aid in subsequent light pollution removal. Additionally, the use of light pollution filters and custom white balance settings can help mitigate light pollution during image capture.
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What You'll Learn
Use a light pollution filter
Light pollution filters can be an effective way to reduce light pollution when using DeepSkyStacker for astrophotography. These filters help to block out artificial light sources, allowing you to capture images with longer exposure times before the image is blown out.
There are a few different types of light pollution filters that you can use with your camera. One option is to use a clip-in light pollution filter, such as the SkyTech CLS-CCD or the Astronomik CLS. These filters can help reduce moonglow and improve the colour accuracy of your images. Additionally, you can use a Hydrogen-Alpha filter, which blocks almost all light except for a narrow band of light at 656nm, which is the natural frequency of hydrogen in starlight. This type of filter can be particularly effective in the presence of a full moon.
When using a light pollution filter with DeepSkyStacker, it is important to set the correct white balance in your camera. For unmodified cameras, use a daylight white balance, while modified cameras or cameras with light pollution filters should use a custom white balance. Set the white balance in the camera for both modified and unmodified cameras with the filter in place, and refer to your camera manual for specific instructions on setting a custom white balance.
It is worth noting that light pollution filters can affect the signal strength and alter the natural colour emitted by stars. To compensate for the weaker signal, you may need to increase your total exposure time or capture more integrated exposure time. Additionally, capturing images during the new moon phase can help improve colour accuracy, as the bright glow of the moon can reduce contrast in deep-sky images.
Using a light pollution filter with DeepSkyStacker can significantly improve your results, especially in light-polluted environments. Experiment with different filters and exposure settings to find the best combination for your specific camera and shooting conditions.
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Shoot custom white balance
Shooting custom white balance is a technique used to calibrate cameras when shooting under artificial lights or in light-polluted skies. This technique helps to remove light pollution from images, especially in deep-sky photography.
To shoot a custom white balance, photographers can use a grey card, which is specifically sold for this purpose in camera stores. The grey card is shot in daylight, yielding a particular Kelvin temperature and tint value. These values are then used as a reference for the camera's white balance setting.
For deep-sky photography, a custom white balance can be achieved by taking a reference image of a relatively blank section of the sky. This allows the camera to correct for the colour of light pollution. It is important to note that the section of sky does not have to be completely empty, but it should not be filled with a deep-sky object.
When shooting in RAW format, a custom white balance is optional. The white balance can be adjusted during image processing, and programs like DeepSkyStacker can apply custom white balance settings if they are available in the image metadata.
Additionally, for JPEG images with incorrect white balance, DeepSkyStacker offers the option to use RGB Channels Background Calibration under the Light Frames Tab of the Stacking Parameters Dialog. This feature helps to correct the white balance and remove the effects of light pollution from the images.
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Reduce brightness
Light pollution can be a tricky issue to navigate when using DeepSkyStacker. The software is unaware of light pollution and processes it as it would any other source of light in the night sky. This can lead to issues such as the horizon being blown out and the multiplication of light pollution.
One way to tackle this is to reduce the brightness of your images. This can be done in pre-processing by making a few adjustments, such as reducing lights and brightness, which will help with subsequent light pollution removal.
When shooting wide-angle shots, using a light pollution filter is recommended. This will improve results by a factor of 2x to 3x. Using a light pollution filter will also reduce the amount of exposure time needed to get good results. For every magnitude brighter the sky is with light pollution, you will need around 2.5x more exposure.
Additionally, you can increase the total brightness of your images by up to 16 without exceeding the maximum threshold. This can be done by modifying the white balance by changing the red and/or blue scales. However, it is important not to go over the 16x multiplier for each channel (brightness multiplied by the channel scale).
It is also recommended to shoot custom white balance images in-camera, with the filter in place, for both modified and unmodified cameras. This will help DeepSkyStacker decode the colours correctly.
By following these steps and reducing the brightness of your images, you can effectively minimise the impact of light pollution on your DeepSkyStacker results.
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Shoot longer exposures
Longer exposures are ideal for capturing the night sky, but they come with their own set of challenges. Firstly, let's understand why longer exposures are beneficial. The faint deep-sky objects you're trying to capture are often hidden by sky glow or light pollution. To capture these faint objects, you need a high signal-to-noise ratio. The signal comes from the photons emitted by the deep-sky object, so the longer the exposure, the more photons you collect, resulting in a better signal.
However, longer exposures come with a trade-off. Light pollution can quickly swamp a long-exposure image. This is where light pollution filters come in. Using a light pollution filter allows you to shoot longer exposures before the histogram is blown out. But, as mentioned earlier, there are drawbacks to using filters. The signal (light) is often weakened, and the natural colour emitted by the stars may be altered.
Additionally, longer exposures require accurate tracking to avoid star trailing. One way to compensate for light pollution and prevent star trailing is to take multiple shorter exposures and combine or "stack" them. This technique, known as "dithering," helps smooth out pattern noise in the sensor and results in a similar outcome to a single long-exposure image.
For example, combining 20 exposures of 30 seconds each will give you a final image comparable to a single 10-minute exposure. This method is especially useful when dealing with hot and humid conditions, as shorter exposures generate less noise and avoid sensor overheating.
In conclusion, while longer exposures provide a better signal-to-noise ratio, they can be challenging due to light pollution and star trailing. To overcome these issues, you can use light pollution filters or opt for stacking multiple shorter exposures, ensuring you capture the beauty of the night sky without sacrificing image quality.
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Remove dark signal
To remove the dark signal from light frames, you need to use a dark frame that contains only the dark signal. Dark frames are created by shooting pictures in the dark with the lens covered. The number of dark frames you should use depends on the source, with one recommending a minimum of 15, and another suggesting between 10 and 20.
The dark frames must be created with the same exposure length, ISO and temperature as your light frames. Dark frames are automatically combined by DeepSkyStacker to create and use a clean master dark or master dark flat.
Dark subtraction can be optimised so that the entropy of the resulting picture (light frame minus dark frame) is minimised by applying a coefficient between 0 and 1 to the dark frame. This optimisation allows for the use of dark frames taken in non-optimal conditions, especially concerning temperature.
It is important to note that DeepSkyStacker will only stack images that contain at least eight stars that are common between all light frames. This means that you should set the Star Detection Threshold in the Settings so that DeepSkyStacker detects 20 or more stars to stand a good chance of finding eight stars in common between all light frames.
If you are shooting with a stock DSLR, you may need to experiment with settings to produce a pleasing result. For example, if you are using a modded DSLR, leave the white balance checkboxes unchecked. Using an auto white balance or the “camera white balance” with a modified camera will produce odd results.
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Frequently asked questions
DeepSkyStacker is unaware of light pollution and processes it as it would any other source of light in the night sky. Therefore, you will need to remove light pollution in pre- or post-processing.
You can do this by reducing lights and brightness.
You can use PixInsight, or Gradient Xterminator for wide-angle shots.
