Greta Jimenez
Light pollution is a major issue for astrophotographers, with over 80% of the world's population living in light-polluted areas. However, it is still possible to capture stunning astrophotographs even in heavily light-polluted cities. The key is to use the right equipment and techniques to mitigate the effects of light pollution. This includes using monochrome cameras, narrowband filters, and increasing exposure time. Additionally, certain targets can be captured without using any filters at all, and some filters, such as light pollution filters, may alter the natural colours of the stars. Techniques like ETTR (Exposure To The Right) can also help improve images taken in light-polluted areas. While dark skies are ideal for astrophotography, with the right tools and knowledge, it is possible to capture incredible images even from your own light-polluted backyard.
| Characteristics | Values |
|---|---|
| Camera | DSLR, monochrome |
| Filters | Light pollution, narrowband, Astronomik CLS, IDAS LPS D-1, Hydrogen Alpha |
| Exposure | Long exposure, high ISO speed, ETTR ("Exposure To The Right") |
| Targets | Milky Way, Orion constellation, Cat's Eye Galaxy (M94), Eagle Nebula (M16), Whirlpool Galaxy |
| Other equipment | Telescope, tripod |
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What You'll Learn
Use a monochrome camera with narrowband filters
Using a monochrome camera with narrowband filters is a popular and effective method for astrophotography in light-polluted areas. This combination can cut through light pollution, allowing you to capture nebulae in great colour and detail.
Narrowband filters are more precise than RGB filters, blocking out all wavelengths of light that interfere with astrophotography images. They only allow a few nanometres of light through, typically transmitting between 3 and 12 nm, whereas RGB filters transmit about 100 nm of light. This means that narrowband filters can pass through the light from gases in deep-sky targets while largely ignoring the intense glow of light pollution.
Monochrome cameras are recommended for narrowband imaging because they generally produce better-quality data, especially in light-polluted skies. While narrowband filters can also be used with one-shot-colour cameras or DSLRs, the built-in colour filters on the pixels (Bayer matrix) will block a lot of the narrowband light, rendering the system less efficient.
To capture a full-colour image with a monochrome camera and narrowband filters, you will need to take a set of photos for each filter. For example, 100 photos with a red filter, 100 with green, 100 with blue, and 100 with luminance. Once you have these different sets, you can stack each one to reduce noise, and then assign the stacked photos to specific colour channels in Photoshop or another image processor.
In addition to narrowband filters, there are various types of light pollution filters that can be used with monochrome cameras. These include the Optolong L-eXtreme filter, which features narrow bandpasses perfect for nebula photography from the city, and the Astronomik CLS and IDAS LPS D-1, which help to reduce moonglow.
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Capture images during the new moon phase
Light pollution is a major challenge for astrophotographers, and the moon can be a significant source of light pollution. The moon's brightness varies depending on its phase, with the full moon being the brightest and the new moon being the darkest. Thus, the new moon phase offers a unique opportunity for astrophotographers to capture stunning images with minimal light pollution.
The days surrounding the new moon phase, about four days before and after, are ideal for astrophotography as they provide the least amount of light pollution. During this period, you can capture the Milky Way in all its glory, with the crispness in the core and a dark foreground. The night skies appear darker, allowing you to see and photograph more stars. This is the perfect time to capture the blackest nights and the most stars.
While the new moon phase offers darker skies, it is important to note that the foreground of your images may appear darker as well. This can be adjusted in post-processing by brightening the foreground while maintaining the quality of the night sky. It is always better to overexpose than to underexpose your images, as it is easier to adjust brightness than to try to add light to underexposed areas.
Additionally, during the new moon phase, you can capture true-color images with more natural colors. The bright glow of the moon can reduce the contrast in deep-sky images, affecting the overall quality. By shooting during the new moon phase, you can avoid this issue and capture the true beauty of the night sky.
Astrophotography during the new moon phase offers a unique perspective on the night sky, allowing you to capture images that showcase the vastness of the universe and the brilliance of the stars. With careful planning and attention to the lunar cycle, you can create breathtaking images that highlight the beauty of the cosmos.
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Use a light pollution filter
Light pollution filters are a popular choice for astrophotographers battling light pollution in their images. They can be a great way to improve your images without spending too much money. The filters work by blocking specific wavelengths of light in the visible spectrum that are associated with artificial light sources, such as street lamps. This allows the important colours and light emitted by your astrophotography subject to reach the camera sensor without the nasty brown or yellow/orange glow of a washed-out sky.
There are two main types of light pollution filters for colour cameras: broadband filters (broad-spectrum, multi-broadband) and multi-narrowband (often dual-narrowband) filters. Broadband filters are great for capturing galaxies, reflection nebulae, dark nebulae, and natural star colours. Narrowband filters are best used on emission nebulae and supernova remnants. They block out all wavelengths except for a small range, for example, only light waves near 656nm will be visible with an H-Alpha narrowband filter. The 589nm wavelength of sodium-vapour lamps, a common source of light pollution, will be completely blocked.
Some popular light pollution filters include the Astronomik CLS and IDAS LPS D-1, which help to reduce moonglow. The Optolong L-Pro Clip-In Filter for Canon EOS R is another good option, but you need to ensure you purchase the correct-sized clip-in filter for your camera body. The NEEWER Natural Night filter is a good budget option, and the K&F Concept Natural Night light pollution filter is also excellent for general nighttime shooting.
It's worth noting that not everyone agrees that using a light pollution filter is the best way to do astrophotography, and it may not be suitable for deep-sky astrophotography. The signal (light) is often weak, and the natural colour emitted by the stars can be altered. Additionally, the type of camera you use will determine the right light pollution filter for you. For example, a stock DSLR camera may be less effective when paired with a multi-narrowband filter.
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Stack multiple images to increase signal and reduce noise
Light pollution is a common issue in astrophotography, and it can be challenging to capture clear and detailed images of the night sky in heavily light-polluted areas. One effective technique to improve the quality of your astrophotography images and reduce the impact of light pollution is to stack multiple images. This process, also known as integration, involves combining several short-exposure images to increase the signal-to-noise ratio (SNR).
Each image you capture contains both the desired signal, such as the stars and celestial objects, and unwanted noise, which can appear as random specks, colour changes, or changes in brightness. By stacking multiple images, you can reduce the noise and enhance the signal, resulting in a clearer and more detailed final image. The stacking process involves aligning and averaging the individual images, which helps to diminish the random noise while keeping the signal consistent.
To stack images, you can use specialised software or manual techniques. Software programs like PixInsight automatically calibrate, align, and stack your images, making the process more efficient. Alternatively, you can use Adobe Photoshop to manually align and layer each exposure on top of each other, giving you more control over the process. Adobe Bridge is another useful tool for file organisation and selection, allowing you to review camera and lens settings for each shot.
When stacking images, it's important to ensure that each image is taken with the same orientation and exposure length. It's also recommended to capture a minimum of 10-12 exposures to create a successful stack. Additionally, consider the ISO settings; while higher ISO sensitivities can help detect weaker details in the night sky, be cautious not to set it too high, as it may wash out your signal.
By stacking multiple images, you can effectively reduce the impact of noise and improve the overall quality of your astrophotography images, even in light-polluted environments. This technique allows you to capture clearer details of the night sky and perform aggressive curve and level adjustments without compromising the data in your images.
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Overexpose images at a high ISO speed
Overexposing images at a high ISO speed is a viable strategy for astrophotography in light-polluted areas. This technique, known as ETTR ("Expose To The Right"), involves pushing ISO sensitivities to relatively high levels to detect the weakest details of light in the night sky. It is particularly useful in light-polluted environments as it allows you to capture faint details that would otherwise be obscured by light pollution.
When using the ETTR technique, it is important to understand the trade-offs involved. Increasing the ISO sensitivity will amplify the signal, resulting in a brighter image. However, this also increases noise or grain in the image. Modern cameras have an electronic sensor that converts light into electronic signals, and the efficiency of this conversion is known as quantum efficiency. Some cameras are ISO invariant, exhibiting less noise trade-off at higher ISO settings.
To strike a balance, you should aim for an ISO setting that provides a healthy amount of signal while keeping noise within acceptable levels. For example, an ISO setting of 1600 for each single sub-exposure of 2-minutes in length can produce a balanced histogram, and the stacking process can help remove noise. The f-ratio of the telescope or lens also comes into play when determining the appropriate ISO setting. A faster lens with a wider aperture will allow more light to reach the sensor, reducing the need for a higher ISO.
Additionally, consider the temperature of the imaging sensor, especially during longer exposures. On cooler nights, the sensor will remain cooler for longer, allowing for longer exposure times without excessive noise. In contrast, on hotter nights, the sensor may heat up faster, resulting in increased noise at higher ISO settings. In such cases, a higher ISO may be preferable to compensate for the sensor heating.
Finally, it is worth noting that overexposing images at a high ISO is just one technique in your astrophotography toolkit. Combining multiple exposures of varying lengths can help capture details of both dim and bright celestial objects. Additionally, using a portable star tracker can enable even longer exposures by compensating for the Earth's rotation.
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Frequently asked questions
Here are some tips for doing astrophotography in light-polluted areas:
- Use a monochrome camera equipped with narrowband filters.
- Use a light pollution filter like the Astronomik CLS or IDAS LPS D-1 to reduce moon glow.
- Capture images during the new moon phase to capture true colour images with more natural colours.
- Stack several images together to increase the signal-to-noise ratio.
- Use a fast telescope or a fast DSLR lens (f/1.4 or f/1.8).
A monochrome camera equipped with narrowband filters is generally recommended for astrophotography in light-polluted areas. This is because monochrome cameras produce better quality data, especially when imaging from light-polluted skies.
Light pollution can make it difficult to capture clear and high-quality images of the night sky. It can also be challenging to find a location with minimal light pollution, as more than 80% of the world's population lives in light-polluted areas.
Light pollution can reduce the contrast in deep-sky images and overwhelm the signal of the target object. It can also alter the natural colour emitted by stars and other celestial objects.
