Greta Jimenez
Light pollution is a major side effect of urbanization, and over 80% of the world's population lives in light-polluted areas. This poses a challenge for astrophotographers, as the glow of light pollution can make it difficult to capture clear and detailed images of the night sky. However, it is still possible to achieve good results even in heavily light-polluted areas. One way to do this is by using a monochrome camera, which has greater sensitivity than a DSLR camera, allowing for shorter exposure times and better dynamic range. Additionally, narrowband filters can be used to enhance the contrast of emission objects by accepting only a narrow range of wavelengths. Light pollution filters can also be used to reduce the orange glow of light pollution, although they may not always produce natural-looking results. Other tips for shooting in light-polluted areas include finding higher vantage points, using a star tracker, and shielding equipment from stray light. With the right techniques and equipment, astrophotographers can capture stunning images of the night sky even in heavily light-polluted urban environments.
| Characteristics | Values |
|---|---|
| Camera | Monochrome CCD camera, astronomy camera, DSLR camera |
| Filters | Light pollution filter, dual bandpass filter, Optolong L-eNhance filter, narrowband filter |
| Targets | Large and bright targets, Orion constellation in Winter, Milky Way galaxy band |
| Telescope | Fast telescope, small refractor telescope |
| Lenses | Fast lenses like f/1.4 or f/1.8 |
| Calibration | Calibration frames, dark frames, bias frames, flats |
| Location | High place like a hill or rooftop, backyard, light pollution-shielded area |
| Post-processing | Photo editing software, graphics editor |
| Planning | Planetarium software and apps like Stellarium |
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What You'll Learn
Use a monochrome camera and quality narrowband filters
If you're shooting astrophotos in light-polluted areas, you may want to consider investing in a monochrome camera and quality narrowband filters. This combination is powerful because narrowband filters are designed for monochrome camera users.
Narrowband filters are excellent for capturing emission nebulae. They enhance the contrast of emission objects by accepting only a narrow range of wavelengths around the emission lines of certain gases within the objects, such as hydrogen-alpha (Ha), oxygen (OIII), and sulphur (SII). These filters block out all the wavelengths of light that normally interfere with astrophotos. Unlike RGB filters, which transmit about 100nm of light, narrowband filters transmit between 3-12nm. The smaller the range, the better, as only the specific wavelengths of light from Ha, SII, and OIII will be visible.
If you're using a monochrome camera, the photos will be in black and white. You will need to map the colours in post-processing. This can be done by taking a set of photos for each filter, for example, 100 photos with a red filter, 100 photos with a green filter, and so on. Once you have these different sets, you can stack each one and assign those stacked photos to specific colour channels in Photoshop or another image processor.
Upgrading to 3nm filters will give you much cleaner and better images when imaging from the city, although they are very expensive. If you love imaging galaxies with hydrogen-alpha, go for a 5nm Ha filter instead of 3nm, as this will give better results.
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Pick the right target, like the Orion constellation
Picking the right target is crucial when shooting astrophotos in light-polluted areas. One of the most popular targets is the Orion constellation, which dominates the winter sky and offers stunning objects like the Orion Nebula (M42) and the Horsehead Nebula.
The Orion Nebula is one of the brightest nebulae in the night sky and is visible to the naked eye. It is part of the larger Orion Molecular Complex, which includes objects such as the Horsehead Nebula, M78, and Barnard's Loop. To capture the Orion Nebula in all its glory, use a lens with a focal length of 50-70mm to frame the most interesting parts of the constellation while also resolving the details that make it special.
The Horsehead Nebula, also known as Barnard 33, is an iconic dark nebula within the Orion constellation. It is located near the easternmost star in Orion's Belt, Alnitak, and is easily identifiable by its shape, which resembles a knight chess piece under perfect conditions. To capture the Horsehead Nebula, you will need a dark sky, a high-power eyepiece for your telescope, and a moonless night.
In addition to the Orion constellation, there are other targets that can be considered when shooting in light-polluted areas. The North America Nebula (NGC 7000) in the constellation Cygnus is a large emission nebula that resembles the shape of the North American continent. The Pacman Nebula (NGC 281) in the constellation Cassiopeia is another bright emission nebula that features glowing hydrogen gas, dark dust lanes, and active star-forming regions, making it ideal for narrowband imaging.
When dealing with light pollution, it is recommended to use narrowband filters that enhance the contrast by accepting a narrow range of wavelengths. These filters can be used to capture images even in the presence of moonlight or from heavily light-polluted locations. Additionally, consider using a monochrome camera as it offers greater sensitivity, requiring shorter exposure times to capture faint details.
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Use calibration frames to reduce noise
Calibration frames are essential to achieving high-quality astrophotography images. They help to correct strange patterns, uneven lighting, and unexpected noise in your images. When captured correctly, they can dramatically reduce noise, fix uneven illumination, and remove other imperfections that may detract from your photos.
There are several types of calibration frames, including light frames, dark frames, bias frames, flat frames, and flat dark frames. Light frames are your main images of celestial objects, but they also include various types of noise and imperfections that need to be corrected. Dark frames are used to increase the signal-to-noise ratio in your photo and reduce "hot or cold" pixels. They are particularly important when taking long exposures, as the chip will introduce "thermal" noise. To capture dark frames, set your camera to the same settings and temperatures as your lights, but keep the cover on. Take at least 15 shots, but generally, the more, the better.
Bias frames are similar to dark frames, but with a faster exposure time. They are not needed when using a dedicated astronomy camera. Flat frames allow you to calculate the correction factor for each pixel so that they all give the same value when exposed to the same quantity of light. This helps to eliminate vignetting, light falloff, and other artifacts in your image due to dust, dirt, or smudges on your sensor or filters. A quick way to capture flat frames is to put a white t-shirt over the end of your scope and point it at a laptop screen.
To integrate all these frames and clean up your images, you can use image processing software such as Deep Sky Stacker. This software can automatically perform all the calibration steps, including combining bias frames, calibrating dark frames, and aligning and stacking light frames to reduce noise and enhance detail.
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Use a star tracker for advanced deep-sky astrophotography
A star tracker is a must-have for advanced deep-sky astrophotography. This technique combines the steps for capturing the Milky Way with those for deep-sky astrophotography. The result is a highly detailed panorama.
Star trackers are a game-changer for astrophotography. They solve the issue of the Earth's rotation by following the stars across the night sky. This allows you to expose your photographs for longer, reducing the ISO and closing the aperture in your lens. With a star tracker, you can use a longer shutter speed and are not limited to the NPF or 500 rules. This results in cleaner images with higher detail and more colour.
To get started with a star tracker, you will need a camera, lens, tripod, and tripod head. A remote shutter release or intervalometer is recommended to prevent camera shake. You may also want accessories like a headlamp or power bank.
When choosing a star tracker, consider its maximum payload, which is the combined maximum weight of your camera, lens, and accessories. The iOptron SkyGuider Pro is a good option for deep-sky astrophotography, accommodating a payload of up to 5 kg. The SAM is another option with a compact and lightweight design, but its app is lacking. If budget is not a concern, the Benro Polaris is a top choice with a huge-capacity battery and a built-in camera interface controller.
With a star tracker, you will be able to capture the beauty of the night sky in all its glory.
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Find a high place with an unobstructed view
When shooting astrophotography in light-polluted areas, one way to improve your results is to find a high place with an unobstructed view. This could be a hill or the rooftop of a building. For example, one astrophotographer on Reddit mentioned that they go to the top of some old medieval walls with a small tower to do their astrophotography. While it may not be ideal, it offers a fairly unobstructed view of the horizon, and they have been able to achieve fantastic results.
Finding a high place with a clear view can help you get above some of the light pollution caused by street lights and buildings. It also gives you a wider field of view, allowing you to capture more of the night sky in your photos. If you can find a place that is away from the busy city centre, you may be able to reduce the amount of light pollution affecting your images.
In addition to finding a high place, you can also try to shield your equipment from stray light by using simple methods such as adding a cardboard cuff to the end of your scope or extending the dew shield. If you have security lights, turn them off while observing, and if your neighbour's security lights are causing issues, politely ask if they can be turned off while you capture your images.
While light pollution can be challenging, it is possible to capture impressive astrophotography images even in heavily light-polluted areas. With the right equipment, techniques, and a bit of creativity, you can take stunning photos of the night sky.
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Frequently asked questions
While it is challenging, it is possible to shoot astrophotos in light-polluted areas. You can try using a monochrome CCD camera, which has a greater dynamic range and is more sensitive than a DSLR, so you can detect faint details in shorter exposures. You can also use narrowband filters to enhance the contrast of emission objects.
You will need a camera with a high dynamic range and sensitivity to light, such as a monochrome CCD camera. You may also want to invest in a telescope with a fast lens (f/5 or under) or a fast DSLR lens (f/1.4 or f/1.8).
Ensure your calibration frames are taken properly. Take at least 15 shots using the exact same settings and temperatures as your lights, but keep the cover of the camera or telescope on. For bias frames, change the exposure time to the fastest your camera will allow.
You can use photo editing software to remove the green tint of light pollution filters. During post-processing, data from each emission line is assigned a colour band—red, green, or blue—and these are combined later in a graphics editor to create stunning images.
