Light Pollution: Strategies For Clear Astrophotography

Light pollution is a major challenge for astrophotographers, but it is possible to capture stunning images even in heavily light-polluted areas. The key to reducing light pollution in astrophotography lies in the right equipment, camera settings, and post-processing techniques.

To minimize light pollution, one can use a monochrome camera with quality narrowband filters, which isolate specific gases in space while aggressively ignoring artificial light. For DSLR users, keeping the ISO low and managing sensor temperature are crucial to controlling noise. Additionally, employing light pollution filters, such as the Astronomik CLS, can significantly reduce the impact of light pollution, especially from streetlights.

When it comes to post-processing, adjusting the white balance, utilizing noise reduction, and manipulating the histogram are effective ways to enhance details and remove unwanted light pollution. While light pollution poses a challenge, with the right techniques and equipment, astrophotographers can capture breathtaking images even in less-than-ideal conditions.

Use a monochrome camera

Using a monochrome camera is a great way to reduce light pollution in astrophotography. Here are some reasons why:

Monochrome Cameras are More Sensitive

Monochrome cameras are far more sensitive than colour cameras. They can capture good data and achieve a better signal-to-noise ratio with shorter exposure times. This increased sensitivity reduces the time demands on the photographer and can delay the need for an autoguider.

Easier for Beginners

Colour imaging should be considered an intermediate to advanced skill. Beginners can achieve success more quickly and confidently by shooting with a monochrome camera. This is because they can focus on learning the fundamentals of astrophotography, such as image calibration, stretching, histograms, and noise reduction, without the added complexity of colour channels.

Narrowband Filters

Monochrome cameras can be equipped with narrowband filters, which are excellent for capturing emission nebulae. These filters aggressively ignore artificial light and isolate the light associated with specific gases in space objects. This helps to reduce the impact of light pollution on your images.

Reduced Exposure Times

Colour images require longer exposure times than monochrome images to achieve similar results. With a monochrome camera, you can achieve great results with shorter exposure times, reducing the impact of light pollution on your images.

Easier to Process

Converting colour images to black and white can be challenging, especially for beginners, as it requires knowledge of colour balancing. With monochrome images, you eliminate this challenge and can focus on capturing high-quality black-and-white images.

Keep your ISO low

Keeping your ISO low is a great way to reduce the effects of light pollution when shooting astrophotography in the city. Here are some tips and tricks to help you master this technique:

• When shooting in a light-polluted city, it is best to keep your ISO low. A good rule of thumb is to use an ISO of 800 or even as low as 400. This will help you capture a cleaner image with less noise, which can be difficult to manage when shooting in light-polluted areas.
• While a low ISO is ideal for reducing light pollution, it will result in darker images. However, this can be easily corrected by stacking multiple images, which will also help to further reduce noise. So, don't be afraid of the dark!
• Another factor to consider when shooting astrophotography in the city is the outside temperature. Try to avoid shooting on hot nights as this can affect your images. The heat can cause your camera sensor to get too warm, resulting in noisy images. If it's a hot night, you can also try adding a few seconds between each shot to let your sensor cool down.
• If you are using a DSLR camera, you might be tempted to increase your ISO to compensate for the low light conditions. However, this will only make light pollution worse. Instead, opt for longer exposure times to let more light in and always remember to keep that ISO low!
• By keeping your ISO low, you are prioritising a clean image over a bright one. This is a great strategy for city astrophotographers as it helps to reduce the impact of light pollution. You can always brighten your images later in post-processing, but it's much harder to remove noise caused by high ISO settings.
• Keeping your ISO low and your exposure times longer will also help you capture the colours of stars more accurately. So, if you're aiming to capture the reddish hue of Betelgeuse, the red giant in the Orion constellation, a low ISO and longer exposure are the way to go!

Use light pollution filters

Light pollution filters can be a great way to reduce light pollution when shooting astrophotography in the city. They work by ignoring 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 subject to reach the camera sensor without the nasty brown glow of a washed-out sky.

There are two main types of light pollution filters for colour cameras: broadband filters and multi-narrowband filters. Broadband filters are used to capture more natural-looking images of objects in the night sky, at the expense of allowing some artificial light through. They are effective for capturing galaxies, reflection nebulae, dark nebulae, and natural star colours.

Multi-narrowband filters, on the other hand, aggressively ignore most wavelengths of light in the visible spectrum, allowing narrow bandpasses of light in key areas to pass through. They are best used on emission nebulae and supernova remnants.

Optolong L-eXtreme

The Optolong L-eXtreme Filter is an improvement over the L-eNhance version. It’s a dual-band filter that isolates two important wavelengths of light emitted by many deep-sky objects: h-alpha at a 7nm bandpass and OIII at 7nm. The resulting images appear more dynamic and have better contrast.

SVBONY SV220

This is a good option if you are on a budget.

Optolong L-Quad eNhance

The Optolong L-Quad Enhance Filter is a great choice for capturing a natural-looking image while blocking some of the major light pollution emission lines. It performs slightly better than the original broadband L-Pro filter.

Antlia Triband RGB Ultra II

The Antlia Triband RGB Ultra II filter focuses on the important RGB bands, making it a practical choice for over 90% of deep-sky objects. It helps to bring out true-to-life hues in areas that naturally contain fainter blue tones, preventing unwanted green influence from overtaking the blue tones.

Astro Hutech IDAS Nebula Booster NB1

The IDAS Nebula Booster NB1 shares a similar profile to the Optolong L-eNhance filter. It collects light in two primary areas of the visible spectrum, H-alpha and OIII. This filter is a great option for those shooting in cityscapes or other light-polluted areas.

Optolong L-eNhance

The Optolong L-eNhance filter has gained popularity among amateur astrophotographers, with fantastic new results popping up daily. It isolates the H-alpha (Ha) and OIII wavelengths of light, making it perfect for nebula photography from the city.

Baader Moon and Skyglow

The Baader Moon and Skyglow filter is a mild light pollution filter that can record colourful stars even in Bortle Scale Class 8 backyards. It is a great choice for those who want to capture natural-looking star colours, particularly the blues and yellows.

Optolong L-Ultimate

The Optolong L-Ultimate is an excellent investment for anyone shooting with a colour camera in severely light-polluted areas. It features an ultra-narrow 3nm bandwidth in Hydrogen Alpha and Oxygen III emission lines for the ultimate contrast in your nebulae images.

When choosing a light pollution filter, it is important to consider your unique imaging location and the types of projects you like to shoot. Additionally, keep in mind that your final result will depend heavily on your image processing skills and how you manipulate the data.

Pick the right target

Picking the right target is crucial when trying to reduce light pollution in astrophotography. Here are some tips to help you choose the right target:

• If you plan to capture an emission nebula, it is best to use a narrowband filter when shooting from home. Narrowband filters are excellent for capturing emission nebulae as they isolate the light emitted by specific gases in the nebula, such as hydrogen-alpha (Ha), oxygen (OIII), and sulphur (SII). This allows you to capture stunning images even in light-polluted areas.
• If you don't have filters, pick a target that is large and bright. There are plenty of these in the Milky Way galaxy band, as well as the Orion constellation in winter, and several others. Some examples of bright and large targets include the Lagoon Nebula and the Triffid Nebula.
• Consider the position of your desired object in the sky. Image when your target is just past the meridian line, as this will ensure better sky conditions and help avoid light pollution.
• Use planetarium software and apps such as Stellarium to plan your imaging session. This will help you work out where your target will be in the sky, how much time you have to capture it, and the best location in your observing site to avoid light pollution and get the best view.
• Take into account the light pollution levels in your area. If you live in a heavily light-polluted area, such as a Bortle 9 zone, it might be more challenging to find suitable targets. However, with the right equipment and techniques, it is still possible to capture incredible images.

Use a fast telescope or lens

Using a fast telescope or lens is a tip that can be applied to most aspects of astrophotography, but it is especially useful when dealing with light pollution. A "fast" telescope or lens refers to one with a low f-number, such as f/1.4, f/1.8, or f/5. Using a fast instrument means that more light will be able to enter your camera with each exposure, resulting in more data being captured. This additional data will then have a significant impact on the final image once the stacking process is complete.

For example, the Zenithstar 73 telescope has a fixed f-ratio of f/5.9, which allows it to collect light faster than most other refractor telescopes. This increased light collection gives the images a boost in signal, which is crucial when dealing with light pollution. Similarly, fast DSLR lenses such as the Canon EF-S 24mm f/2.8 can be used to great effect when capturing images from light-polluted areas.

When shooting from light-polluted areas, it is important to remember that longer exposure times will often result in overexposed images. By using a fast telescope or lens, you can keep exposure times shorter while still capturing a significant amount of light data. This will help you obtain cleaner images with less noise, making it easier to process the data and obtain the desired results.

Frequently asked questions