Telescopes And Light Pollution: Seeing Through The Haze

Light pollution is a common problem for astronomers, and it can significantly impact the visibility of the night sky. It is caused by unwanted light from towns and cities, which fills the sky with a bright, diffuse glow. This light pollution can drown out the light from faint objects in space, such as galaxies and nebulae, and makes it harder to see stars. However, some celestial bodies, such as planets, are bright enough to be seen even under extreme light pollution.

So, what can be done to improve visibility when using a telescope in a light-polluted area? Firstly, it is possible to buy telescopes with in-built technology to filter out light pollution, such as the UNISTELLAR telescopes. Additionally, light pollution filters can be added to telescopes to reduce unwanted light. However, these methods will not eliminate all light pollution, and the image quality will not be optimal. To get the best view of the night sky, it is recommended to travel to a dark-sky location away from cities, which are the main source of light pollution.

Using a light pollution filter

Light pollution filters can be a great way to reduce the effects of light pollution when stargazing from the city. While they cannot eliminate light pollution entirely, they can significantly improve the visibility of celestial objects that would otherwise be obscured.

There are two main types of light pollution filters: broadband light pollution filters and ultra-high contrast or narrowband OIII filters. Broadband filters work by attenuating the wavelengths of light associated with older mercury vapour and low-pressure sodium lighting. This improves the contrast of the sky, making it easier to observe deep-sky objects such as nebulae. However, these filters do tend to dim the overall view, although this is less of a problem when observing brighter objects like the moon and planets.

Ultra-high contrast or narrowband OIII filters work by increasing the contrast of the objects you wish to observe, allowing you to select the specific wavelengths you want to focus on. These filters can be very effective for observing emission objects like nebulae, even in areas with significant light pollution.

When choosing a light pollution filter, it's important to consider the type of telescope you have and the specific objects you want to observe. The filter should be compatible with your telescope's eyepiece size, and you may also want to consider the filter's cell size to ensure it fits securely in your telescope. Additionally, some filters are designed for visual astronomy, while others are meant for astrophotography, so be sure to select a filter that suits your intended use.

• Sky-Watcher UHC Filter: This filter performs well at reducing the effects of light pollution, producing a good, dark background sky and allowing you to observe nebulosity in the Orion Nebula.
• Lumicon's UHC Filter: This filter provides extra detail when observing the Orion Nebula, with a strong hint of blue-green colouration to the overall view.
• Orion UltraBlock Narrowband Filter: This filter retains more of the Orion Nebula and brings out details in M43, part of the Orion Nebula complex. It offers a good dark background with a characteristic blue-green hue, while still allowing stars to stand out.
• Baader UHC-S L-Booster Filter: This filter gives a slight hint of red to the typically blue-green tint of this type of filter, and it allows a good amount of detail to be seen in nebulae.
• Burgess Optical Broadband Nebula Filter: Despite being a budget option, this filter provides reasonable views of the Orion Nebula. The background sky is not as dark as some other filters, but the green-blue hint is also less overpowering.
• Orion Optics EHC Photo Visual Filter: This filter has a low profile and significantly reduces the background glow, although it does dim the view slightly, especially with nebulous objects.

Understanding transparency

Transparency is the opacity of the atmosphere, or how clear it is. It is a measure of the atmosphere's turbulence and is usually measured in arcseconds. If the seeing was 4 arcseconds, it would mean that the stars can be expected to move in a circle with a diameter of 4 arcseconds.

Moisture and humidity lower transparency, as does smoke or other kinds of pollution. Poor transparency washes out the fainter details of astronomical targets and scatters light around, making light pollution worse.

Transparency usually gets better with altitude, as you are looking through less air. This is why high altitudes are prized for observatories and star parties. It also tends to be good after a rainstorm, which clears particulates from the air.

Transparency is also affected by geography. Places with very flat geography tend to have better transparency as air masses can flow smoothly over the land. Conversely, air masses moving over mountains become turbulent, which makes for poor transparency.

Light pollution is unwanted light that projects into the night sky, drowning out the light from faint objects like galaxies and nebulae. It is caused by inefficient or misused light fixtures that scatter light above the horizontal plane.

Telescopes with integrated technology can filter out light pollution, improving the viewing experience. However, light pollution can be amplified by air pollution, so transparency still plays a role in how well you can see through a telescope.

Observing the planets

However, several planets and the Moon are bright enough that they can be seen using any telescope, even under extreme light pollution. Planets are relatively brighter than most stars and require less of a dark sky to see clearly. They are ideal for observation in light-polluted areas, lending themselves to high magnification and revealing intricate details that can escape the naked eye. For example, under the right conditions, even city smog can't smother the rings of Saturn or the festoons on Jupiter.

Selecting targets wisely is the first line of defence. Focus on brighter objects that can still pierce through the light-drenched heavens—planets, the Moon, double stars, and some prominent nebulae. These can still provide breathtaking sights and valuable experiences despite the diluted backdrop.

Telescopes with a larger aperture (diameter of the primary mirror or lens) can gather more light and can be more effective in urban areas. For casual urban stargazing, a good-quality refractor or reflector with at least a 4-inch aperture can be very useful. However, the effectiveness of any telescope in light pollution will also depend on the techniques and tools used in the observing process.

Using a telescope from a light-polluted city

Understand Light Pollution and Transparency

Firstly, it's important to understand the impact of light pollution and atmospheric transparency on your observations. Light pollution from cities and towns obscures stars and creates a diffuse nocturnal glow, drowning out the light from faint celestial objects. Additionally, haze or air pollution can further reduce clarity, resulting in murky skies. Therefore, it's essential to monitor transparency levels and plan your observing sessions accordingly.

Choose the Right Equipment

When selecting a telescope for light-polluted areas, consider its aperture—the diameter of the main lens or mirror. Telescopes with larger apertures typically provide more impressive views but may be less convenient to use and transport. Additionally, optical and mechanical quality are crucial factors to ensure clear and precise observations.

Utilize Light Pollution Filters

To mitigate the effects of light pollution, consider using a light pollution filter tailored to visual observing. These filters are specifically designed to suppress the glow from artificial light sources, enhancing your ability to observe deep-sky objects. While they may not work effectively with broadband LED lighting, they can still improve your viewing experience.

Optimize Your Observing Position

Your observing position can also make a difference. Avoid observing from inside a building with open doors or windows, as escaping warm air can create air currents that distort your view. Similarly, avoid targets directly above rooftops or hot air vents to minimize air current interference.

Allow Your Eyes to Adjust

Just as you would in a darker location, allow your eyes to adapt to the darkness for at least 30 minutes before beginning your observations. Keep any artificial lights out of your direct line of sight to help your eyes adjust and improve your night vision.

Focus on Suitable Targets

While light pollution can make observing faint objects challenging, there are still plenty of celestial bodies that can be seen from light-polluted cities. The planets, for example, are largely unaffected by light pollution. You can get great views of Jupiter, Saturn, Mars, and Venus with even a small telescope. Bright star clusters, double and multiple-star systems, and some variable stars are also within reach from light-polluted locations. Additionally, summer and winter showpieces like the Orion Nebula and the Lagoon Nebula can be observed with a small telescope or a good pair of binoculars.

In summary, while light pollution can be a hindrance, it doesn't have to stop you from enjoying astronomy. With the right equipment, careful planning, and a focus on suitable targets, you can still explore the wonders of the cosmos from the heart of a light-polluted city.

Understanding your city's light pollution

Light pollution is the presence of any unwanted, inappropriate, or excessive artificial lighting. It is a major side effect of urbanization and has been blamed for compromising health, disrupting ecosystems, and spoiling aesthetic environments. It is most severe in highly industrialized, densely populated areas of North America, Europe, and Asia, and in major cities in the Middle East and North Africa.

The first step to understanding your city's light pollution is to determine its severity. This can be done by referring to the Bortle scale, a nine-level numeric scale that measures the night sky's brightness in a particular location. The Bortle scale ranges from Class 1, which is classified as excellent dark sky viewing with no light interference, to Class 9, which is inner-city sky viewing where light significantly reduces the number of objects one can see.

There are several online tools available to help you assess your city's light pollution level. Dark Site Finder provides a map that shows your city's Bortle scale rating. Bing's Light Pollution map allows you to find small relative differences in light pollution within your city. Google Earth also has a light pollution feature that can be used to identify areas with lower radiance.

Once you have determined the severity of light pollution in your area, you can start exploring ways to mitigate its effects on stargazing. While light pollution can make it challenging to view faint objects like galaxies and nebulae, there are still plenty of engaging targets for astronomers to enjoy under urban and suburban night skies.

• Use a light pollution filter tailored to visual observing. These filters are specifically designed to suppress the glow from artificial light, making it easier to observe deep-sky objects.
• Consider your observing position. Avoid observing from inside a building with open doors or windows, as the escaping warm air will create air currents and cause the view to shimmer. Try to avoid targets that are directly over rooftops or above hot air vents.
• Allow your eyes to adapt to the darkness. Give yourself at least 30 minutes to adjust to the available light, and keep any artificial lights out of your direct line of sight.
• Choose the right targets based on light conditions. On nights with poor transparency, focus on brighter targets like the Moon and planets. When transparency is good, you can attempt to observe fainter objects like star clusters.
• Use a star atlas, a planetarium computer program, or a stargazing app to navigate your way around the stars and locate specific objects. Many planetarium programs and apps have tools that allow you to limit the magnitude of the stars displayed, making it easier to find fainter objects.
• Choose the right equipment. A small telescope or a good pair of binoculars will allow you to observe a respectable number of brighter deep-sky targets, including star clusters, bright planetary nebulae, and summer and winter showpieces like the Orion Nebula and the Lagoon Nebula.

By understanding your city's light pollution and implementing these strategies, you can enhance your stargazing experience even in light-polluted urban environments.

Frequently asked questions