Light Pollution For Telescope Observation – An Essential Guide

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Written By: Ed Anderson
Category: Learn

Everyone who looks skyward eventually learns that light pollution is a growing problem for astronomers, be they professionals or amateurs. This growing problem diminishes what can be seen and the amount of detail that can be seen as we marvel at the wonders of the universe.

But don’t give up. This is one of those situations where a little knowledge and a little planning can help you deal with light pollution to provide the best observing experience. So, what’s light pollution?

Two Types of Light Pollution

When we talk about light pollution, we are talking about two forms: ground and sky. Ground light pollution comes from your immediate surroundings. Sky light pollution, or sky glow, is caused by the projected glow, a collective orange-yellow haze, of the cumulative light in the area, projected for dozens or hundreds of kilometers outward. We often talk about the sky glow dome over a large city that comes from the collective glow of the highway lights, lights from buildings, and lights from other sources.

We will discuss each kind of light pollution and how you can deal with it to plan and enhance our observing experience. We will also talk about the types of targets that are more impacted by light pollution and those that are less affected by light pollution. With a little planning and picking your targets carefully, we can have a wonderful observing experience that is optimized for the local conditions.

Glare and Dark Adaption

Ground light pollution, usually referred to in scientific literature and by DarkSky as glare, is the light pollution that comes from your house lights, your neighbor’s house lights, and the street light in front of your house. This is the stuff that shines directly into your eye through your telescope, ruining your night vision or washing out the view in a sea of gray-brown muck. It can come from a nearby parking lot, hospital, or other commercial site where they keep big lights on all the time. For the most part, we are talking about lights you can see or that are close enough to light the area immediately around you.

Before and during the 2003 Northeast blackout, a massive power outage
Before and during the 2003 Northeast blackout, a massive power outage, Pic Credit – Todd Carlson

If you live in the country, where houses are spread out and there are no nearby commercial buildings or street lights, then this may not be a problem for you unless someone in your family has a habit of leaving on a particular light that shines out your window (blackout curtains are an astronomer’s friend). But if you are like most of us and live in a city or a bright suburb, then ground light pollution is something you will have to deal with when you are observing the sky. Ground light pollution is also something you can directly affect and perhaps reduce with some effort.

The effect of this type of light pollution is on your eyes. Your iris, the colored part of your eye, controls how much light is allowed to reach your retina, which is where your light sensor cells reside. It does that by expanding or contracting the size of your pupil, the black center of your eye. If too much light reaches the retina, your eye can be damaged.

In order to see as much as possible when looking at the night sky through binoculars or through a telescope eyepiece, we want our eyes to fully dark adapt, letting in the most light possible. As we are looking at things in the sky that are dim, we want our eyes to be as light-sensitive as possible.

To experience dark adaptation in order to understand how it affects what we see, stand in a brightly lit room in your home at night. Close the shades or the blinds to minimize any light from outside. Now, turn off the lights.

Initially, you may not be able to see anything as your eye adjusts to the bright room, like you were in a moment ago. The room may appear completely black.

After a few moments, your eye will begin dark adapting, picking up the traces of light that are coming through the window, past the shades. Perhaps light will leak in under the door. The iris will contract, and your pupil will expand to let in more light. The full process can take 20 minutes or longer until your eyes fully dark adapt.

As your eyes adapt, the faint amount of light in the room will begin to allow you to see things. First, the outline of the covered window. Then maybe the chair across the room. The longer you stay in the dark, the more your eyes will dark adapt and the more you can see.

To test this outside, pick a night when there is no moon. Step from your brightly lit home to the darkest location you can find immediately outside your door. As you look up at the sky, you may see very few stars, but slowly more will appear. This will progress over the next 20 minutes until your eyes dark adapt to your local ground lighting level, letting in more light and letting you see more stars than when you first stepped out the door. Of course, this assumes your outside area is darker than the inside of your lit home.

This will happen at the eyepiece too. Try the same experiment, stepping from the house, but this time put your eye to your binoculars or the eyepiece of your telescope and just watch. Over time, you will see more and more stars in the eyepiece as your eyes dark adapt.

Dealing with Glare

In order to achieve the best dark adaptation of your eyes, you need to find an observing location where there are the fewest lights in your line of sight. If possible, pick a spot where passing car headlights cannot be seen. Unfortunately, a quick flash of passing car headlights can undo 20 minutes of dark adaptation. Now you have to wait through the dark adaptation process again.

If you forget something in the lighted house and run in to get it, you will completely lose your dark adaptation, and you will have to wait another 20 minutes for them to get as adapted as they can. When your eyes reach their best dark adaptation for your surroundings, you will see the most detail in the sky and at the eyepiece.

If you can’t avoid all light sources, consider ways to block them. Place a tree or a fence between you and the light. Consider putting up some kind of screen. Place a building between you and that street light.

Wear a hood to block as much surrounding light as possible and place it over your head as you look through the eyepiece. Not only will this allow your eyes to better adapt to the dark, but it will also prevent stray light from reflecting off the surface of the eyepiece.

Some people put an eyepatch over their observing eye to allow it to achieve maximum dark adaptation. When they go to the eyepiece, they keep that eye closed, flip up the eyepatch, and only open the eye when they are at the eyepiece. Again, a hood can help preserve the dark adaptation of the eye.

Sky Light Pollution

This type of light pollution is harder to address. The only way to deal with skylight pollution is to drive to a location where there is less skylight pollution.

You may see advertisements for light pollution filters. At the time these were developed, the majority of municipal lighting was based on sodium and mercury vapor lights. These emit light at very specific wavelengths. Light pollution filters could filter out those specific wavelengths. They were helpful in dealing with light pollution. However, as soon as sodium and mercury vapor lights gave way to other designs (including LEDs), these filters became irrelevant. In addition, broadband filters never blocked the metal halide lamps often still used in car dealerships and sports venues. If you’ve ever driven past a sporting event or a car dealership at night, you should be aware of how much sky glow they produce. Since broadband filters have always been ineffective against one of the prime sources of sky glow anyway, this fact makes buying one especially pointless.

However, the trend in municipal lighting for buildings, streets, and roads is to go to white LED lights. They use a lot less power, so the incentive to switch over is high, and the trend is gaining speed. White LEDs cover the full light spectrum, so light pollution filters cannot filter them out. As a result, light pollution filters have lost most of their value.

If you know someone who has a light pollution filter, ask them to give it a try. It might help in your area. But for the most part, they have become ineffective in dealing with skylight pollution. For this reason, you may hear that the best light pollution filter is gasoline, meaning that you get in the car and drive to a darker location.

Finding Darker Sky Locations

There are a variety of tools available to help you identify areas that have less sky light pollution. is the most comprehensive and up to date of these. You can pinpoint any location you want and get an estimated Sky Quality Meter and Bortle Scale reading. This can give you a general idea of where you should be going. Your local astronomy club probably has a dark sky site or two they recommend visiting or have even purchased land on. Alternatively, has a list of International Dark Sky Parks that are worth visiting, as well as Urban Night Sky Places.

The SQM, or Sky Quality Meter, is a device used to measure the brightness of the night sky, providing a quantifiable measure of the impact of light pollution. It measures the luminance of the sky in magnitudes per square arcsecond, a measure that indicates how bright each small patch of sky appears to the observer. A higher SQM reading implies a darker, less light-polluted sky, while a lower SQM reading indicates a brighter sky with more significant light pollution. The SQM isn’t perfect, however, and will give different readings near and far from the horizon as well as when the Milky Way, a bright planet, or the Moon are up.

Another way to talk about the level of light pollution in a given area is with the Bortle scale. This scale was developed by John Bortle, who published it in 2001. Like an SQM reading, you can describe the level of sky glow based on whether you can see the Milky Way, how dim the dimmest stars you see with your naked eye may be, and so forth. The center of a brightly lit city, like New York City, would be classified as Bortle 9. This would be a white zone on the Dark Site Finder Map. The Bortle Scale is only a rough estimate, however, and varies based on the observer and sky conditions such as transparency, which cannot always be adequately accounted for.

Table approximating limiting magnitude, Bortle scale, limiting magnitude (naked eye limiting magnitude, or NELM folllowed by with instruments). This is not perfect, but it should give you a rough idea.

Map colorBortle ScaleSQM ReadingNELM(Expert)NELM (Beginner)50mm Binoculars Magnitude Limit8” Dobsonian Limiting Magnitude25” Dobsonian Limiting Magnitude
Yellow-dark yellow420.8-21.476.511.514.2517
Red-pale red619.5-2065101315.5

Of course, a map does not address ground conditions. So you might go to a park that is in a darker zone, but if they have the ball field all lit up, that is not going to be indicated by the dark site finder map. That is a ground light pollution/glare issue. 

You may hear or read comments on astronomy forums about people being in red or green zones. They are referring to a light pollution map.

Surface Brightness vs. Magnitude

I am going to tell you which targets work best for a light-polluted area and which are better reserved for darker locations. To do this, I need to explain magnitude and surface brightness.

Magnitude is a measure that tells us how bright a stellar object shines. The higher the number, the dimmer the object. The scale is built around Vega, one of the brightest stars in the sky. It is assigned a value of zero. The Sun is magnitude -26.74; notice the negative. The full moon is the second brightest object at -12.7.

The key thing to know is that one of the ways to measure the light pollution in your area is by checking to see what magnitude stars you can see with your naked eye near the zenith, which is straight up. At a truly dark site, you can see down to magnitude 7, and it gets quite a bit fainter with practice. But at more light-polluted locations, magnitude 4 stars, which are far brighter, may elude you.

Where things get tricky is when a magnitude number is assigned to a deep sky object like a galaxy or a nebula. While the total amount of light from a magnitude 8 nebula may be equal to that of a magnitude 8 star, the star is a point light source, while the light from a nebula is spread out over a larger area. Through some complex math, this can be translated to a value for surface brightness. The bottom line is that a mag 8 star appears to be much brighter than a mag 8 galaxy or nebula because the light is spread out over a larger area.

Selecting Your Targets According to Your Sky Light Pollution Conditions

If you are in a dark area with few direct sources of glare and little to no sky glow, you are the envy of those who live in the glow of the modern city or suburbs. All targets will be accessible to you and will look their best in your telescope. For the rest of us, we need to take light pollution into our planning for what we want to observe.

All stars, deep-sky objects, faint Solar System objects, and the naked-eye Milky Way are affected by light pollution levels, but some are more affected by light pollution than others. Targets that are less sensitive to light pollution are those that are point light sources, like stars or planets. Or deep sky objects that are made up of stars, where we can resolve the individual stars in the eyepiece of the binoculars or telescope. Good examples would be:

  • Sun
  • Moon
  • Planets
  • Open Star Clusters
  • Double Stars

These targets are essentially points of light or a collection of points of light. As such, they shine brightly in contrast to the glow of the sky. Fainter stars, as well as faint Solar System objects (e.g., dim dwarf planets, and icy moons), can be washed out by light pollution even though they are still point sources. Your best views of open star clusters will be under dark skies, and you need at least moderately dark skies to go after Pluto, the faint moons of Saturn and Uranus, etc.

Targets that are more sensitive to light pollution are those that have their light spread out over a larger area where we may not be able to resolve point light sources. Here, surface brightness numbers are more indicative of how easily they can be seen. A magnitude 8 galaxy is much dimmer than a magnitude 8 star. So we turn to the surface brightness numbers for guidance. Targets where surface brightness is a better indicator are:

  • Galaxies
  • Nebulae
  • Globular star clusters
  • Comets

These are targets that have their light spread out over an area as opposed to being a point of light. This lowers their contrast with the surrounding sky, making them harder to see. An example may be helpful.

The Andromeda Galaxy, also called Messier 31, has a magnitude rating of 3.44. If it were a star, it would shine brightly and be visible to the naked eye, even in light-polluted locations. However, its surface brightness is magnitude 13.3, which would indicate that it is much dimmer than a magnitude 3.44 star. How does this translate to what we see with our eyes, with binoculars, or with a telescope?

Let’s say you live in a Bortle 8 location. This would be white or off-white on a light pollution map. When you look at the Andromeda Galaxy from this location using a typical 8” telescope, it is just a small, slightly grayish, fuzzy patch in the sky. But if you go to a dark red zone, approximately Bortle 6 that smudge becomes a much larger oval-shaped object with a bright core, more like what one might expect of a galaxy.

If you were to go to a Bortle 4 location, Andromeda would suddenly be visible to the naked eye, albeit not very well. Binoculars or a small telescope will reveal dust lanes above/below the core of the galaxy. The two small companion galaxies M32 and M110 are also readily apparent in any telescope under these conditions.

Under a Bortle 1 sky, Andromeda clearly stretches across an area of sky larger than the Moon to the naked eye, with a brighter core and a fuzzier, broader area. Binoculars show the dust lanes and companion galaxies, while a telescope can reveal the star cluster NGC 206, multiple dust lanes, and, of course, the full expanse of the disk of the galaxy. Additional galaxies also become visible to the naked eye under a Bortle 1 sky, namely M33 and M81, which are only visible with binoculars under even Bortle 4 skies.

Planning an Observing Session

The bottom line is that it is good to know what your local light pollution situation is so you can plan your observing session. If you are at a very light polluted location, say Bortle 6 to 9, you may wish to focus on open clusters, planets, stars, and double stars. There are some brighter nebulas, such as the Orion Nebula or the Ring Nebula, that can still be viewed from a light polluted location with good detail.

If you plan a trip to a darker location, that is the time to focus on galaxies, nebulae, and other targets with low surface brightness. You may be able to detect them at the light polluted site, but they will be more satisfying at the darker location.

Learn about the light pollution level in your observing area. Take this into account, and you will learn to enjoy the sky and avoid the frustration of not being able to see your targets.

Many urban locales dim their streetlights after 10 p.m. or midnight. Even if this is not the case, as people go to sleep and businesses close, there is inevitably a decrease in light pollution from many of the worst sources of sky glow. Absent additional complicating factors like moonlight or humidity, if you are dealing with any source of sky glow, it is likely that you will see a significant decrease in its brightness if you observe it in the very early hours of the morning compared to right after it gets dark.

Things that can make light pollution’s effects worse

  • Your telescope

If you are dealing with any light pollution and using an open-tubed telescope, a tightly fitting shroud or baffle is a must. You should not be able to see anything looking down your telescope’s empty focuser drawtube but the reflection from your secondary mirror or out through the lens, surrounded by a sea of black. If your telescope fails in this department, adjustments are needed. Paint exposed glass edges black, paint inside your focuser drawtube, line your tube walls with black velvet flocking, cover the back of your mirror cell, and/or add an extended front baffle or dew shield to your tube if need be. Also, make sure that you are using decent eyepieces (and if not a Newtonian, a decent star diagonal). Poor optics can scatter light and make sources of glare or sky glow appear far worse at the eyepiece than their deleterious effects should actually be.

  • Scatter – Smoke, Dust, Moisture

Smoke and dust particles in the atmosphere can make the problem of light pollution significantly worse by scattering artificial light. When light encounters these small particles, it can be redirected in various directions, a process known as scattering. This effect increases the overall brightness and glow of the sky, reducing contrast and obscuring faint celestial objects. This is one reason why the night sky in urban areas or regions affected by wildfires or dust storms is often brighter and less clear than in rural or unpolluted areas. 

Humidity also scatters light. While you might be tempted to go observing right after rain has taken dust and other pollutants out of the air, it’s possible that the scattering of excess moisture obviates this advantage (as will the dew or frost that you’ll inevitably encounter). Observers in humid climates can easily lose 0.5 magnitudes on the SQM or an entire 1-2 Bortle classes to high humidity. Thin clouds can also form without warning, often obstructing the sky (and reflecting light back down).

  • Reflection – Snow, Clouds, the Moon

Snow is, of course, extremely reflective. How reflective exactly? Upwards of 90%. For comparison, asphalt, water, dirt, and grass or foliage are all less than 20% reflective. That means that over 4 times as much sky glow is bouncing back off the ground when there’s a thick coating of snow. If you live anywhere near a large source of light pollution, snow is going to completely and utterly ruin your ability to see the Milky Way, stars, and deep-sky objects.

Likewise, clouds are also fairly reflective. Thin, high-altitude clouds are the worst of all, as they tend to reflect light pollution back down while also absorbing light from the sky above (and you might be none the wiser). Higher clouds can reflect light pollution from further away and are more likely to stick around, which makes them particularly obnoxious.

While it’s largely unavoidable and very far away, the Moon is also a reflective object (albeit duller than asphalt), and it reflects the worst light pollution of all – that of the Sun. Observing on a night when the Moon is above the horizon and even a thin crescent is equivalent to adding light pollution. Brightness goes up exponentially thanks to the inverse-square law and the incident angle of light hitting the Moon, so a 25% illuminated Moon is 4 times dimmer than a half-full one. If the Moon is more full than a slender crescent, either wait for it to set (or observe before it rises) or adjust your observing expectations accordingly, as the Moon will wash out most of the sky just like a source of light pollution would.

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