Light pollution and satellites

Light pollution is artificial light that goes where it is not needed. For astrophotography the worst kind is skyglow: light that escapes upward, scatters off air molecules, dust and moisture, and brightens the whole sky. It lifts the background in every frame, which buries faint detail and adds noise.

The usual suspects

• Street and road lighting. Old fixtures that show the bulb or a glass globe throw light in every direction, including up.
• Parking lots and commercial lighting. Very bright, often left on all night, and sometimes lit far beyond what safety needs.
• Sports fields, industrial sites and ports. Floodlights aimed at a large area, so a lot of light goes past its target.
• Signs and billboards. Lit from below or inside, and often the brightest thing on a block.
• Homes and security lights. Small each, but millions of them add up, especially unshielded and motion-triggered ones.
• Ground and cloud reflection. Snow, wet roads and low cloud bounce city light back down, which is why overcast skies glow orange over towns.

Why LEDs changed things

Air scatters blue light far more than red: scattering strength rises steeply as the wavelength gets shorter (roughly with the inverse fourth power). Cool-white LEDs (4000 K and up) contain a lot of blue, so for the same brightness they brighten the sky more than warm amber lamps. LEDs are also cheap to run, which has encouraged more lighting overall. Warm (3000 K or lower), fully shielded, dimmable fixtures are the dark-sky-friendly choice.

How big is the problem?

• A 2016 world atlas of artificial sky brightness found that more than 80% of the world’s people, and over 99% of people in the US and Europe, live under light-polluted skies. The Milky Way is hidden from more than a third of humanity.
• Citizen-science star counts from 2011 to 2022 showed the night sky brightening by about 10% a year on average, far faster than satellite measurements suggested, because satellite sensors miss much of the blue and horizontal light.

What it means for you: a brighter background means a worse signal-to-noise ratio for the same exposure. The fixes, in order of power, are a darker site, narrowband or dual-band filters for emission targets (see the filters guide), and more total integration time (try the exposure calculator).

Amateur astronomer John Bortle published a nine-step scale in 2001 to describe how dark a site is, from Class 1 (the darkest skies on Earth) to Class 9 (an inner-city sky). It started as a description of what you can see with your eyes and has since been matched to instrument readings. The sky quality column is the brightness of the sky overhead in magnitudes per square arcsecond: a higher number is a darker sky.

What it means for imaging

When the sky glow is the main noise source, the exposure you need for the same image quality grows roughly in step with sky brightness. A Bortle 8 sky is around 60 times brighter than a natural dark sky, which is why the same target can take many nights from the city and a single night in the country. Class 4 and darker skies make galaxies and faint nebulae comfortable; Class 7 and brighter push you toward narrowband imaging of bright emission nebulae, star clusters, the Moon and planets.

Dark Window does not yet rate your own site’s Bortle class; use one of the maps below to find it.

• Light Pollution Map

  A world map of satellite-measured night light, with the Bortle-style World Atlas layer. Zoom in on any spot to judge how dark it is.

• Dark Site Finder

  A light pollution map with colors tuned for finding dark sky, plus user-submitted observing sites.

• DarkSky International

  Home of the International Dark Sky Places program: certified parks, reserves and communities with protected skies.

• Clear Dark Sky

  Forecast charts for thousands of observing sites in North America: cloud, transparency and seeing at a glance.

• Globe at Night

  A citizen-science project where people report how many stars they can see, building a map of real sky brightness.

• National Park Service Night Skies

  Night-sky information and measurements for US national parks, many of which are among the darkest accessible places.

Picking a site

• Look for dark areas on the map that are away from the glow of every nearby town, not just yours. Towns 30 miles away still raise the horizon in their direction.
• Public land is the easiest start: national and state parks, national forests, wildlife areas. Check whether the park is open at night and whether you need a permit.
• On private land, ask the owner first. A farmer’s field a short drive out is often darker than a busy park.
• Visit once in daylight if you can. Check the ground, the horizon, safe parking and whether you have phone signal.
• Pair the trip with a forecast: a perfect dark site under cloud is a wasted drive. The planner’s GO / NO-GO verdict covers any coordinates you enter.
• Tell someone where you are going, bring warm layers, red light and water, and be careful with your headlights and phone around other observers.

Sooner or later every long exposure catches something crossing the field. A satellite in orbit shines by reflected sunlight, so it appears as a straight, steady line as it moves across the sensor during the exposure. The most common culprits:

• Satellite constellations such as Starlink: many small, similar satellites, often crossing in sequence.
• The ISS and other large satellites: a single very bright trail. Use the ISS pass finder to see when it will cross your sky.
• Aircraft: dashed or blinking trails in red, green and white from navigation lights and strobes.
• Meteors: a short, bright streak that begins and ends mid-frame, unlike a satellite that crosses the whole field.

What a trail does to a sub

A trail adds a line of extra brightness across the frame, covering the pixels it passes over. It does not damage the sensor and affects only the sub that caught it. How bad it looks depends on how bright the satellite is compared with the sky and your target, and on how long the exposure is: the longer the sub, the higher the odds one will cross it. Wide fields and sub-exposures near dusk and dawn catch the most, because satellites are sunlit then.

How stacking removes them

Stacking software lines up all your subs, then looks at each pixel position across the whole set. A star or nebula pixel has about the same value in every sub. A trail pixel is far brighter in one sub than in the others, so it stands out as an outlier. Rejection algorithms (sigma clipping, winsorized sigma clipping, linear fit clipping and similar) throw that one value away and average the rest.

• Turn rejection on. Plain average stacking keeps the trails, only dimmer. Pick a rejection method in DeepSkyStacker, Siril, PixInsight, AstroPixelProcessor or Kappa-sigma in your stacker of choice.
• You need enough frames. Rejection compares a pixel against its siblings, so about 10 subs is a practical minimum and 20 or more works better. With three or four subs, a trail can survive.
• Dither. Shifting the frame a few pixels between subs moves the trail relative to the stars and spreads hot pixels, so clipping separates them cleanly.
• Check the rejection map. If a very bright trail leaves a faint ghost line, either lower the clipping threshold a little or delete the worst sub by hand before stacking.
• Cross your fingers on overlap. The one case rejection cannot fix is several trails passing the same pixels in the same stack. Dithering makes that rare.

Counts below are a snapshot from October 2026 and keep changing as launches continue and old satellites come down.

Starlink satellites in orbit

11,137

As of Oct 10, 2026. 11,123 operational.

Starlinks launched since 2019

12,988

Older ones have already re-entered.

All satellites tracked in orbit

16,700+

Starlink is about two thirds of them.

• Other programs are growing fast. Amazon Leo (formerly Project Kuiper) and China’s Qianfan and Guowang systems each have hundreds of satellites up and thousands planned.
• A study of Hubble Space Telescope images from 2002 to 2021 found about 2.7% were crossed by a satellite trail, a share that was rising and expected to keep rising.
• Typical operational Starlinks are around the limit of naked-eye visibility, but vastly brighter than the faint galaxies you are imaging. Newly launched batches, still low and in a tight train, are brighter and easier to spot. These brightness figures are approximate and have been improved by darkening measures SpaceX added to later satellites.
• Astronomers raise concern most about survey telescopes. For backyard imagers the practical answer is the stacking method above, plus avoiding the hour after dusk when satellites are brightest.

How do I find the Bortle class of my backyard?

Three ways, from simplest to best. Look up your address on a light pollution map and read the color. Count the faintest stars you can see in a known constellation (the naked-eye limit column in the table). Or use a Sky Quality Meter, a small device that reads sky brightness in magnitudes per square arcsecond, and compare it to the table.

Do filters fix light pollution?

They help with the right target. Narrowband and dual-band filters block most artificial light because streetlights glow in a broad range of colors, while emission nebulae glow in only a few narrow ones. Broadband targets such as galaxies and reflection nebulae gain much less. Read the filters guide for what each type does.

Can I fix a bright, gradient-filled sky in processing?

Gradient removal tools can flatten the sky background and make a bright-sky image look clean, but they cannot bring back the signal that the sky glow drowned in noise. Light pollution adds noise as well as brightness, so a Bortle 8 image needs far more total exposure than a Bortle 3 one to look equally smooth.

Should I use shorter exposures to avoid satellite trails?

Shorter subs mean each trail ruins less of any single frame, and you can discard an affected frame without losing much. But a trail is still one frame's worth of rejected pixels, and the odds that a given sub is crossed grow with its length. In practice, stack enough subs with rejection turned on and the trails vanish whatever the exposure.

Is Starlink really a problem for deep-sky imaging?

For most backyard work it is an annoyance, not a ruin: a trail through a few subs that stacking removes. It matters far more for professional survey telescopes with wide fields and long exposures, and for imaging near twilight, when satellites are sunlit and brightest. The count keeps growing, so the number of affected frames will too.

• KeepTrack: Starlink satellite count (Oct 10, 2026) and the 10 largest constellations (July 2026).
• azmth.space: satellites in orbit for the total tracked count.
• Space.com: Hubble images and satellite streaks (study of 2002 to 2021 images).
• Falchi et al., “The new world atlas of artificial night sky brightness,” Science Advances, 2016.
• Kýba et al., “Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022,” Science, 2023.
• Bortle, J. E., “The Bortle Dark-Sky Scale,” Sky & Telescope, February 2001.