Light Pollution Back | Up | Next

Light pollution is the bane of astronomers and astrophotographers.

It can make the night sky so bright that even stars are hard to see, let alone faint nebulas and galaxies.

Mouse Over
Hold your mouse cursor over the image to see a comparison between images taken under clear skies at a light polluted observing site and at a true dark-sky observing site. The images show exactly the same area of sky in the region of Sagittarius. They were exposed so the sky background would have the same brightness.

In the mouse-over image comparison above of Sagittarius, we can see the incredible difference that light pollution makes in ruining the sky for astrophotography. The first image was taken where I live in the suburbs of Philadelphia. It has an ugly red-brown sky from light pollution. There are some stars recorded in the image, but they are hard to see.

The second image in the mouse-over comparison above was taken at a true dark-sky observing site at Cherry Springs, Pennsylvania at the Black Forest Star Party where the sky has a naked-eye limiting magnitude greater than 7th magnitude. Here, the majesty of the Milky Way is revealed in all its glory. Because of the spread of light pollution, for me to reach these true dark skies, it is a 6-hour, 275-mile drive.

Dark skies are not important for imaging the Sun, Moon or bright planets, but they can make a tremendous amount of difference in the quality of your long-exposure deep-sky images. Darker skies produce images with higher contrast between the deep-sky object and the sky background, and also a higher signal-to-noise ratio.

Hold your mouse cursor over the image to see a comparison between a single 2.5-minute exposure shot from a light-polluted suburban observing site, and a single 2.5-minute exposure shot from a decent dark-sky observing site. We can see the tremendous difference that a dark sky makes, even in a single exposure of galaxy M31.

The first image immediately above of galaxy M31 was shot from my driveway in the suburbs of Philadelphia. This site has a sky brightness with an limiting magnitude of about mag 4.5. And this was on a really good night that was about as good as it gets for this light-polluted observing site. No Milky Way is ever visible from here.

The second image in the mouse-over above was shot from a moderately dark-sky site about 48 miles (77 kilometers) and an hour's drive from downtown Philadelphia, a metropolitan area of about six million people. This site has a sky brightness with a limiting magnitude of about sixth magnitude. The Milky Way was visible from this dark-sky site. Although it is not nearly as dark as a true dark-sky site such as Cherry Springs, it is still much darker than where I live.

Keeping the exposure times equal, they both recorded the same amount of signal from the photons from M31. The photon signal from M31 the same at both sites. It is the brightness of the sky background that is different. The sky at the suburban site with light pollution is brighter. Therefore there is less contrast between the background sky and the galaxy. There are also many more photons from the sky background, therefore there is much more photon noise from the sky background. The signal-to-noise ratio has gone down in the shot from the light-polluted observing site because whereas the signal is the same from M31, there is more noise in the image from the suburban location.

An image of equivalent quality could have been taken from the suburban location by using more total exposure, but it would have required four to five times as much exposure. This doesn't seem like much in a 2.5-minute exposure, but to produce a high quality image, the total exposure at the dark-sky site should have been much longer as in this photo of M31. This means that at the suburban site, to equal the quality of a one-hour exposure at the dark-sky site, four to five hours of exposure would have been needed.

Since it's only two hours total driving time for me, it is a much better investment, both time-wise and quality-wise, to drive to a dark-sky site and shoot from there. You can spend the extra time at a dark-sky site shooting more frames of a single object, making the picture even better, or you can spend the extra time shooting different objects.

Light pollution from street lights, homes and businesses make the night sky bright, making astrophotography of faint objects very difficult. This is the street in front of my house in suburban Philadelphia.

  • Solution:
    • Drive outside of the city to the rural countryside to get as far away from light pollution as you can.

    • Use a light pollution filter to cut out undesirable wavelengths from street lights.

    • Stack a LOT of shorter exposures to overcome the photon noise from the bright sky background and improve the faint signal-to-noise ratio in deep-sky objects.

    • Take up solar, lunar, or planetary astrophotography. These are not really affected by the brightness of the sky.

Because of the quantum nature of light, photons do not arrive with exact regularity. Their arrival has a random element to it. This random element creates noise in the image called "photon noise". Skies that are really bright from light pollution have a lot of photons, and they have a lot of associated photon noise.

The signal from deep-sky objects however, stays fixed. So the signal-to-noise ratio is very low under bright skies. The only way to combat this is to use the methods outlined above. But the best way to deal with the problem of light pollution is to shoot from a dark-sky observing location.

Problem: Light Pollution - The Bottom Line

Light pollution lowers the contrast between deep-sky objects and the sky background, and adds noise to the image.

Using a light-pollution filter can help. Stacking more exposures can also help greatly improve your images.

The best solution is to use an observing site with darker skies that is as far away from light pollution as you can get.

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