9/20/2023 0 Comments On1 effects make sky more vibrant![]() The brightness of the sky is measured in magnitudes per square arcsecond. This is a 15 second exposure at f/2.8 at ISO 1600. Here, the Milky Way is visible, but is beginning to be washed out by light pollution from Philadelphia 35 miles to the northwest, and Atlantic City some 22 miles to the southeast. To compensate, you’ll just need to shoot many short images to increase the desired signal in the final image, as discussed in my last column, "Of Signals and Noise." Calculating Exposure Times Under Light-Polluted Skies The sky at my rural observing location in the Pine Barrens of New Jersey is not as dark at magnitude 20.8. The background will be extremely bright in a single long exposure. It can be a bit confusing because your individual exposures have to be short when shooting bright skies. To overcome this loss of contrast, you need more total integration time. Under a bright sky, the contrast is severely lowered between the sky and your target. If you shoot under a dark sky, you’ll have the best contrast you can get between the object and the sky foreground. The amount of photons arriving from the deep-sky object is fixed, whereas the number of photons arriving from the foreground sky can vary depending on the amount of light pollution you’re shooting through. You also get photons from the foreground sky, mostly Earth’s atmosphere scattering bright lights from urban locations. In a single exposure, you get a certain number of photons from the deep-sky object you are interested in. Today with digital, we can combine 12 five-minute exposures into one image to equal one hour of integration time. For example, in the old days of film, you might need to expose a single image for one hour. ![]() Integration is a fancy word for total exposure created by combining many shorter exposures. The bad news is that as the sky gets brighter, you’ll need much longer integration times to compensate. The good news is yes - yes, you can shoot from your backyard, whether it’s far out in the country away from the city lights or in the suburbs with light pollution, or even from downtown under really strong light pollution. Well, this is one of those “I’ve got good news and bad news” answers. So you’re just getting into astrophotography, and you’re wondering if you can shoot from your backyard and duplicate those great pictures that you see in magazines and online. This is a 55-second exposure at f/2.8 at ISO 3200 on a fixed tripod taken with a 16mm lens. Each of these images shows the same section of sky towards Sagittarius (Teapot asterism outlined). Cherry Springs has a sky brightness of magnitude 21.8 per square arcsecond. ![]() ![]() Can you photograph galaxies and star clusters from your backyard? That depends - how light-polluted is your yard, and how much time do you have? Imaging at a true dark-sky observing site allows fainter objects to be more easily seen and photographed due to the contrast between the object and sky background. ![]()
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