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Expedition 31 Flight Engineer Don Pettit relayed some information about photographic techniques used to achieve the images:
“My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then ‘stack’ them using imaging software, thus producing the longer exposure.
Over the past two months NASA has been releasing a number of wonderful long exposure photographs taken by astronaut Don Pettit aboard the International Space Station. While there are many photos like these taken from the perspective of the Earth’s surface, Pettit’s images are unique in that they incorporate the passing blur of entire illuminated cities, aurora, and the sporadic flashes of lightening from thunderstorms. Check out many more photos from the series here. (via petapixel)
"Lately we've been having some extraordinary auroras," he reports. "They meander like big green amoebas crawling across the sky. Sometimes there is a faint touch of red layered above the green. These lights are constantly changing. They swirl. Bright spots come and go. Green blobs transform into upward-directed rays topped by red feathery structures."