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A composite image of Comet Lemmon, showing where bright pixels were rejected by an image stacking algorithm. | Photo credit: Dan Bartlett
In recent months, space fans have enjoyed a wealth of spectacular astrophotography showing the evolution of Comet Lemmon’s glowing coma and spinning tail as it travels through the satellite-filled Northern Hemisphere’s night sky.
While the vast majority of photographers have chosen to publish sanitized views of Comet Lemonothers have deliberately compiled their images to reveal the incredible number of them Satellites that crossed the night sky over the course of several exposures.
In any case, Comet Lemmon is surrounded by a crazy network of thin, web-like lines, each representing the path of an orbiting satellite over the course of a short camera exposure.
“From a photography perspective, if someone is trying to capture a single image of a target and the image needs to be “clean,” — free of artificial objects — “Well, this image is almost impossible to get,” said astrophotographer Dan Bartlett Space.com in an email. “Every single subframe I capture (before stacking) now contains at least one, and usually more than one, satellite stripe.”
There are currently around 13,000 operational satellites orbiting the Earth Earthof which approx 8,900 are SpaceX Starlink spacecraft. This number is expected to grow exponentially SpaceX alone aims to put up to 42,000 of its internet-beaming spacecraft into orbit, while rival companies aim to add thousands more satellites to their own “mega-constellations.”
Comet Lemmon passes over Spain. | Photo credit: Anadolu/Getty Images
Luckily, astrophotographers have access to powerful editing tools that allow them to easily remove the unsightly streaks.
“Despite the huge increase in satellite traffic in recent years, with the right tools, it is actually quite easy to eliminate satellite tracks in post-production,” explains Bartlett. “I certainly don’t like seeing so many satellites everywhere I point my telescope or even a good pair of binoculars, but we’re still not at the point where you can’t enjoy the night sky anymore (it can be a visual thrill when a satellite flies through your field of view).”
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Astrophotographers combat satellite streaks by taking a variety of short exposures over the course of a single session. The images are then combined and subjected to an algorithm that determines a median value for each pixel before rejecting outlier pixels with values that exceed the specified parameters.
“If you stack at least a dozen images, you can use a combination method called sigma rejection,” said astrophotographer Chris Schur in an email to Space.com. “This is available in almost all astronomical imaging software. With a single capture of up to about six images, the Sigma does not have enough data to discard all traces. For this reason, as a normal process, comet and deep sky photographers capture sets of at least a dozen images, so the batch software has a good set to run its algorithm.”
Pictured in a star field at the bottom left is a bright green comet against a black sky, with a long tail that extends to the top right corner of the screen.
Pictured in a star field at the bottom left is a bright green comet against a black sky, with a long tail that extends to the top right corner of the screen.
Pictured in a star field at the bottom left is a bright green comet against a black sky, with a long tail that extends to the top right corner of the screen.
Pictured in a star field at the bottom left is a bright green comet against a black sky, with a long tail that extends to the top right corner of the screen.
Pictured in a star field at the bottom left is a bright green comet against a black sky, with a long tail that extends to the top right corner of the screen.
The end result is a beautiful composite image of an ancient building Solar system Comet trapped in an orbital dance around the Sun since the planets formed over four billion years ago.
Stargazers looking to capture their own views of the night sky should check out our picks for the best cameras and lenses for astrophotography, as well as our guide to spotting and photographing comets.
Editor’s note: If you would like to share your comet astrophotography with Space.com readers, please send your photos to spacephotos@space.com along with your comments, your name, and the times and dates you took them.
