Hubble scientists create tool for erasing satellite trails

Hubble scientists create tool for erasing satellite trails from images

With ever-increasing numbers of satellites in the sky, astronomers have repeatedly expressed worry over how these satellites could impact scientific research. Earlier this year, a study of Hubble Space Telescope observations showed how some images were being ruined by streaks of light coming from satellites — and while only a small percentage of Hubble images were affected, the authors raised concerns that with the projected number of satellites set to explode in the next decade, the problem could become serious.

Now, astronomers at the Space Telescope Science Institute (STScI), which runs Hubble, have come up with a tool to deal with satellite streaks in Hubble images. “We developed a new tool to identify satellite trails that is an improvement over the previous satellite software because it is much more sensitive. So we think it will be better for identifying and removing satellite trails in Hubble images,” said Dave Stark of STScI in a statement.

This image captures the streak of an Earth-orbiting artificial satellite crossing Hubble's field of view during an observation of "The Mice" interacting galaxies (NGC 4676). A typical satellite trail is very thin and will affect less than 0.5% of a single Hubble exposure. Though in this case the satellite overlaps a portion of the target galaxy, the observation quality is not affected. That's because multiple exposures are taken of the same target. And the satellite trail is not in other frames. Developers at the Space Telescope Science Institute in Baltimore, Maryland, have software that identifies the bad pixels from the satellite photobombing, the extent to which they affect the image, and then flags them. When flagged, scientists can recover the full field of view. Even as the number of satellites increases over the decade, these tools for cleaning the images will still be applicable. — This image captures the streak of an Earth-orbiting artificial satellite crossing Hubble’s field of view during an observation of “The Mice” interacting galaxies (NGC 4676). Developers at the Space Telescope Science Institute in Baltimore have software that identifies the bad pixels from the satellite photobombing, the extent to which they affect the image, and then flags them. When flagged, scientists can recover the full field of view. NASA, ESA, STScI

The tool works by looking for trails in images from one of Hubble’s cameras, the Advanced Camera for Surveys. This camera has a very wide field of view, meaning it captures a large portion of the sky in one go. That’s useful in this case because it means that trails from satellites will only interrupt a very small percentage of the image.

“The average width I measured for satellites was 5 to 10 pixels. The ACS’ widest view is 4,000 pixels across, so a typical trail will affect less than 0.5% of a single exposure,” said Stark. “So not only can we flag them, but they don’t impact the majority of pixels in individual Hubble images. Even as the number of satellites increases, our tools for cleaning the pictures will still be relevant.”

When Hubble takes an image, it actually captures multiple exposures of its target. So a satellite trail would usually be on just one exposure, and the tool can look at multiple exposures and use that data to highlight the affected area. Then the researchers can combine data from the different exposures to edit out the streak.

This kind of approach will be even more important as the number of satellites in the sky continues to grow, the team writes in their paper describing the research. However, for now, Hubble remains only minimally affected.

“To date, these satellite trails have not had a significant impact on research with Hubble,” said Tom Brown, head of STScI’s Hubble Mission Office. “The cosmic rays that strike the telescope’s detectors are a bigger nuisance.”

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