Scrubbing Hubble images of satellite light tracks

Scrubbing Hubble images of satellite light tracks
by Staff Writers
Baltimore MD (SPX) Jun 06, 2023

When the Hubble Space Telescope was launched in 1990, there were about 470 artificial satellites orbiting Earth. By 2000, that number doubled. But by 2023, the rising number has grown almost exponentially to nearly 8,000 satellites. For Hubble this means that satellites photobomb about 10% of its exposures on celestial targets. But a typical satellite trail is very thin and will affect less than 0.5% of a single Hubble exposure.

Nevertheless, these denizens leave annoying pencil-thin, white streaks across a Hubble image as they zoom overhead. And, they are not the only image artifacts Hubble astronomers have to contend with. Cosmic rays rain onto Hubble's camera detectors. These leave what looks like "scratch marks" too. In fact, they are a bigger nuisance than satellite trails.

Astronomers at the Space Telescope Science Institute in Baltimore, Maryland have developed tools for cleaning up this clutter. Hubble observations consist of more than just one exposure. And so, artifacts can be identified and subtracted between exposures because they are not in the same place on a detector.

It's estimated that by 2030 there could be ten times as many satellites circling Earth as there are now. But even as the number of satellites increases, the Space Telescope Science Institute's tools for cleaning the Hubble pictures will still be useful. To date not one Hubble science program has been affected by satellite trails.

Artificial satellites are photobombing the Hubble Space Telescope's snapshots as much as every two to four hours, according to researchers at Baltimore's Space Telescope Science Institute (STScI).

As they whirl around Earth, the satellites leave streaks across an image, like scratches on photographic film. Hubble is in a low-Earth orbit and so many satellites in higher orbits sweep overhead. As many as 8,000 satellites circle Earth - more than half for telecommunications.

But not to worry - experts say that they are not a threat to the celebrated telescope's ongoing observations of the universe.

"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.

Stark applied the new tool, based on the image analysis technique known as the Radon Transform, to identify satellite trails across Hubble's camera with the widest field of view, the Advanced Camera for Surveys (ACS).

In 2002 the satellite streaks were present in five percent of ACS exposures, with many of those too faint to discern easily. This rose to ten percent by 2022, although the typical brightness of the detected trails remained unchanged.

As the number of artificial satellites encircling Earth rises, sky contamination for all telescopes based on the ground or in Earth orbit becomes increasingly worse.

"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."

Radiation from space hits the ACS electronic detectors on every exposure, leaving streaks. These are easy to identify from exposure to exposure. The same holds true for artificial satellites. "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. 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," said Stark.

A Hubble science observation is assembled from a collection of multiple exposures on the same celestial target. So a satellite streaking across the sky can appear in one frame and not the next consecutive frame. Stark and collaborators developed a masking routine that identifies where the bad pixels are, the extent to which they affect the image, and then flags them. '"When we flag them, we should be able to recover the full field of view without a problem, after combining the data from all exposures," said Stark.

The Radon Transform software tool Stark used is applied in other sciences as well, such as reconstructing images from medical CT scans, and reconstructing a map of a planet's polar regions gleaned from a spacecraft. The software is ideal for identifying and characterizing linear features in an image because it sums up all the light along every possible straight path across an image. This approach combines all the light from a satellite trail, making them "pop out" in the transformed image, even many of those that are very faint in the original image. Previous studies regarding Hubble do not pick up the fainter satellite trails. The new software is up to ten times more sensitive than prior software developed by STScI to detect satellite trials, and it identifies roughly twice as many trails as other studies.

"We have a toolbox of things that people use to clean Hubble data and calibrate it. And our new application is another tool that will help us make the best out of every Hubble exposure," said Stark.

Research Report:Improved Identification of Satellite Trails in ACS/WFC Imaging Using a Modified Radon Transform