Sharp-eyed Telescope Tracks Satellites During Daylight

Australia’s Huntsman uses CMOS detectors for unprecedented exposures

4 min read

small blue dome with numerous lenses inside against a blue sky

A panorama of the Huntsman Telescope and the dome it’s housed in is located at Siding Spring Observatory, Coonabarabran, Australia.

Sarah Caddy

Daytime astronomy: Amateur nighttime stargazers might wonder, what’s the point? But astronomers at Macquarie University in Sydney have developed a new technique that makes possible daylight observation of structures in the Milky Way galaxy, potentially doubling available observation time and increasing our knowledge of the cosmos. This advance also promises to improve space safety by tracking orbiting satellites during the day, a vital addition to satellite tracking as the total number of satellites in orbit—and the risk of cascading collisions—grows.

The technique employs a custom-built telescope assembled from commercially available components. In a salute to the Dragonfly Telephoto Array telescope at the New Mexico Skies Observatory in Mayhill, N.M. that inspired the project, the Macquarie astronomers have named the telescope Huntsman, after the speedy spider of the same name.

Huntsman boasts rapid exposure times and frame rates

The Huntsman features an array of 10 Chinese ZWO-brand ASI183 Pro Series cameras, each equipped with a CMOS sensor and paired with a Canon 400 mm f/2.8 lens. Each lens is fitted with a Chroma Technology Sloan red filter to block out the blue light from the sun while allowing the light from faint astronomical sources to pass through. The lens array is configured to cover the same field of view to enhance image sensitivity and enable efficient data collection.

numerous different lenses next to each other Huntsman consists of an array of 10 lenses, each with individual cameras that were originally designed for low-surface-brightness imaging.Sarah Caddy

“It’s like a Lego telescope in that most of the components can be bought off the shelf,” says Sarah Caddy, a Ph.D. candidate at Macquarie University. Caddy helped design and build the system and coauthored a paper published on the project in Publications of the Astronomical Society of Australia in May. “We can add and remove parts to suit new projects or take advantage of new technology advancements.”

Case in point is the researchers’ choice of CMOS detectors in place of the traditional charge-coupled devices (CCDs) for image sensors in the cameras. While CCDs are favored by astronomers because of their high sensitivity, Caddy says CMOS detectors are closing the gap in image quality and have the added advantage of significantly higher frame rates. The Huntsman sports an exposure time as short as 32 microseconds, and frame rates up to 271 per second.

“High frame rates are usually not needed in astronomy, because things in the sky can take years or centuries or longer to change or move,” says Lee Spitler, an associate professor of space-related projects at Macquarie University. “We’ve been homing in on exciting aspects of astronomy where you do need such high data rates, like the work Sarah is leading.”

The telescope is located in Coonabarabran, New South Wales, a wearisome six-hour drive west of Sydney and its bright lights. But because the Huntsman is robotically operated, a program’s sequences—such as opening and closing the dome, choosing a target for observation, and adjusting the focus—can all done autonomously under the control of the astronomers in Sydney. “So we only go out there if things go wrong,” says Caddy.

The astronomers don’t even need to make the trip to test procedures: Before configuring a specific program for the Huntsman, they use a single-lens Huntsman in the Sydney campus to test procedures like optimal exposure times and the tracking of a target in a controlled environment.

Unlike nighttime imaging, which often requires tens of minutes of exposure time, daylight imaging is performed in microsecond bursts to prevent overexposure. “We focus on one object for several minutes, taking thousands and thousands of exposures,” says Caddy. When the observation is completed, the astronomers process the data by manually removing artifacts and then stacking the data arrays, one on top of the other, to produce a quality image. This work can take up to a couple of hours, says Caddy, but they plan to automate the process.

Huntsman can also track near-Earth objects

The Huntsman can also track orbiting satellites during the daytime. The astronomers first tested this technique by observing the International Space Station and China’s Tiangong space station, as well as the Hubble telescope—the brightest objects in low Earth orbit—during the day and then predicted their orbits. When one of these objects reaches a predicted point in the orbit that the Huntsman is focused on, the telescope is programmed to pick it out, and then the telescope tracks the object as it moves along its orbit.

a small white dot against a black backgroundThe International Space Station taken by the Huntsman Telescope Pathfinder at midday, only a few degrees away from the sun.Sarah Caddy

The same method is used for tracking satellites. When a satellite reaches an expected point in its orbit, the Huntsman is waiting and takes a rapid number of images until it locks onto the satellite.

The orbits of satellites in low Earth orbit can change with variations in the Earth’s atmosphere or when solar storms occur, so orbits can become unclear over time. With 10,000 satellites already in operation and with up to 50,000 expected to be launched in the next decade, the situation could turn dangerous. One crash already occurred in 2009, and the resulting space shrapnel is still a cause for alarm for space stations and their occupants, as well as other satellites.

The Huntsman’s field of view is large relative to other telescopes, spanning 1 degree by 2 degrees of the sky, according to the astronomers. And its mount is fast enough to sweep through the sky to find and keep a satellite in view. “So even if the satellite’s orbit is a little uncertain, we’re still likely to see it because of the large field of view,” says Caddy.

The astronomers are still refining this technique, but the technology has already attracted attention. “The space industry is already showing interest in our work,” says Caddy.

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