LP-Infinity: overcoming limits to the number of lifetime positions in the COS FUV channel

Deep in the history of the universe, light from distant galaxies travels across billions of years to reach us, carrying secrets about how stars and planets formed. To read these secrets, astronomers rely on instruments that can catch this faint light and break it apart into a rainbow of colors, a process called spectroscopy. One of the most powerful tools for this job is the Cosmic Origins Spectrograph, a device attached to the Hubble Space Telescope. Since its installation in 2009, it has captured thousands of images of the cosmos, specifically looking at a part of the light spectrum called the Far Ultraviolet. This invisible light is crucial for studying hot, young stars and the gas between them, but it is also the most energetic and damaging part of the spectrum for the telescope's sensors.

The sensor inside the Cosmic Origins Spectrograph works like a highly sensitive camera that counts individual particles of light. Over time, however, the constant bombardment of these particles wears down the sensor's ability to respond, much like a tire slowly losing air pressure. As the sensor weakens, the images it produces become blurry or lose detail. To fix this, the telescope team has developed a strategy of moving the target to a fresh, unused spot on the sensor every few years. This keeps the instrument sharp, but the number of available spots was strictly limited by the software running inside the telescope. Now, a new method has been devised to break that limit, ensuring the telescope can continue its work well into the 2030s.

For over a decade, the Cosmic Origins Spectrograph has been the workhorse of the Hubble Space Telescope, capturing detailed views of the universe in ultraviolet light. The instrument uses a special detector that counts photons, or particles of light, as they hit the sensor. Each time a photon arrives, the detector records exactly where it landed and how strong the signal was. Over years of continuous use, the sensor's sensitivity naturally declines, a phenomenon known as gain sag. When the sensor becomes too weak in one area, the faint signals from distant objects can no longer be distinguished from the background noise, and the data is lost. To compensate, the telescope team adjusts the pointing of the observatory, shifting the view of the target to a different, unused section of the detector. These specific locations are called Lifetime Positions.

Since the instrument was installed, the team has successfully moved the observations to new spots on the sensor several times. By 2025, they had defined eight of these positions, with two more planned for the near future. However, a hard limit existed in the telescope's onboard software: it could only store instructions for eight positions. As the sensor continued to age, the team realized that eight spots would not be enough to keep the instrument running at its best quality for the next decade. The most obvious solution would have been to rewrite the software to allow for more spots, but this would have required a massive, risky overhaul of the telescope's computer systems while it was orbiting Earth. Another option was to simply reuse the old spots, but that would have complicated the data processing and confused the timeline of the observations.

Instead of fighting the software limit, the researchers found a clever way to work around it. They developed a new strategy, which they call LP-Infinity, that allows the telescope to use as many positions as needed without changing the onboard computer code. The method works by temporarily updating the instructions for a single observation just before it begins. Imagine the telescope's computer has a small notebook with eight pages, each dedicated to a specific location. Instead of adding more pages to the notebook, the team now changes the content of the last page right before each new observation. They swap the old instructions for the new location, take the picture, and then the system is ready for the next one. This dynamic patching happens so quickly that the telescope behaves as if it has an unlimited number of spots available.

This approach required no physical changes to the Hubble Space Telescope itself, but it did demand a significant update to the ground-based systems that send commands to the satellite. The team had to ensure that the new instructions were sent correctly and that the telescope's software could handle the rapid changes without error. The new system was implemented quickly, with the first observations using this method taking place in late 2025. By skipping the number eight in the sequence and starting the new series at ten, the team avoided any confusion between the old, fixed positions and the new, flexible ones. This change allows them to define new positions based on the specific needs of each observation, such as the type of light being studied or the condition of the sensor at that moment.

The benefits of this innovation are substantial. It gives the team the freedom to move the telescope's view to the best possible spot on the sensor for every single observation, rather than being forced to use a pre-defined list of locations. They can now place the view of a target on a spot that has just the right amount of sensitivity for a specific wavelength of light, or avoid areas of the sensor that have developed defects. This flexibility means that the Cosmic Origins Spectrograph can continue to produce high-quality data even as the sensor continues to age. The team has already mapped out a plan for using these new positions through the 2030s, ensuring that the instrument remains a vital tool for astronomy long after its original five-year design life has passed.

The success of this project relies on a deep understanding of how the sensor degrades over time and how the telescope's optics interact with different parts of the detector. The researchers used detailed models to predict where the sensor would remain strong enough to capture clear images and where it would fail. They also had to account for physical limitations, such as the mechanical range of the telescope's aperture and the presence of "hot spots" or dead areas on the sensor that cannot be used. By carefully selecting new positions that avoid these problem areas, the team ensures that the data collected remains reliable. The new method also allows for multiple positions to be defined for the same target, depending on the specific grating or filter being used, which provides a level of control that was previously impossible.

Looking ahead, the Cosmic Origins Spectrograph is expected to operate well into the next decade, providing a continuous stream of data that will bridge the gap between the Hubble Space Telescope and its future successor, the Habitable Worlds Observatory. The LP-Infinity strategy ensures that the detector will not be the limiting factor in obtaining high-quality ultraviolet spectra. As the sensor continues to age, the team will continue to refine their models and adjust the positions as needed, keeping the instrument at the forefront of astronomical discovery. This work demonstrates that even with aging hardware, creative engineering and software solutions can extend the life and capability of space telescopes, allowing us to keep looking deeper into the universe than ever before.