Breaking the brightness barrier: JWST/NIRCam DHS spectroscopy for high-precision time-series observations

Imagine the universe as a giant, noisy concert hall where stars are the loudest singers. For decades, astronomers have wanted to listen closely to the "voice" of planets orbiting these stars to understand what they are made of. To do this, they use a super-powerful tool called a telescope that acts like a giant prism, splitting starlight into a rainbow of colors. This rainbow, called a spectrum, reveals hidden ingredients like water or methane in a planet's atmosphere. However, there's a catch: if the star is too bright, it's like trying to hear a whisper while standing next to a jet engine. The telescope's sensors get overwhelmed, the "microphone" gets blown out, and the data is ruined. This has left many of the most interesting, nearby planets off-limits to our best space telescopes because their host stars are simply too dazzling.

This paper tells the story of how the James Webb Space Telescope (JWST) learned a clever trick to listen to these bright stars without getting deaf. The team developed a new way to use the telescope's camera (NIRCam) that combines two smart ideas. First, they use a special sensor called the Dispersed Hartmann Sensor (DHS), which acts like a set of tiny, narrow windows that only let a small fraction of the star's light through, dimming the "jet engine" down to a manageable hum. Second, they invented a new way to read the camera's data called "multistripe" mode. Instead of reading the entire giant camera sensor every time (which takes too long and lets the light build up too much), the camera now only reads the tiny, specific strips where the light actually lands, skipping the empty space in between. This makes the camera read data much faster, preventing it from getting saturated. The result is a new observing mode that allows JWST to take high-precision measurements of planets orbiting very bright stars, a feat that was previously impossible.

The paper reports on the successful "on-orbit commissioning" of this new mode, which means the team tested it in space to make sure it works as planned. They didn't just simulate it; they actually pointed the telescope at a real exoplanet system called WASP-18b and watched a full transit (when the planet passes in front of its star). The results showed that the new mode is stable and ready for use. By combining the light-dimming DHS with the fast "multistripe" reading, the team demonstrated that they can now observe stars as bright as magnitude K ∼2.5, a significant jump from the previous limit of K ∼5.7. This opens the door to studying the atmospheres of many nearby planets that were previously too bright to analyze, marking a major step forward in our ability to explore the worlds right next door.

The Brightness Problem and the "Jet Engine" Solution

Think of the James Webb Space Telescope as the most sensitive camera ever built, designed to take pictures of the faintest, most distant objects in the universe. But, like any camera, it has a limit. If you point it at something too bright, the image gets washed out, or "saturated," just like your eyes get blinded if you stare directly at the sun. For a long time, this was a major problem for astronomers studying exoplanets (planets outside our solar system). Many of the most exciting planets to study orbit very bright, nearby stars. These stars are so bright that they overwhelm the telescope's sensors, causing the data to become useless before a single measurement can be taken.

The standard way telescopes handle bright objects is to read the sensor data very quickly. However, even with the fastest standard reading speed, the JWST sensors would fill up with light (photoelectrons) faster than they could be emptied for the brightest stars. It's like trying to fill a bucket with a firehose while only having a tiny cup to scoop the water out; the bucket overflows before you can measure anything. The paper explains that to fix this, you need to either reduce the amount of water (light) hitting the bucket or make the cup (the reading process) much bigger and faster.

The Two-Part Trick: Narrow Windows and Fast Stripes

The team behind this paper came up with a two-part solution to solve the "overflowing bucket" problem.

Part 1: The Narrow Windows (DHS)
The first part of the trick uses a tool called the Dispersed Hartmann Sensor (DHS). Originally, this was built to help align the telescope's mirrors, but the team realized it could also be used to dim bright stars. Imagine the telescope's main mirror as a giant window. The DHS acts like a mask with ten tiny, narrow slits cut into it. Instead of letting the whole window shine through, it only lets light pass through these small slits. This reduces the amount of light hitting the sensor by about 75%, turning that blinding "jet engine" into a manageable "vacuum cleaner."

But there's a bonus: because the slits are arranged in a specific way, the light that gets through is split into multiple separate rainbows (spectra). Instead of one big, blurry rainbow, the sensor sees eight distinct, smaller rainbows of the same star. This spreads the light out even more, making it even easier to measure.

Part 2: The Fast Stripes (Multistripe Readout)
Even with the light dimmed by the DHS, the sensor could still get overwhelmed if the telescope took too long to read the data. This is where the second part of the trick comes in: the "multistripe" readout mode.

Normally, when a camera takes a picture, it reads every single pixel on the sensor, even the ones that are just showing empty black space. This takes time. For the JWST, reading the whole sensor takes about 10.74 seconds. For a very bright star, that's too long; the light builds up and ruins the measurement.

The new multistripe mode is like a smart camera that knows exactly where the light is. Instead of reading the whole sensor, it only reads the specific narrow strips (or "stripes") where the rainbows are landing. It skips all the empty space in between. It's like a librarian who only checks the specific shelves where books are being returned, ignoring the empty shelves. By doing this, the time it takes to read the data drops dramatically from 10.74 seconds down to just 1.36 seconds. If they only need to read the two brightest strips, they can go even faster, down to 0.22 seconds. This speed is crucial because it prevents the sensor from filling up with light before the measurement is finished.

Putting It to the Test: The WASP-18b Experiment

The paper describes the process of testing this new system in space, a phase called "commissioning." The team didn't just hope it would work; they had to prove it with real observations. They selected a target that was perfect for testing: a planet called WASP-18b orbiting a very bright star. They chose this specific planet because it doesn't have a complex atmosphere with lots of weird features. This was a smart move because it meant that any "noise" or weird patterns in the data would likely come from the telescope itself, not the planet. This allowed the team to see exactly how well their new tools were performing.

The observations took place in June 2026. The team watched the planet pass in front of its star for about 4 hours. During this time, the new DHS and multistripe mode worked together to capture a continuous stream of data. The result was a set of "light curves" (graphs showing how the star's brightness changed over time) that were incredibly stable. The team saw the planet's transit clearly, proving that the system could handle the brightness of the star without getting saturated.

What This Means for the Future

The paper concludes that this new mode is a success. It has successfully extended the range of stars that JWST can study. Before this, the telescope could only handle stars up to a brightness of about K ∼5.7. With the new DHS and multistripe mode, it can now observe stars as bright as K ∼2.5. This is a huge deal because the brightest stars are often the ones with the most interesting, nearby planets.

The authors note that while the initial tests are complete, there is still work to be done. They are currently refining the calibration (making sure the measurements are perfectly accurate) for all the different filter combinations. They are also analyzing the data to measure exactly how precise the new mode is and how much "noise" remains in the measurements. However, the initial results are very promising. The paper suggests that this "multistripe" technique isn't just for the NIRCam camera; it could be applied to other instruments on the telescope as well, potentially allowing JWST to study even more bright targets in the future.

In short, this paper shows that by using a clever combination of light-blocking slits and a super-fast reading technique, astronomers have broken through a brightness barrier. They have turned the telescope's biggest weakness (sensitivity to bright light) into a strength, opening up a new chapter in the study of the planets closest to us.