MiraSOL: a DMD-based spectrograph for resolved solar spectroscopy

Imagine the Sun as a giant, churning pot of soup, constantly bubbling with hot plasma rising and cool plasma sinking. This isn't just a pretty picture; it's a noisy mess that makes it incredibly hard for astronomers to listen to the quiet whispers of distant worlds. When we look at other stars to find Earth-like planets, we usually treat the whole star like a single, blurry point of light. But just like the Sun, those stars have their own "weather"—sunspots, magnetic storms, and bubbling surfaces—that creates false signals. These signals can trick our instruments into thinking a planet is there when it's actually just the star wiggling. To solve this, scientists need to stop looking at the whole star at once and start peeling back the layers, looking at specific spots on the surface to understand exactly how the star is moving and changing. This is the difference between listening to a whole orchestra play at once versus isolating the violin to hear if it's out of tune.

Enter MiraSOL, a new experimental tool designed to do exactly that for our own Sun, acting as a practice ground for studying other stars. The team behind this project, led by researchers at Penn State, is building a device that uses a Digital Micromirror Device (DMD)—think of it as a high-tech, super-fast version of a kaleidoscope made of thousands of tiny, tilting mirrors. Instead of using a physical mask to block out parts of the Sun, MiraSOL uses these mirrors to digitally "paint" a mask in real-time. It can instantly decide which parts of the solar disk to send to a high-precision spectrograph (to study the light) and which parts to send to a camera (to see what's happening). This allows them to isolate specific features like sunspots or simulate a planet passing in front of the Sun, helping them understand how stellar "noise" affects the search for alien worlds.

The paper presents the science goals, the initial design, and a working prototype of this instrument. The researchers discovered a tricky "flicker" in the device's electronics—a 60 Hz signal likely caused by the commercial board used to control the mirrors—that they need to account for. However, they successfully built a proof-of-concept prototype that can image the Sun and dynamically create patterns, like spelling out "PSU" in the dark spots of the solar disk. This proves the concept works: we can use a digital mirror array to slice and dice the Sun's light, paving the way for future instruments that can finally see through the stellar noise to find those hidden, habitable planets.

The Big Idea: Why We Need to Look Closer

For decades, astronomers have been hunting for planets around stars like our Sun using a method called the radial velocity technique. It's like trying to hear a mosquito buzzing next to a roaring jet engine. The "jet engine" is the star itself, which isn't a calm, steady light bulb. It has granulation (bubbling plasma), sunspots, and magnetic activity that make it wobble and change brightness. These changes create "false positives," making it look like a planet is tugging on the star when it's actually just the star's own surface acting up.

To find truly Earth-like worlds, we need to reach a level of precision called Extreme Precision Radial Velocity (EPRV), aiming for stability better than 10 centimeters per second. But right now, the star's own "weather" is the biggest obstacle. The solution? Stop looking at the whole star as one blob. If we can look at specific regions of the star's surface, we can map out exactly how the different parts are moving and subtract that noise from our data. This is where MiraSOL comes in. It's designed to be a "smart mask" for the Sun, allowing scientists to pick and choose which parts of the solar disk to study in detail.

The Magic Mirror: How MiraSOL Works

At the heart of MiraSOL is a Digital Micromirror Device (DMD). You might know these from projectors that throw images onto a wall. Inside, there are thousands of microscopic mirrors, each smaller than a human hair. In a projector, these mirrors tilt to reflect light either toward the screen (ON) or away from it (OFF). MiraSOL repurposes this technology for astronomy.

Instead of projecting a movie, the telescope focuses an image of the Sun onto this chip of mirrors. The computer can then tell each individual mirror to tilt.

• The ON state: Mirrors tilt to send light from a specific spot on the Sun (like a sunspot) into a fiber optic cable, which feeds a super-precise spectrograph. This gives us a detailed chemical and velocity map of that specific spot.
• The OFF state: Mirrors tilt to send light from the rest of the Sun into a camera. This creates a real-time image where the parts being studied look dark, letting scientists verify exactly where they are looking.

This setup allows for two main types of experiments:

1. Surface Mapping: Scientists can isolate active regions to measure their specific radial velocities, helping them understand how the Sun's rotation and convection affect the light we see.
2. Simulating Transits: By rapidly switching mirrors on and off, the device can "erase" a patch of the Sun to mimic a planet passing in front of it. This helps scientists study the "transit light source effect"—a phenomenon where a planet's atmosphere looks different depending on what part of the star it's crossing.

The Hiccups and the Hardware

Building this wasn't without its surprises. The team used a commercial, off-the-shelf (COTS) evaluation board to control the DMD. While these boards are great for projectors, they aren't designed for scientific precision. During testing, the researchers discovered a persistent 60 Hz flicker in the signal. They suspect this is an automated "reset" sequence programmed into the board's firmware to keep the tiny mirrors from getting stuck (a problem called "stiction").

When they measured the light, they saw a signal that looked like a 120 Hz wave, but when they analyzed the timing closely, they realized it was actually two offset 60 Hz signals. This flicker degrades the contrast of the image, making it harder to see the faint details. The authors suspect this is a limitation of the specific commercial board they are using, not the mirrors themselves. They also noted that the glass window on the DMD is optimized for visible light, but for some of their experiments (using near-infrared light), a different window would be needed to improve efficiency.

The Prototype: Spelling "PSU" on the Sun

To prove this idea wasn't just a dream, the team built a working prototype on the roof of a building at Penn State. They used a small telescope, a solar filter to protect the equipment, and the DLP801RE DMD chip.

The results were promising.

• Imaging: They successfully captured images of the Sun. When they set all the mirrors to the "OFF" state (sending all light to the camera), they could clearly see sunspots on the solar surface.
• Dynamic Masking: In a fun demonstration, they programmed the mirrors to create a dark shape spelling out "PSU" (Penn State University) across the solar disk. This showed that the device could dynamically change the shape of the "mask" in real-time.
• The Banding: The images showed some horizontal banding. The team suspects this is caused by the camera's short exposure times clashing with the 60 Hz mirror reset sequence they discovered earlier. They plan to add filters to fix this in future versions.

What This Means for the Future

The paper concludes that the core concept of using a DMD for solar observations is viable. The prototype successfully demonstrated that we can control the solar image pixel-by-pixel. While the 60 Hz flicker and the need for better optical windows are challenges to solve, the team has a clear path forward.

By refining the design and integrating this system with high-precision spectrographs like NEID or HPF, MiraSOL could become a powerful tool. It won't just help us understand our own Sun better; it will provide the "ground truth" data needed to build better models for studying other stars. If we can learn to filter out the Sun's noise, we'll finally be able to hear the quiet signals of Earth-like planets orbiting other stars, bringing us one step closer to answering the age-old question: Are we alone?