🔭 astrophysics

Fabrication of focusing optics for TA-MOONS: Micro-MOONS

This paper demonstrates the successful fabrication and integration of gold-coated, spherically concave micro-mirrors, created via two-photon polymerization on silicon substrates, to serve as a critical calibration and positioning component within the Micro-MOONS robotic arm system for the TA-MOONS multi-object spectroscopic survey of young stellar objects.

Original authors: Ceiwynn Longworth, Megan Delamer, Suvrath Mahadevan, Joe P. Ninan, Kathleen Gehoski, Krushna Jadhav

Published 2026-07-16

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ceiwynn Longworth, Megan Delamer, Suvrath Mahadevan, Joe P. Ninan, Kathleen Gehoski, Krushna Jadhav

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Time Machine and the Tiny Mirrors

Imagine trying to understand how a family grows up, but you can only take a single, blurry photo of one child at a time. You can't watch them from babyhood to adulthood; you only get a snapshot. This is the challenge astronomers face when studying how planets are born. Planets form inside swirling disks of gas and dust around young stars, but these disks disappear in just a few million years—a blink of an eye in cosmic time. To figure out the recipe for planet formation, scientists need to take thousands of these "snapshots" of different young stars to piece together the story.

To get these snapshots, they need special cameras called spectrographs. Think of a spectrograph as a prism that splits starlight into a rainbow, revealing hidden details like the temperature, chemistry, and movement of the star and its surrounding disk. However, looking at many stars at once is like trying to read eight different books simultaneously while wearing blinders. You need a way to grab the light from eight specific stars and funnel it into one camera without mixing them up. This is where a new instrument called TA-MOONS comes in. It uses robotic arms to pick up light from eight stars at once. But to make sure those robotic arms are pointing exactly right, they need a tiny, built-in GPS system made of mirrors. This is the story of how scientists built those tiny mirrors.

The Paper: Building a Microscopic GPS for Star-Hunting Robots

The paper you're reading details the creation of Micro-MOONS, a set of tiny, custom-made mirrors designed to act as the "eyes" for the TA-MOONS instrument. The goal was simple but incredibly difficult: create a grid of four microscopic mirrors for each of the eight robotic arms on the telescope. These mirrors need to be perfect spheres, 400 micrometers in diameter (that's about the thickness of a human hair), spaced 500 micrometers apart from center to center, and they must be shaped so precisely that they can tell the telescope exactly where a star is located.

The Magic Ink and the Laser Pen

To build these mirrors, the team didn't use traditional glass grinding or metal casting. Instead, they used a technique called two-photon polymerization (2PP). Imagine a 3D printer, but instead of melting plastic, it uses a super-fast laser pen to turn liquid "ink" (called photoresist) into solid plastic, one tiny dot at a time.

The magic happens because the laser is so fast that it only hardens the ink at the very center of its focus, like a microscopic sculptor chipping away at a block of ice. This allowed the team to print curved, bowl-shaped mirrors with a level of detail that would be nearly impossible and prohibitively expensive to make any other way. They printed these mirrors on silicon wafers, which are the same material used in computer chips, and then coated them with gold to make them shiny enough to reflect infrared light from distant, dusty stars.

The Bumpy Road to Perfection

The journey wasn't smooth. The team faced several "monster" challenges that threatened to ruin their tiny mirrors:

The Floating Ghosts: At first, the mirrors wouldn't stick to the silicon wafer. They were like little boats floating in the liquid ink instead of being glued to the bottom. The team realized the laser was getting confused by the shiny silicon surface. They solved this by tweaking the printer's software to "look" deeper, forcing it to focus on the bottom of the liquid layer rather than the top.
The Temperature Tango: The liquid ink was sensitive to temperature. If the ink was cold when they started printing but warmed up halfway through, the mirrors would shrink or warp, making them the wrong shape. It was like baking a cake where the oven temperature keeps changing. They fixed this by letting the ink and the silicon sit together for 30 minutes to reach the same temperature before printing.
The Bubble Trouble: The biggest enemy turned out to be tiny air bubbles trapped in the ink. These bubbles were like invisible potholes on a road, ruining the smooth surface of the mirrors. The team tried different methods to remove them, including putting the ink under a vacuum (a "burping" process) to pop the bubbles. They found that letting the ink sit under a vacuum for 30 minutes worked best, shrinking the bubbles so they wouldn't ruin the mirror's surface.

The Gold Standard

Once the mirrors were printed, they had to be coated with gold. Gold is special because it reflects infrared light very well, which is crucial for seeing the dusty, reddish stars the telescope is designed to study. They also added a thin layer of titanium underneath the gold to act like double-sided tape, ensuring the gold wouldn't peel off.

The Final Verdict

The team successfully fabricated eight sets of these mirror arrays. They tested them using powerful microscopes and lasers to measure their shape. The results showed that the mirrors were incredibly accurate, with their curvature varying by less than 100 micrometers—a tiny margin that meets the strict requirements of the telescope.

However, the paper is honest about what remains unsolved. While they managed to create high-quality mirrors, the "bubble problem" wasn't completely defeated. About 42% of the printed arrays still had at least one mirror with a bubble that disrupted the surface roughness at the center. The authors admit they don't know exactly why these bubbles form, but they have proven that 2PP printing is a viable way to make these complex optical parts.

In the end, this paper shows that we can now "print" custom-made, high-tech mirrors for giant telescopes. While the bubble issue needs more work, the team has demonstrated that this technology can help astronomers take those crucial snapshots of young stars, bringing us one step closer to understanding how our own solar system—and perhaps others—came to be.

Technical Summary: Fabrication of Focusing Optics for TA-MOONS

Problem Statement
The TIFR-ARIES Multi-Object Optical to Near-infrared Spectrometer (TA-MOONS) is designed to conduct a large spectroscopic survey of young stellar objects (YSOs) across 360 nm to 2.5 µm wavelengths. A critical component of this instrument is a robotic arm system capable of picking up to eight sources simultaneously within a 12 arcmin field of view. To achieve the required 10 µm placement accuracy for these pickup arms, the system utilizes a Micro-MOONS array: a set of micro-optics that act as position-sensitive retro-reflectors. These mirrors must be precisely positioned on the telescope focal plane to provide absolute position calibration. The challenge lies in fabricating these micro-mirrors with specific geometries (spherical concave, 400 µm diameter, 3–4 mm radius of curvature) and tight tolerances (±100 µm radius of curvature uniformity within an array) at a scale and cost that conventional optical manufacturing struggles to achieve. Furthermore, the mirrors must maintain surface quality sufficient to avoid beam obstruction and flux loss, with a surface tolerance of approximately $\lambda/4$ in the I-band.

Methodology
The authors employed Two-Photon Polymerization (2PP) via a NanoScribe Photonic Professional GT system to fabricate the Micro-MOONS arrays. The process involved the following key steps:

Design: A 2x2 array of spherical, concave mirrors was designed using Autodesk Fusion. Each mirror has a 400 µm diameter, a 500 µm pitch, and a target height of 11.67 µm (including a 6.67 µm sag and 5 µm base).
Substrate Preparation: Silicon substrates (25 x 25 x 0.7 mm) were selected for cost and dicing ease. They underwent chemical washing (acetone/IPA), nitrogen blow-drying, and plasma etching (O2, 5 minutes) to promote adhesion and remove residues.
Resin Handling: IP-S photoresist was used. To mitigate bubble formation and temperature-induced variations, the resin was stored refrigerated (4°C). A critical pre-processing step involved a 30-minute thermalization period to equalize temperatures between the substrate and resin, followed by a 30-minute degassing cycle with periodic "burping" (vacuum release) to remove micro-bubbles.
Fabrication: Printing was performed using a 780 nm femtosecond laser with a 25x objective. Key parameters included a laser power of 17.5 mW (35 LP), a write speed of 40k, a piezo settling time (PST) of 250 ms, and a slice/hatch size of 0.2 µm with a hatching angle offset of 137.508° to ensure uniform distribution. A specific software override (InterfaceMin 1000.090) was implemented to force the printer to focus on the photoresist-substrate interface rather than the air-resin interface, addressing the high reflectivity of silicon.
Post-Processing: Prints underwent heat curing (65-95-65°C), development in mr-Dev 600, an IPA rinse, and a 10-minute UV cure to minimize shrinkage.
Coating: To maximize reflectivity in the infrared and I-band, the arrays were coated with a 0.053 Å titanium adhesion layer followed by a 0.951 Å gold layer via E-beam evaporation.
Characterization: Arrays were analyzed using an optical microscope, a Zygo NexView 3D Optical Surface Profiler (for radius of curvature, surface roughness, and profile), and a Field Emission Scanning Electron Microscope (FESEM) to verify structural features like bubbles.

Key Contributions and Results

Fabrication Success: The team successfully fabricated eight sets of 2x2 micro-mirror arrays, one for each of TA-MOONS' pickup arms. All arrays met the critical requirement of having a radius of curvature (ROC) variation within ±100 µm across the array.
Process Optimization: The study identified and resolved several fabrication challenges:
- Focusing: The InterfaceMin override successfully resolved focusing issues on reflective silicon substrates.
- Uniformity: Reducing the piezo settling time from 500 ms to 250 ms and implementing a 30-minute thermalization period significantly reduced ROC deviations within arrays (from variations up to 800 µm to <32 µm).
- Bubble Mitigation: While bubbles remained a persistent issue, the combination of refrigerated storage, upright positioning, and a 30-minute degassing cycle reduced their size and frequency.
Performance Validation:
- Surface Quality: Profilometer data confirmed that the surface roughness met the $\sim\lambda/4$ tolerance required for the I-band.
- Coating Impact: Gold deposition resulted in negligible changes to the ROC (median change of -0.5 µm for the inner 360 µm), ensuring the optical figure remained stable.
- Optical Performance: Point Spread Function (PSF) measurements using a Point Source Microscope (PSM) indicated that the mirrors are "seeing limited." The spot diameter of the fabricated mirrors is significantly smaller than the best seeing conditions at the Devasthal Optical Telescope (DOT), meaning the mirrors do not introduce additional aberrations.
Defect Rate: Despite optimizations, 42% of arrays produced after the initial test set contained at least one mirror with a bubble disrupting the surface roughness at the center. The root cause of these bubbles remains unknown.

Significance and Claims
The paper claims to demonstrate the feasibility of integrating advanced, custom-printed micro-optics into multi-object spectroscopic instruments. By utilizing 2PP, the authors achieved the fabrication of complex, sub-micron scale spherical mirrors that meet the stringent calibration requirements of the TA-MOONS instrument. The work validates 2PP as a viable, flexible, and cost-effective alternative to conventional manufacturing for producing bespoke micro-mirror arrays.

However, the authors maintain a modest tone regarding the technology's readiness for mass production. They explicitly state that while they successfully fabricated eight high-quality, bubble-free sets, the "major challenge that remains is reaching reliable replication of arrays." The persistence of bubbles in a significant portion of the production run suggests that further investigation into replication reliability is necessary before 2PP can be considered a fully dependable manufacturing tool for next-generation astronomical instrumentation. The paper concludes that addressing this defect rate is essential for the broader adoption of the technology.