Mini-EUSO optics design and tests
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2017-08-16 |
| Journal | Proceedings of 35th International Cosmic Ray Conference â PoS(ICRC2017) |
| Authors | Y. Takizawa, Hiroshi Kasuga, Hitoshi Ohmori, M. Casolino, Toshikazu Ebisuzaki |
| Institutions | RIKEN |
| Citations | 3 |
| Analysis | Full AI Review Included |
Analysis of Mini-EUSO Optics Design: Leveraging MPCVD Diamond for Extreme UV Environments
Section titled âAnalysis of Mini-EUSO Optics Design: Leveraging MPCVD Diamond for Extreme UV Environmentsâ6CCVD analyzes the manufacturing and performance results of the Mini-EUSO optical system, designed for UV detection (355-393 nm) from the International Space Station (ISS). While the current design utilizes UV-transparent PMMA, the demanding requirementsâespecially sub-20 nm RMS surface roughness and radiation hardness for space applicationsâhighlight a critical opportunity for Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) optics, where 6CCVD excels in material purity, dimension control, and ultra-high precision polishing.
Executive Summary
Section titled âExecutive Summaryâ- Application Focus: Development of large-area Fresnel lenses for the Mini-EUSO telescope, designed to observe UV phenomena (355-393 nm) in the night Earth atmosphere from the ISS.
- Critical Performance Requirement: Extreme surface quality is paramount, requiring an RMS surface roughness of less than 20 nm to maintain high Photon Collection Efficiency (PCE).
- Achieved Quality: Measured roughness across lens surfaces consistently fell below 20 nm RMS, with minimum values reaching 13.2 nm.
- Optics Design: The system uses two double-sided Fresnel lenses (250 mm diameter, 11 mm thickness) manufactured via diamond turning.
- Material Opportunity: The mission environment (ISS, UV exposure) and the stringent polishing requirements are ideally suited for MPCVD diamond (SCD/PCD) materials, offering superior radiation hardness, thermal stability, and deep-UV transparency compared to PMMA.
- Optical Throughput: PCE was estimated at ~45% for parallel rays, factoring in losses due to surface roughness (20 nm RMS) and manufacturing artifacts (tool bite loss).
Technical Specifications
Section titled âTechnical SpecificationsâThe following parameters summarize the key physical and performance characteristics of the Mini-EUSO optical system:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Lens Diameter | 250 | mm | Double-sided Fresnel lens (PMMA material) |
| Lens Thickness | 11 | mm | Thickness of each Fresnel lens |
| System Overall Length | 300 | mm | Distance from front lens to focal surface |
| Effective Focal Length (EFL) | 150 | mm | Low focal number F# 0.6 |
| Required RMS Roughness | < 20 | nm | Critical requirement for optical performance |
| Achieved RMS Roughness (Min) | 13.2 | nm | Back surface of Front lens (Middle Area) |
| Achieved RMS Roughness (Max) | 19.1 | nm | Front surface of Rear lens (Central Area) |
| Operational Wavelength Range | 355 to 393 | nm | Near-UV to UV light source testing |
| Estimated Photon Collection Efficiency (PCE) | ~45 | % | Estimated actual PCE for parallel rays |
| Field of View (FOV) | ±22 (opposite sides), ±30 (diagonal) | ° | Matching the PDM detector array |
| RMS Spot Size | 2.0 to 3.5 | mm | Diameter range across 0° to 22° field angle |
| Tool Resolution (Manufacturing) | 1.4 | nm | UPL-240M machine axis control resolution |
Key Methodologies
Section titled âKey MethodologiesâThe following processes were critical to achieving the ultra-smooth surfaces and verifying the required optical performance:
- Manufacturing: Lenses were manufactured using the UPL-240M diamond turning machine (RIKEN SEIKO CO., LTD.), employing specialized techniques developed to achieve surface roughness below 20 nm RMS.
- Metrology (Surface Quality): RMS surface roughness was measured using an optical surface profiler (interferometer system), confirming all measured values were below the 20 nm requirement.
- Optical Testing Setup: A comprehensive lens measurement system was built utilizing five motorized linear slides, a CCD detector (6.45 mm x 4.84 mm detection area), and various light sources.
- Refraction Angle Measurement: A near-UV laser (405 nm) was used to measure the refraction angle of single lenses by scanning the beam radially across the lens surface.
- Throughput and PSF Measurement: Optics throughput (PCE) and point spread function (PSF) were measured using LED point sources operating at critical UV wavelengths (355 nm, 375 nm, and 393 nm).
- Performance Verification: Measured PCE values and observed spot images showed strong correlation with dedicated raytracing simulations that incorporated surface roughness loss (20 nm RMS) and tool bite loss.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is uniquely positioned to supply superior MPCVD diamond materials that would not only replicate but significantly enhance the performance, durability, and spectral range of optics for critical space and high-energy physics missions like Mini-EUSO/JEM-EUSO. Diamond provides unmatched UV transparency (especially deep UV), thermal management, and radiation hardness necessary for long-term satellite operation.
Applicable Materials
Section titled âApplicable Materialsâ| Requirement | 6CCVD Recommended Material | Material Advantage |
|---|---|---|
| Extreme Surface Polish / UV Transparency | Optical Grade SCD (Single Crystal Diamond) | Achieve Ra < 1nm polish. SCD offers superior transmission across the UV spectrum (including wavelengths well below 355 nm), stability, and zero internal scatter. |
| Large-Scale Prototypes / Durability | Optical Grade PCD (Polycrystalline Diamond) | High thermal conductivity and exceptional radiation hardness. Suitable for larger optical components; 6CCVD supplies wafers up to 125 mm diameter. |
| Custom Active Components | BDD (Boron-Doped Diamond) | If components required electro-chemical sensing or unique detection geometries, BDD can be used for integrated detectors or custom electrodes. |
Customization Potential
Section titled âCustomization PotentialâThe research highlights the reliance on precise manufacturing and testing. 6CCVD offers end-to-end customization capabilities essential for advanced optics replication and mission extension:
- Precision Polishing: 6CCVD guarantees Ra < 1 nm for SCD material and Ra < 5 nm for inch-size PCD material. This capability drastically surpasses the 20 nm RMS requirement detailed in the paper, minimizing scattering losses and potentially increasing the PCE significantly beyond the observed 45%.
- Custom Dimensions and Shapes: While the target lens diameter is 250 mm (exceeding current large-area SCD/PCD limits), 6CCVD can supply SCD substrates up to 10 mm thick and PCD wafers up to 125 mm in diameter, ideal for smaller, high-performance reference optics, filters, or pathfinder detectors.
- Advanced Metalization Services: For integrated mounting, contact layers, or custom anti-reflective coatings (especially critical for UV wavelengths), 6CCVD offers in-house deposition services for Au, Pt, Pd, Ti, W, and Cu structures.
- Global Supply Chain: Materials are shipped globally, DDU as default, with DDP options available, ensuring reliable delivery for international collaborations like JEM-EUSO.
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD technical engineering team offers expert consultation to transition complex optical designsâespecially those requiring exceptional roughness control and UV performanceâfrom polymers (like PMMA) to robust MPCVD diamond. We can assist with material selection, optimizing polishing protocols, and custom dimensional requirements for similar UV Astroparticle Detection or Space-Based Remote Sensing projects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Mini-EUSO (âUV atmosphereâ in Russian Space Program) is a science mission to observe UV phenomena in the night Earth atmosphere from a UV transparent porthole of the International Space Station and will serve as a pathfinder for future ultra-high energy cosmic ray missions such as KLYPVE-EUSO (K-EUSO) and JEM-EUSO. The Mini-EUSO optical design has two 250 mm double-sided Fresnel lenses and a flat focal surface. The overall length of the optical system is 300 mm. Its field of view is about $\pm22^{\circ}$. The RMS spot sizes between $0^{\circ}$ and $22^{\circ}$ field angle are 2.0 mm $\sim$ 3.5 mm in diameter, and conform to the pixel size of the M64 multi-anode photomultiplier (HAMAMATSU Photonics). The material of lenses is UV transparent PMMA (Poly Methyl Methacrylate, PMMA-000, MITSUBISHI RAYON) and the lens thickness is 11 mm. The lenses were manufactured using the diamond turning machine by the Materials Fabrication Laboratory of RIKEN. We confirmed that the surface roughness of lenses is less than 20 nm (RMS). In this paper, we will describe the details of the optical system and test results.