The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2017-04-07 |
| Journal | arXiv (Cornell University) |
| Authors | P. R. Christensen, V. E. Hamilton, G. Mehall, Daniel Pelham, William OâDonnell |
| Citations | 1 |
| Analysis | Full AI Review Included |
Technical Analysis: CVD Diamond in OSIRIS-REx OTES Spectrometer
Section titled âTechnical Analysis: CVD Diamond in OSIRIS-REx OTES Spectrometerâ6CCVD Ref: OTC-2024-0SIRS-REx-OTES
Executive Summary
Section titled âExecutive SummaryâThe OSIRIS-REx Thermal Emission Spectrometer (OTES) leverages advanced Chemical Vapor Deposited (CVD) diamond components to achieve mission-critical infrared (IR) measurements in the extreme thermal and radiation environment of space.
- Diamond Enables Advanced Interferometry: The Michelson interferometer at the core of OTES utilizes a 38-mm diameter, 1-mm thick CVD diamond beamsplitter, replacing traditional, less robust materials (CsI or KBr) used in previous missions.
- Thermal and Mechanical Superiority: Diamondâs low thermal expansion coefficient and high heat conductivity ensure precise optical alignment and mechanical integrity across the wide non-operational survival range of -25 °C to +55 °C, crucial for long-duration space missions.
- High Throughput Achieved: Optical efficiency is maintained by minimizing surface reflections via an Antireflection Microstructure (ARM) etched directly onto the diamond beamsplitter surface, providing >78% transmission over the critical 7 to >25 ”m spectral range.
- Low Dispersion Simplifies Design: The low dispersion properties of diamond allow the use of a single substrate beamsplitter without the need for a separate compensator plate, significantly simplifying the optical train and reducing weight/complexity.
- Exceptional Radiometric Performance: OTES achieved a Noise Equivalent Spectral Radiance (NESR) of â€2.3 x 10-8 W cm-2 sr-1/cm-1 between 300 and 1350 cm-1, demonstrating the materialâs purity and effectiveness for high-precision mineralogy and thermal physics analysis.
- Proven Radiation and Robustness: The instrument design, relying on the robust properties of CVD diamond, is built to survive estimated radiation doses exceeding 20 krad, establishing diamond as a qualified material for deep space IR instrumentation.
Technical Specifications
Section titled âTechnical SpecificationsâThe table below summarizes the key material and performance metrics enabled by the CVD diamond components within the OTES instrument (data extracted primarily from Table 3 and associated text).
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material Type | CVD Diamond | Substrate | Beamsplitter and Detector Lens |
| Beamsplitter Diameter | 38 | mm | Mission-critical optical element |
| Beamsplitter Thickness | 1 | mm | Optimal for required spectral range |
| Spectral Range | 1750 to 100 | cm-1 | (5.71-100 ”m) |
| Spectral Resolution | 8.66 | cm-1 | Sampling interval |
| ARM Reflectance Goal | <22 | % | Surface reflection minimization |
| Operational Temp Range | 10 to 40 | °C | Performance in specification |
| Non-Operational Survival | -25 to +55 | °C | Required for proto-flight integrity |
| NESR Requirement (300-1350 cm-1) | â€2.3 x 10-8 | W cm-2 sr-1/cm-1 | Radiometric precision requirement |
| NESR Measured (772 Hz, 1350 cm-1) | 2.17 x 10-8 | W cm-2 sr-1/cm-1 | Measured noise equivalent spectral radiance |
| Yarkovsky Accuracy | 1.5 | % | Absolute integrated radiance error (6-50 ”m) |
| Thermal Inertia Accuracy | ±10 | J m-2 K-1 s-1/2 | Requires ±2 K surface temp knowledge (220-350 K) |
| Metrology Laser Wavelength | 0.849 | ”m | Used for precise mirror control (metrology interferometer) |
Key Methodologies
Section titled âKey MethodologiesâThe successful deployment of high-performance CVD diamond components relied on advanced fabrication, integration, and thermal-vacuum testing protocols.
- CVD Growth and Fabrication:
- Diamond Materials fabricated the large, optical-quality CVD diamond beamsplitter (38 mm diameter, 1 mm thick) and the small (4.4 mm) CVD diamond detector lens.
- This beamsplitter represented the largest precision optical-quality diamond yet produced by the manufacturer at the time of development.
- Antireflection Microstructure (ARM) Processing:
- TelAztec developed and applied a specialized antireflection microstructure (ARM) etch to the beamsplitter surface.
- The ARM process required several iterations to perfect the necessary size and uniformity requirements to achieve the optical throughput (>78% transmission).
- High-Precision Mechanical Mounting:
- The beamsplitter was installed in a radial three-point mount designed to maintain precise optical alignment across the operational temperature range (10 °C to 40 °C).
- The mount specifically accommodated diamondâs low thermal expansion coefficient and high heat conductivity while ensuring mechanical integrity during launch/pyro shock.
- Digital Servo Control and Metrology:
- A redundant 0.849 ”m VCSEL laser metrology interferometer, sharing the diamond beamsplitter, was used to precisely track the moving mirror position and trigger IR signal sampling at a controlled velocity (0.321 mm/s) and frequency (772 Hz).
- Thermal Vacuum Radiometric Calibration (TVAC):
- Radiometric calibration was performed in a vacuum across a range of instrument temperatures (-10 °C to 40 °C) and target temperatures (85 K to 380 K).
- The extensive TVAC testing confirmed that the diamond components, coupled with digital servocontrol, met the absolute temperature accuracy requirements (e.g., ±2 K for 220-350 K surfaces).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe OTES instrument demonstrates the critical enabling role of high-quality CVD diamond in next-generation thermal emission spectroscopy, particularly in demanding aerospace and high-reliability industrial applications. 6CCVD is positioned to supply and engineer the precise materials required to replicate or advance this technology.
Applicable Materials
Section titled âApplicable MaterialsâTo achieve the stringent optical and thermal stability requirements seen in OTES, researchers and engineers require materials offering exceptional homogeneity, low strain, and superior thermal characteristics.
| OTES Requirement/Application | 6CCVD Recommended Material | Material Rationale & Capability |
|---|---|---|
| Beamsplitters & Windows (38mm diameter, 1mm thick) | Optical Grade SCD or High-Purity PCD Wafer | CVD diamond plates up to 125mm (PCD) or large-area Single Crystal Diamond (SCD). Provides maximum thermal stability and purity essential for low noise. |
| IR Detector Lenses (4.4mm diameter) | Optical Grade SCD | SCD offers superior homogeneity and crystal quality, ensuring minimal absorption loss across the critical IR spectral range (5.71-100 ”m). |
| High Thermal Sink/Spreader (Passive Cooling) | High Thermal Conductivity PCD Substrate | 6CCVD provides substrates with thermal conductivity exceeding 2000 W/mK, ideal for rapidly dissipating heat from detector assemblies (D* maintenance) or electronics in vacuum. |
| Metrology Interferometer (0.849 ”m) | Optical Grade SCD or PCD | Diamond transparency extends into the visible/near-IR, allowing the same beamsplitter to be used for the IR scene and the metrology laser (as demonstrated in OTES). |
Customization Potential: Meeting Space-Grade Specifications
Section titled âCustomization Potential: Meeting Space-Grade SpecificationsâThe OTES project highlighted the need for customized material dimensions and advanced surface processing, capabilities central to 6CCVDâs offering:
- Custom Dimensions and Thicknesses: The OTES beamsplitter (38mm diameter, 1mm thick) is well within 6CCVDâs manufacturing sweet spot. We offer wafers/plates up to 125mm diameter in PCD, and thicknesses ranging from 0.1 ”m to 500 ”m for optical components, and up to 10mm for substrates.
- Precision Polishing for ARM Preparation: High-performance optics like beamsplitters require ultra-low surface roughness before specialized AR structures (like ARM) can be applied. 6CCVD guarantees surface roughness (Ra) of <1nm for SCD and <5nm for inch-size PCD wafers, providing the ideal foundation for advanced micro-structuring techniques.
- Metalization Services: While OTES used a Ge beam-dividing coating, many future optical systems require tailored coatings for specific spectral splitting. 6CCVD offers in-house metalization services including Au, Pt, Pd, Ti, W, and Cu, allowing for custom thin-film coatings compatible with extreme thermal cycling.
- Rapid Global Supply Chain: 6CCVD ensures high-reliability delivery of flight-ready materials worldwide, managing logistics via DDU default or DDP available shipping terms.
Engineering Support
Section titled âEngineering SupportâThe challenges encountered by the OTES teamâparticularly the need to increase servo velocity to mitigate IMU-induced vibrationsâdemonstrate that material performance is inextricably linked to system-level integration.
6CCVDâs in-house PhD-level engineering team specializes in material selection and optimization for complex scientific instruments. We can assist with material and dimension specifications for projects requiring:
- High-Stability Spectrometers: Ensuring minimal thermal drift and alignment sensitivity across wide temperature ranges for thermal emission spectroscopy projects.
- Radiation-Hardened Optics: Selecting diamond grades proven to withstand high-dose environments typical of planetary missions or fusion research.
- Microphonic/Vibration Mitigation: Consulting on mounting designs and material properties that minimize sensitivity to system-level noise sources.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
The OSIRIS-REx Thermal Emission Spectrometer (OTES) will provide remote measurements of mineralogy and thermophysical properties of Bennu to map its surface, help select the OSIRIS-REx sampling site, and investigate the Yarkovsky effect. OTES is a Fourier transform spectrometer covering the spectral range 5.71 - 100 ÎŒm (1750 - 100 cm-1) with a spectral sample interval of 8.66 cm-1 and a 6.5-mrad field of view. The OTES telescope is a 15.2-cm diameter Cassegrain telescope that feeds a flat-plate Michelson moving mirror mounted on a linear voice-coil motor assembly. A single uncooled deuterated L-alanine doped triglycine sulfate (DLATGS) pyroelectric detector is used to sample the interferogram every two seconds. Redundant ~0.855 ÎŒm laser diodes are used in a metrology interferometer to provide precise moving mirror control and IR sampling at 772 Hz. The beamsplitter is a 38-mm diameter, 1-mm thick chemical vapor deposited diamond with an antireflection microstructure to minimize surface reflection. An internal calibration cone blackbody target provides radiometric calibration. The radiometric precision in a single spectrum is <= 2.2 x 10-8 W cm-2 sr-1/cm-1 between 300 and 1350 cm-1. The absolute integrated radiance error is <1% for scene temperatures ranging from 150 to 380 K. The overall OTES envelope size is 37.5 x 28.9 x 52.2 cm, and the mass is 6.27 kg. The power consumption is 10.8 W average. The OTES was developed by Arizona State University with Moog Broad Reach developing the electronics. OTES was integrated, tested, and radiometrically calibrated on the Arizona State University campus in Tempe, AZ.