The Lucy Thermal Emission Spectrometer (L’TES) Instrument
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-12-19 |
| Journal | Space Science Reviews |
| Authors | P. R. Christensen, V. E. Hamilton, G. Mehall, Saadat Anwar, H. Bowles |
| Institutions | SpaceTech (United States), Arizona State University |
| Citations | 22 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”The Lucy Thermal Emission Spectrometer (L’TES) is a high-precision Fourier Transform Spectrometer (FTS) designed for the NASA Lucy mission to characterize the thermophysical properties of Trojan asteroids.
- Mission Objective: Determine asteroid regolith properties (primarily grain size and thermal inertia) using thermal infrared spectral observations (5.71-100 µm).
- Instrument Heritage: L’TES is a build-to-print mechanical copy of the OSIRIS-REx OTES instrument, leveraging flight-proven uncooled DLATGS pyroelectric detector technology.
- Key Optical Components: Features a 15.2-cm Cassegrain telescope and a 38-mm Chemical Vapor Deposited (CVD) diamond beamsplitter, enhanced with an Antireflection Microstructure (ARM) coating for increased throughput.
- Performance Metrics: Achieves an absolute temperature accuracy of <2 K for scene temperatures >75 K, meeting the requirement necessary to determine thermal inertia to ±15%.
- Operational Flexibility: Incorporates redesigned electronics with a digital servo control loop (FPGA/FPU) allowing selectable spectral sampling (8.64, 17.3, and 34.6 cm-1) and faster scan times (0.5, 1, and 2 seconds).
- Noise Mitigation: Post-delivery testing required increasing the IR sampling frequency from 656 Hz to 772 Hz to shift spacecraft Inertial Measurement Unit (IMU)-induced microphonic interference out of the instrument’s information band.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Spectral Range | 1750 to 100 | cm-1 | Corresponds to 5.71-100 µm |
| Spectral Sampling (Selectable) | 8.64, 17.3, 34.6 | cm-1 | Achieved via variable Michelson mirror travel |
| Telescope Aperture | 15.2 | cm | Cassegrain Ritchey-Chretien design (f/3.91) |
| Field of View (FWHM) | 7.3 | mrad | Azimuth and Elevation |
| Detector Type | DLATGS | Pyroelectric | Uncooled, deuterated L-alanine doped triglycine sulfate |
| Detector D* (22 °C) | 1.2 x 109 | cm Hz1/2 W-1 | Measured at 10 Hz |
| NESR (2-sec scan) | 1.6 x 10-8 | W cm-2 sr-1/cm-1 | Precision range: 300 to 1350 cm-1 |
| Absolute Temp. Error | <2 | K | For scene temperatures >75 K |
| Michelson Mirror Travel | ±0.281, ±0.141, ±0.070 | mm | Corresponds to selectable spectral sampling |
| Mirror Velocity | 0.319 | mm/s | Nominal operational speed |
| IR Sampling Frequency | 772 | Hz | Used for precise moving mirror control |
| Beamsplitter Material | CVD Diamond | 38 mm dia, 1 mm thick | Features Antireflection Microstructure (ARM) |
| Cal Blackbody Emissivity | 0.98 ± 0.005 | Unitless | Internal calibration target |
| Mass | 6.47 | kg | Overall instrument mass |
| Power Consumption | 12.6 (avg), 18.0 (peak) | W | Operational power |
| Operational Temp. Range | -10 to +30 | °C | In-specification operation |
| Thermal Stability Requirement | <0.1 | °C per minute | Required for calibration maintenance |
Key Methodologies
Section titled “Key Methodologies”The L’TES development and calibration involved stringent procedures focused on thermal stability, optical alignment, and noise mitigation:
- Interferometer Alignment Stabilization: Initial thermal cycling caused alignment degradation (a 13 arcsec tilt in the fixed mirror). This was corrected by optimizing the torquing and re-torquing procedure for the fixed mirror mounting nuts, followed by injection bonding to prevent movement during launch.
- Metrology Laser Redesign: The metrology laser lens was redesigned to reduce its signal sensitivity to interferometer tilt, preventing saturation of the analog-to-digital converter (ADC) signal across the operational temperature range.
- Digital Servo Control Implementation: The moving mirror control loop was implemented using a custom processor core (FPU) within the FPGA, enabling high-frequency rate estimation (2 kHz) based on the metrology fringe sinusoid model, ensuring precise IR signal sampling.
- Thermal Vacuum Radiometric Calibration: Calibration was performed in TVAC using two NIST-calibrated Bench Checkout Unit (BCU) blackbody targets (accuracy ±0.1 °C). One BCU simulated space (85 K), and the other simulated scene temperatures (100 K to 330 K) across the expected range.
- Absolute Accuracy Verification: Absolute temperature performance was verified by computing the brightness temperature from the calibrated radiance spectra, filtering noise, and confirming the derived kinetic temperature matched the BCU target temperature within the <2 K requirement.
- IMU Noise Mitigation: Following delivery, system-level testing identified microphonic interference from the spacecraft IMUs. The solution was to increase the IR sampling frequency from 656 Hz to 772 Hz, shifting the resulting beat frequencies outside the L’TES information band (10-120 Hz).
Commercial Applications
Section titled “Commercial Applications”The technologies developed for L’TES, particularly in high-precision thermal sensing, advanced optics, and robust digital control systems, have broad commercial relevance:
- Infrared Optics and Spectroscopy:
- CVD Diamond Components: Use of large-format, high-purity CVD diamond as beamsplitters and lenses, leveraging its low dispersion and high thermal conductivity for broadband IR applications.
- Antireflection Microstructures (ARM): Application of ARM coatings on diamond optics to maximize throughput (up to 50% increase in the L’TES system) for high-efficiency IR windows and domes.
- Precision Metrology and Control:
- Digital Servo Systems: Implementation of FPGA/FPU-based digital servo control loops for ultra-precise, high-speed linear motion (voice-coil motors), applicable in advanced manufacturing, lithography, and high-resolution scanning systems.
- Fringe Counting Interferometry: High-accuracy position tracking using monochromatic laser diodes (0.851 µm) for industrial metrology and quality control.
- High-Reliability Sensing:
- Uncooled Bolometric Detectors: Use of uncooled DLATGS pyroelectric detectors for long-wave infrared (LWIR) sensing, offering lower mass and complexity compared to cooled photon detectors, suitable for industrial monitoring and security systems.
- Thermal Management:
- Ultra-Stable Thermal Design: Techniques for achieving extreme thermal stability (<0.1 °C per minute drift) in sensitive instruments through conductive isolation and radiative coupling, critical for high-accuracy calibration in laboratory standards and industrial process control.
View Original Abstract
Abstract The Lucy Thermal Emission Spectrometer (L’TES) will provide remote measurements of the thermophysical properties of the Trojan asteroids studied by the Lucy mission. L’TES is build-to-print hardware copy of the OTES instrument flown on OSIRIS-REx. It is a Fourier Transform spectrometer covering the spectral range 5.71-100 μm (1750-100 cm −1 ) with spectral sampling intervals of 8.64, 17.3, and 34.6 cm −1 and a 7.3-mrad field of view. The L’TES 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 to a single uncooled deuterated l -alanine doped triglycine sulfate (DLATGS) pyroelectric detector. A significant firmware change from OTES is the ability to acquire interferograms of different length and spectral resolution with acquisition times of 0.5, 1, and 2 seconds. A single ∼0.851 μm laser diode is 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, together with observations of space, provides radiometric calibration. The radiometric precision in a single spectrum is ≤2.2 × 10 −8 W cm −2 sr −1 /cm −1 between 300 and 1350 cm −1 . The absolute temperature error is <2 K for scene temperatures >75 K. The overall L’TES envelope size is 37.6 × 29.0 × 30.4 cm, and the mass is 6.47 kg. The power consumption is 12.6 W average. L’TES was developed by Arizona State University with AZ Space Technologies developing the electronics. L’TES was integrated, tested, and radiometrically calibrated on the Arizona State University campus in Tempe, AZ. Initial data from space have verified the instrument’s radiometric and spatial performance.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 1988 - Theory and practice of radiation thermometry