Miniature Thermal Emission Spectrometer (Mini-TES)
The Miniature Thermal Emission Spectrometer (Mini-TES) will provide remote measurements of mineralogy and thermophysical properties of the scene surrounding the Mars Exploration Rovers, and guide the Rovers to key targets for detailed in situ measurements by other Rover experiments. The Mini-TES is a Fourier Transform Spectrometer covering the spectral range 5-29 micrometers (339.50 to 1997.06 cm-1) with a spectral sample interval of 9.99 cm-1. The Mini-TES telescope is a 6.35-cm diameter Cassegrain telescope that feeds a flat-plate Michelson moving mirror mounted on a voice-coil motor assembly. A single deuterated triglycine sulfate (DTGS) uncooled pyroelectric detector with proven space heritage gives a spatial resolution of 20 mrad; an actuated field stop can reduce the field of view to 8 mrad. Mini-TES is mounted within the Rover's Warm Electronics Box and views the terrain using its internal telescope looking up the hollow shaft of the Pancam Mast Assembly (PMA) to the fixed fold mirror and rotating elevation scan mirror in the PMA head located ~1.5 m above the ground. The PMA provides a full 360 degree of azimuth travel and views from 30 degrees above the nominal horizon to 50 degrees below. An interferogram is collected every two seconds, and transmitted to the Rover computer where the Fast Fourier Transform, spectral summing, lossless compression, and data formatting are performed prior to transmission to Earth. Radiometric calibration is provided by two calibration V-groove blackbody targets instrumented with platinum thermistor temperature sensors with absolute temperature calibration of +/-0.1 K. One calibration target is located inside the PMA head, the second is on the Rover deck. The Mini-TES temperature is expected to vary diurnally from 263 to 303 K, with most surface composition data collected at scene temperatures >270 K. For these conditions the radiometric precision for two-spectra summing is +/-1.8 x10-8 W cm-2 sr-1 /cm-1 between 450 and 1500 cm-1, increasing to ~4.2 x10-8 W cm-2 sr-1 /cm-1 at shorter (300 cm-1) and longer (1800 cm-1) wavenumbers. The absolute radiance error will be less than 5 x10-8 Watt cm-2 sr-1 /cm-1, decreasing to ~1 x10-8 Watt cm-2 sr-1 /cm-1 over the wavenumber range where the scene temperature will be determined (1200-1600 cm-1). The worst-case sum of these random and systematic radiance errors correspond to an absolute temperature error of ~0.4 K for a true surface temperature of 270 K, and ~1.5 K for a surface at 180 K. The Mini-TES will be operated in a 20-mrad panorama mode and an 8-mrad targeted mode, producing 2-dimensional rasters and 3-dimensional hyperspectral image cubes of varying sizes. The overall Mini-TES envelope size is 23.5 cm x 16.3 cm x 15.5 cm and the mass is 2.40 kg. The power consumption is 5.6 W average. The Mini-TES was developed by Arizona State University and Raytheon Santa Barbara Remote Sensing (SBRS). Information in this instrument description is taken from The Miniature Thermal Emission Spectrometer for the Mars Exploration Rovers paper [CHRISTENSENETAL2003]. See this paper for more details.
The chief scientific objectives of the Mini-TES are:
- determine the mineralogy of rocks and soils, and
- determine the thermophysical properties of selected soil patches, and
- determine the temperature profile, dust opacity, water-ice opacity, and water vapor abundance in the lower boundary layer of atmosphere
The initial Mini-TES calibration and test was performed at SBRS prior to delivery to JPL, and a subset of these tests was performed on the integrated Mini-TES/PMA assembly. The objectives of these tests were to determine:
- the field-of-view definition and alignment;
- the out-of-field response;
- the spectrometer spectral line shape and spectral sample position; and
- the spectrometer radiometric calibration.
Bench-level testing of the Mini-TES instrument was performed at SBRS in two phases. The first phase consisted of piece-part and system-level testing of the spectral performance of each sub-section under ambient conditions. The second phase consisted of field of view and out-of-field tests conducted before and after vibration and thermalvacuum testing to determine and confirm the instrument field-of-view and alignment. Mini-TES I was operated for a total of 166 hours and Mini-TES II was operated for 594 hours at SBRS prior to initial delivery to JPL. The Mini-TES spectrometer, without the PMA, was tested and calibrated in vacuum at SBRS at instrument temperatures of -243, 263, 283, and 303 K. A matrix of calibration tests were performed viewing two precision calibration reference blackbody standards, one set at 223 K, 243 K, 263 K, and 283 K. While the second was varied at temperatures of 145 K, 190 K, 235 K, 280 K, and 325 K. The Mini-TES/PMA systems were radiometrically calibrated in 6 mbar of nitrogen at instrument temperatures of 243, 273, and 303 K over a range of calibration blackbody temperatures. These tests determined:
- the emissivity and effective temperature of the internal reference surface;
- the instrument response function and its variation with instrument temperature;
- the absolute radiometric accuracy;
- the spectrometer noise characteristics; and
- the spectrometer gain values.
The Mini-TES has many performance requirements, that if not met could significantly compromise the quality of the data obtained.
Mineralogic mapping has three measurement requirements:
- radiometric accuracy and precision necessary to uniquely determine the mineral abundances in mixtures to within 5% absolute abundance;
- spectral resolution sufficient to uniquely determine the mineral abundances in mixtures to within 5% absolute abundance; and
- spatial resolution of less than 25 cm at 10 m distance (25 mrad) necessary to resolve and identify individual rocks 0.5 m in size or larger in the rover near field.
The determination of atmospheric temperature profiles, aerosols, water vapor, condensates has two measurement requirements:
- radiometric accuracy and precision necessary to determine the opacities of atmospheric dust and ice to +/-0.05 and temperature to +/-2 K; and
- spectral resolution sufficient to uniquely identify dust, water-ice, water-vapor, and sound the atmosphere, and monitor their physical and compositional properties.
The Mini-TES uses uncooled detectors to reduce the complexity of the fabrication, testing, operation, and rover interface of the instrument, while meeting the scientific requirements for the investigation. The Mini-TES has a single deuterated triglycine sulfate (DTGS) uncooled pyroelectric detector with proven space heritage that gives a spatial resolution of 20 mrad; an actuated field stop reduces the field of view to 8 mrad.
Mini-TES uses two Datel DC to DC power converters that accept +11 to +36 volts unregulated input voltage and supply +/-5 and +/-15 volts regulated output voltage. The Datel converters went through significant screening by Raytheon and NASA to validate them for use on the MER Mini-TES instruments. The power converters are mounted on the same circuit card as the two SDL 80 mWatt 978 nm laser diode assemblies. These laser diodes have also been through significant screening for the Mini-TES instruments. The laser diodes are coupled into the optics via 1m fiber optic cables. The power connections to the spacecraft power bus are through the 21-pin Cannon micro-D flight connector located at the base of the Mini-TES interferometer baseplate.
Mini-TES uses an uncooled DTGS pyroelectric detector with an integrated FET detector package. The bias voltage applied to the FET by the pre-amplifier ensures that the DTGS detector's crystals are properly poled when power is applied to the instrument. Pre-amplification and front-end filtering is performed on the preamplifier circuit board amplify the signal and to AC couple the detector output to block high frequency oscillations. A +/-12 volt regulator supplies power the detector and preamplifier electronics.
The spectrometer circuit board performs the bulk of the analog electronics processing. The analog detector signal is passed through dual post-amplifier chains, performing the high-frequency boost, 3-pole Bessel filtering, amplifier gain, and analog signal track/hold. The interferogram signal due to the scene is 'boosted' to account for the '1/f' roll-off of the detector response and is amplified to fill the 16-bit analog to digital converter. The filtering is performed to achieve the desired IR signal bandpass of 5 to 220 Hz. In addition, the analog signals from the two Hammamatsu silicone photodiode fringe signal detectors are passed through the fringe post-amplifier and fringe detection circuitry on the spectrometer board. The fringe detection electronics use a zero crossing comparator to generate the sampling pulse and the constant velocity servo feedback fringe clock. The amplified and filtered IR signal, fringe analog signal amplitude and the internal instrument analog telemetry is then fed into a 16:1 analog multiplexer followed by a 16-bit analog to digital converter. The 16-bit digital IR data are then transferred to the data buffer on the command and control circuit board for formatting and transfer to the Mini-TES interface electronics.
The low level command, control and data flow tasks of the Mini-TES are controlled by logic in the command and control Field Programmable Gate Array (FPGA). The interface electronics parse out the low level instrument command parameters that control various Mini-TES hardware functions. The Mini-TES command parameters are: interferometer motor on/off, amplifier gain high/low, amplifier chain primary/redundant, target (shutter) open/close, laser diode1 on/off, laser heater2 on/off, start-of-scan optical switch primary/redundant, and laser heaters on/off.
The flow of the digital interferometer data is controlled by additional logic in the command and control board FPGA. After each interferometer scan, the 16-bit interferogram data and 16-bit telemetry data are moved from the A/D to the input memory buffer on the 16-bit parallel data bus. These 16-bit parallel data are then sent to the digital multiplexer and serializer electronics where the three header words and fourteen digital telemetry words are serialized with the 16-bit IR data. The multiplexer, serializer and data formatting logic are included in the command and control FPGA. The three data header words include: 8-bit sync, 8-bit commanded parameter status, 16-bit scan count, and 16-bit interferogram sample count. The fourteen 16-bit telemetry words include: +5V power, -5V power, +15V power, -15V power, +10V power, -10V power, +12V power, -12V power, detector temperature, motor temperature, beamsplitter/optics temperature, laser diode1 temperature, laser diode2 temperature, and fringe signal amplitude.
The Mini-TES timing sequencing electronics are implemented in the command and control board FPGA. These electronics generate the timing waveforms necessary to control and synchronize instrument operation. The timing electronics provide the control and synchronization of the amplification, track/hold, multiplexing, and analog to digital conversion of the analog signals. They also control and synchronize the interferometer servo electronics with the data acquisitions. The timing sequencing electronics include the fringe delay electronics which are used to correct the sampling error due to the phase delays between the fringe and IR analog channels. All clocks in the timing sequencer are generated from the master clock crystal oscillator which operates at a frequency of 14.5152 MHz.
The Mini-TES interferometer servo electronics are located on the command and control board and include the digital motor control logic and the analog servo drive electronics. The interferometer digital drive electronics, located in the FPGA, receive scan timing clocks from the timing sequencer electronics and the fringe clock from the fringe detection electronics. The motor control logic uses these clocks to synchronize the mirror movement with the spectrometer data acquisitions. The interferometer analog servo drive electronics generate the analog signals that control the movement of the TES interferometer moving mirror actuator. The moving mirror uses a direct drive Schaeffer linear motor with tachometer feedback. The moving mirror tachometer signal is returned to the interferometer control electronics to allow active feedback control of the actuator. The start of scan is monitored using primary and redundant single and double scan optical-interrupters that are connected to the moving mirror assembly.
The Mini-TES optical system uses a compact Cassegrain telescope configuration with a 6.35 mm diameter primary mirror that defines the system's aperture stop. Light reflects off the secondary mirror, forming the f/12 focal ratio. The 1.12 cm diameter secondary obscures the clear aperture reducing the effective collection area. The use of baffles around the telescope housing and secondary mirror and the use of diffuse black paint around the optics and within the cavity minimizes stray light affects. An anti-reflection coated Cadmium Telluride (CdTe) window is located between the exit of the telescope's optical path and the entrance of the interferometer optical system. This window is tilted so that an internal etalon is not created between this surface and the beamsplitter. A flat mirror folds the radiance into the plane of the interferometer. All mirror surfaces are diamond-turned and gold-coated.
Mini-TES utilizes the identical Michelson interferometer design as the TES instruments. The radiance from the main fold mirror passes through a 0.635 cm thick Potassium Bromide (KBr) beamsplitter and its amplitude is split in two and reflected/transmitted to each arm of the interferometer. This beamsplitter is installed in a radial 3-point mount that allows the beamsplitter to maintain alignment over a 373 K operational range (223 K to 323 K). Due to the hydroscopic nature of KBr, a dry nitrogen purge during ground testing is required to maintain its transmission properties. In order to maintain positive purge without over-pressurization, the Mini-TES housing has a CdTe window, described above, an exhaust port, and check valve.
A fixed mirror is in the reflected path of the interferometer, while a constant velocity moving mirror is in the transmission path. The moving mirror moves +/-0.25 mm to achieve the spectral sampling requirement of 10 cm-1. The wavefronts recombine at the beamsplitter and pass through a compensator of identical thickness to the beamsplitter to preserve the optical path difference. This recombined radiance is directed by a fold mirror through the 20-mrad field stop towards the parabolic focus mirror. This mirror reimages the optical pupil onto the on-axis DTGS detector element, which is protected by thin (0.05 cm) chemical vapor deposited diamond window.
Within the Rover's Warm Electronics Box, at the base of the Pancam Mast Assembly
- Full 360 20-mrad panoramic mode
- 8-mrad field of view mode
- Single spectrum per pixel, 20-mrad mode
- Partial panorama mode
The Mini-TES takes thermal infrared spectra of the target by viewing wavelengths from 5 to 40 micrometers. The Mini-TES calibrated radiance is the primary data product for the MER mission. These data will be converted to effective emissivity and surface temperature by fitting a Planck blackbody function to the calibrated spectrum. The emissivity spectra will be converted to mineral abundance using a linear deconvolution model and a matrix of mineral spectra from the ASU Mineral Library and other sources. The derived surface temperature will be used to produce thermal inertia images via a thermal model, using data from multiple times of day where possible. Attempts will be made to coordinate these diurnal observations with the times of TES or THEMIS direct overflights, providing simultaneous temperature observations that can be extended to broader regions surrounding the rovers.