| DESCRIPTION |
Instrument Overview
===================
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 <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.
Scientific Objectives
=====================
The chief scientific objectives of the Mini-TES are:
1) determine the mineralogy of rocks and soils, and
2) determine the thermophysical properties of selected soil patches,
and
3) determine the temperature profile, dust opacity, water-ice
opacity, and water vapor abundance in the lower boundary layer of
atmosphere
Calibration
===========
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:
(1) the field-of-view definition and alignment;
(2) the out-of-field response;
(3) the spectrometer spectral line shape and spectral sample
position; and
(4) 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:
(1) the emissivity and effective temperature of the internal
reference surface;
(2) the instrument response function and its variation with
instrument temperature;
(3) the absolute radiometric accuracy;
(4) the spectrometer noise characteristics; and
(5) the spectrometer gain values.
Operational Considerations
==========================
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:
(1) radiometric accuracy and precision necessary to uniquely
determine the mineral abundances in mixtures to within 5%
absolute abundance;
(2) spectral resolution sufficient to uniquely determine the mineral
abundances in mixtures to within 5% absolute abundance; and
(3) spatial resolution of <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:
(1) radiometric accuracy and precision necessary to determine the
opacities of atmospheric dust and ice to +/-0.05 and temperature
to +/-2 K; and
(2) spectral resolution sufficient to uniquely identify dust,
water-ice, water-vapor, and sound the atmosphere, and monitor
their physical and compositional properties.
Detectors
=========
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.
Electronics
===========
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.
Optics
======
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.
Location
========
Within the Rover's Warm Elecronics Box, at the base of the Pancam
Mast Assembly
Operational Modes
=================
1. Full 360 20-mrad panoramic mode
2. 8-mrad field of view mode
3. Single spectrum per pixel, 20-mrad mode
4. Partial panorama mode
Measured Parameters
===================
The Mini-TES takes thermal infrared spectra of the target by viewing
wavelenghts 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.
|
.png)
.png)