Chemistry Camera (ChemCam)
The ChemCam instrument suite includes both the LIBS (Laser Induced Breakdown Spectrometer) and the RMI (Remote Micro-Imager) components. The instrument suite consists of two major sections, the Mast Unit, housing the laser, RMI, telescope, and associated electronics, and the Body Unit, consisting of an optical demultiplexer, three spectrometers, the thermo-electric cooler, CCD electronics, spectrometer electronics, the data processing unit (DPU), and the low-voltage power supply. The ChemCam mast unit is contained in the remote warm electronics box (RWEB) at the top of the rover's mast. The body unit is located in the rover body, attached to the rover accommodation mounting plate (RAMP). Besides the mast and body units, there are two other sections of the instrument. A fiber optic cable transmits the plasma light between the two ChemCam units. Finally, included on the rover deck is a calibration target assembly. The following text describes both the RMI and LIBS components, as they work together.
The ChemCam LIBS instrument's objective is to obtain major element compositions for rocks and soils within seven meters of the instrument to an accuracy of +/-10%. The instrument must be capable of removing dust and weathering rinds, and must be able to profile to a depth of 1 mm into rocks.
The ChemCam RMI instrument provides context imaging with sub-millimeter resolution comparable to a close-up camera, but at metric distances.
The reader is referred to the ChemCam calibration paper [WIENSETAL2013].
During the observation of the Sun by other instruments, the telescope is focused at 2 meters to avoid damages on the RMI detector or other optics. Two exclusion zones have also been defined: 1.20 m - 1.36 m, 1.942 m - 2.217 m to avoid laser reflections on some optical parts. These two exclusion zones do not apply to activities implying RMI only.
For detectors the ChemCam LIBS spectrometers employ E2V 42-10 back illuminated 2048 x 515 pixel CCDs that are operated in low-noise advanced inverted mode. Each pixel is 13.5 microns square, for an image area of 27.6 x 6.9 mm. The grade-zero units used in ChemCam were essentially off-the-shelf in terms of specifications with the exception of the anti-etalon coating mentioned above for the VNIR CCD. The full wells were measured at 255k, 190k, and 216k electrons for the UV, blue, and VNIR spectrometers, respectively.
The ChemCam imager is a heritage unit from the CIVA experiment onboard the ESA Rosetta mission. The imager is composed of a camera head (3D cubes based on the TH 7888A CCD image sensor manufactured by Atmel) with specific optical systems. It is a frame transfer CCD with a useful zone of 1024x1024 square pixels (14x14 microns square). Four extra lines and sixteen extra columns are provided for dark referencing. The 3D cube is equipped with FPGA based controller, which manages the CCD clocking and the analog to digital converter (10-bit ADC).
There are three identical CCD cards, one for each of the optical spectrometers. The spectrometers were required to be thermally isolated from the rest of the body unit, so a separate CCD card for each spectrometer accommodated the thermal isolation. The CCD card provides the first stage analog signal amplification, provides some of the required CCD voltages, and interfaces electrically to the CCDs. The CCD cards connect to the spectrometer electronics using a flex circuit, which also helps minimize the thermal path.
The CCDs underwent normal factory testing provided for commercial units plus one hundred thermal cycles between -55 C and +70 C, as well as a 72 hour burn-in at 125 deg C. These were done at the factory prior to delivery. The units underwent characterization at JPL for dark current, gain, full well, optimal operating voltages, and any gross anomalies. Because of differences in clock speeds between the initial characterization and flight operation the units had to be tested again with the flight electronics in a socket prior to soldering them to the boards.
The spectrometer electronics board operates the CCDs used by the three LIBS spectrometers. The functions include providing proper reference voltages to drive the CCDs, operating the CCDs, receiving the data from the CCDs, converting the signals to digital data, and passing the data to the DPU.
Reference voltages are provided separately to each CCD, allowing preflight adjustment to optimize the voltages for each CCD as required to optimize performance. Adjustments are provided for the vertical and horizontal clock speeds. These are made via commands sent to the spectrometer field-programmable gate array (FPGA) where the CCD clocking control is performed. Nominal operation for 1D LIBS spectra uses a 56 kHz vertical transfer rate and a 550 kHz horizontal transfer rate as these appear optimal in terms of overall noise performance. The total transfer time for this setting is a maximum of ~13 ms if all rows are summed.
When power is applied to the CCBU, all three CCDSs are turned on but are not clocked. This saves power while warming up the CCDs to equilibrium temperature if required by performance measurements. Even though nominal operation uses all three CCDs, power to each unit is controlled separately in case of a failure of one in flight with one serendipitous exception: UV and VNIR combination cannot be operated without the VIS spectrometer. The exact timing of each A-D conversion can be adjusted with a parameter sent to the spectrometer board FPGA, allowing optimization of performance. The clocking parameters (horizontal, vertical and A/D sample point) are the same for all three CCDs.
The required supply voltages are +5, and +15 V, and receiving a +5 V command from the MU FPGA switches on the camera.
Data Processing Unit (DPU)
The DPU carries out the functions of taking commands from the rover, performing these commands, storing the data, and sending the data to the rover. In addition, the DPU sends commands to the mast unit and receives signals and data back in return.
The DPU communicates with the mast unit through two communication links. Commands and telemetry data between the body unit and mast unit are performed using a programmable universal asynchronous receiver transmitter (UART) link. The UART link can run at 4 different speeds: 9.6 kbaud, 19.2 kbaud, 38.4 kbaud and 115.2 kbaud. Images taken by the mast unit camera are sent from the mast unit to the body unit using a low voltage differential signal (LVDS) high speed serial link (HSS) that runs at 8.25 Mbps.
The DPU communicates with the rover through a 2-way LVDS high speed serial link. Command data from the rover to the body unit run at 4.125 Mbps. Telemetry data from the body unit to the Rover runs at 8.25 Mbps. The DPU contains a UTMC 80C196 microcontroller, which is supported by two Actel FPGAs. One FPGA, called the Micro FPGA, contains the microcontroller logic control, the MU UART interface logic, state of health (SOH) interfacing and ground based test port. The second DPU FPGA, called the Memory FPGA, handles the interfaces to the 6 Megabyte DPU data memory bank, the spectrometer interface, the mast unit HSS link and the rover HSS links. Program memory is redundantly stored in two banks of EEPROM. One bank of EEPROM is hardware locked, in that the write line is tied off to the inactive state to guard against any memory corruption during flight. The other bank of EEPROM is capable of being reprogrammed in flight if the need arises, but is also protected from corruption by the EEPROM's built-in software data protection algorithm. In addition the DPU also has a bank of scratchpad SRAM and a bank of data EEPROM.
Some changes have already been made to the reprogrammable side. The handling of interrupts was modified to avoid conflicts which result in approximately 1% of commands rejected on the locked side. The locked side also operates with an RMI data transmission rate to the rover compute element (RCE) of only 2 MHz, while the reprogrammable side can transfer data at up to 8 MHz. The locked side also is missing headers on the RMI images, which was fixed in the reprogrammable side's program.
Low-Voltage Power Supply
The low-voltage power supply receives the nominal 28 V input from the rover, passes it on to the Mast Unit, and provides the Body Unit with +5 V digital, +/-5 V analog, +24 V digital, and +/-12 V analog. Each of these has a maximum of 5 W except the +12 V and +24 V, which are 2.5 W maximum. The DPU uses only the +/-5 V lines, while the spectrometer board uses all five supplies. Design goals were noise levels < 5 mV rms and spikes of < 20 mV amplitude. The LVPS board also provides connections for the PRT temperature sensors, two of which are on the spectrometers and two of which are on the CCD board covers. The LVPS board uses commercially available Interpoint power converters with inline filters to achieve its voltage levels. The units were heat sunk to aluminum bars which extended to the edge of the board. The board is housed in a separate compartment accessed from the rover attachment side of the electronics box, where heat is most easily transmitted to the rover.
Thermal modeling of the LVPS indicated that at the hot qualification temperature limit of 70 deg C, with the board averaging 77 deg C the +5 V converter, if operating at 120% with a 36 V bus input, could exceed 125 deg C. Additionally, in this scenario, two other converters could exceed 110 deg C. While the instrument was successfully tested at these levels, it is not expected to operate near these levels on Mars.
There are no filters on ChemCam.
The ChemCam telescope focalizes the system on a distance rock to (1) create a LIBS plasma and collect light from the plasma into an optic fiber (LIBS); or (2) to image the context around the laser pit (RMI). The telescope is designed to perform LIBS in the range of 1-7 m, and RMI in the range of 2-20 m, using a moving secondary mirror. The primary mirror of the telescope is 110 mm in diameter. The RMI field of view is 20 mrad. The optical system includes also a continuous laser and a photodiode to perform autofocus.
ChemCam is designed and planned to be used daily on Mars. Operations planning divides the rover activities into reconnaissance sols (sol
Mars day), drive sols, approach sols, contact sols during which arm-mounted instruments APXS and MAHLI (hand lens imager) are used, and in-situ analysis sols during which CheMin and SAM analyses are carried out. ChemCam can be used to perform reconnaissance on both of the first two types of sols. During approach sols ChemCam data will help select the precise sample for in-situ analysis, and during contact and in-situ analysis sols ChemCam can perform additional surveys of surrounding materials. A typical analysis will consist of a number of laser pulses sufficient to remove surface dust, followed by a number of laser pulses (e.g., 30) that will be averaged together for better statistics. At times depth profiles will be performed to understand the compositions of dust and weathering layers as well as the rock composition. Because the LIBS analysis spot size is by necessity
0.5 mm in diameter, determining a whole-rock composition requires multiple analysis spots on the same rock for medium- and coarse-grained lithologies.
Each analysis spot is expected to take about six minutes and two Watt-hrs, but in most cases it will be necessary to warm the laser from the -40 deg C survival temperature of the mast to its operation temperature of -10 deg C, taking about 20 minutes before the first analysis of the day. There is also some time required at the end of analyses to slowly cool the unit and to defocus to a sun-safe configuration and stow the mast. Of the actual analysis, the laser shots will normally operate at 3 Hz, taking only ~20 s, while up to 3 minutes is required for focusing and some tens of seconds are required for transferring the data. RMI context images will be taken before and after each LIBS analysis, transmitting only the pixels corresponding to the LIBS spot on the second image.
The major subsystems are:
- RMI subsystem - provides context images in visible light of target area sampled by LIBS system or can be used to provide narrow FOV images of other targets of interest.
- LIBS subsystem - provides LIBS spectra over the wavelength range from UV to VNIR of selected targets. Can also be used for passive spectroscopy of selected targets.
ChemCam Mast Unit
- Optical Box consisting of optical telescope, focus mechanism and focus laser
- LIBS laser
- Electronics box with laser capacitors, LVPS, HVPS, I/F electronics, FPGA and associated electronics
- Fiber optic cable that transmits LIBS light from Mast Unit to Body Unit
ChemCam Body Unit
- Demultiplexer that accepts LIBS photons from the fiber optic cable and separates light into UV, VIS and VNIR bands
- Spectrometers: UV, VIS and VNIR spectrometers that record the intensity of LIBS light in these wavebands.
- Electronics box with LVPS, CPU and I/F electronics board and spectrometer control board
- Thermoelectric cooler: used to cool Spectrometer CCDs
- Decontamination heaters: used to prevent contamination of the Body Unit optics
In addition there are a variety of survival and laser heaters, temperature monitors and other minor subsystems in both the Mast and Body Units.
ChemCam Calibration Targets: assembly mounted on Rover deck with rock and optical calibration targets
The reader is referred to the ChemCam Instrument papers MAURICEETAL2012 and WIENSETAL2012A published in Space Science Reviews; complete citations are in the reference catalog file CCAM_REF.CAT in the Catalog directory of the archive. Copies of these papers are provided in the Extras directory.