Panoramic Camera (Pancam)
The Panoramic Camera (Pancam) investigation is part of the Athena science payload launched to Mars in 2003 on NASA's twin Mars Exploration Rover (MER) missions. The scientific goals of the Pancam investigation are to assess the high resolution morphology, topography, and geologic context of each MER landing site; to obtain color images to constrain the mineralogic, photometric, and physical properties of surface materials; and to determine dust and aerosol opacity and physical properties from direct imaging of the Sun and sky. Pancam also provides mission support measurements for the rovers, including Sun-finding for rover navigation; hazard identification and digital terrain modeling to help guide long-term rover traverse decisions; high resolution imaging to help guide the selection of in situ sampling targets; and acquisition of education and public outreach products. The Pancam optical, mechanical, and electronics design were optimized to achieve these science and mission support goals. Pancam is a multispectral, stereoscopic, panoramic imaging system consisting of two digital cameras mounted on a mast 1.5 m above the martian surface. The mast allows Pancam to image the full 360 degrees in azimuth and +/-90 degrees in elevation. Each Pancam camera utilizes a 1024 x 1024 pixel active imaging area frame transfer charge-coupled device (CCD) detector array. The Pancam optics have an effective focal length of 43 mm and a focal ratio of f/20, yielding an IFOV of 0.27 mrad/pixel and a FOV of 16 degrees x 16 degrees. Each rover's two Pancam 'eyes' are separated by 30 cm and have a 1 degree toe-in to provide adequate stereo parallax. Each eye also includes a small 8-position filter wheel to allow surface mineralogic studies, multispectral sky imaging, and direct Sun imaging, in the 400-1100 nm wavelength region. Pancam was designed and calibrated to operate within specifications on Mars at temperatures from 218 K to 278 K. An onboard calibration target and fiducial marks provide the capability to validate the radiometric and geometric calibration on Mars.
Information in this instrument description is taken from the Mars Exploration Rover Athena Panoramic Camera (Pancam) Investigation paper [BELLETAL2003]. See this paper for more details.
The chief scientific objectives of the Pancam are:
- to obtain monoscopic and stereoscopic image mosaics to asses the morphology, topography, and geologic context of each MER landing site,
- to obtain multispectral visible to short-wave near-IR images of selected regions to determine surface color and mineralogic properties,
- to obtain multispectral images over a range of viewing geometries to constrain surface photometric and physical properties, and
- to obtain images of the Sun and martian sky to constrain aerosol physical and radiative properties.
A rigorous test and calibration program has been conducted to derive and monitor Pancam instrument performance and calibration parameters and to validate the instrument calibration pipeline. These tests were conducted at the individual component level (CCDs, filters), at the assembled (standalone) camera level for each flight unit, at the subsystem level when the cameras were integrated with the camera bar and Pancam Mast Assembly (PMA), and finally at the system level during fully assembled rover testing. Tests were performed at both (Earth) ambient temperature and pressure conditions and under thermal vacuum conditions simulating the range of expected conditions on Mars. The most critical flatfield, throughput, and responsivity, calibrations were performed under near-vacuum (P less than 10-6 torr) and at three temperatures (218 K, 263 K, and 278 K) chosen to bracket the expected nominal daytime range of surface conditions expected during the mission. Co-alignment tests among Pancam, Navcam, Hazcam, and Mini-TES were conducted in ambient conditions and in near-vacuum at temperatures of 178 K, 218 K, 243 K, and 273 K.
The Pancam will also be calibrated inflight. Monitoring of the stability of the Pancam spatial response pattern (flatfield) will be performed during flight by occasional imaging of the martian sky (which, if the azimuth and elevation relative to the Sun are chosen properly, should be acceptably flat over the field of view of Pancam). If variations are detected because of, for example, dust particles on the Pancam external sapphire window, then these flatfields may be used in place of the pre-flight ground flatfields in the Pancam calibration pipeline.
Monitoring of the Pancam radiometric calibration stability will be performed on Mars by frequent imaging of the Pancam calibration target under repeatable illumination conditions, and by occasional downlinking of reference pixel and dark current images. Imaging of the Sun and potentially certain bright standard stars at night may also provide additional information on calibration stability.
A baseline for Mars surface calibration performance of Pancam (and other MER instruments) will be established during a 'calibration campaign' to be performed shortly after each rover's landing. Observations planned for this campaign include baseline calibration target imaging, dark current and reference pixel images, sky flatfields, and reassessment of PMA pointing by imaging surveyed fiducial marks on the rover deck and lander. In addition to validating (or not) the pre-flight calibration coefficients and overall camera and PMA performance, these baseline measurements will be used to monitor the potential build-up of dust over time on either the camera optics or the surfaces of the Pancam calibration targets or magnetic properties experiment magnets.
Pancam is a very versatile instrument, and it will be used in a number of different ways during operations. One of the most important operational roles will be to acquire full 360 degree panoramas. One of these per rover, in RGB color (L2: 753 nm, L5: 535 nm, and L6: 483 nm) and stereo (R2: 754 nm), is called for by the formal MER Level 1 Mission Success requirements. We plan to meet this requirement while each rover is still on the lander preparing for egress. The approach for such a panorama will be to acquire red-filter images at full resolution in both eyes, along with green- and blue-filter images at reduced resolution (using compression and/or downsampling) in the left eye. Such a panorama provides morphologic and textural information at the highest possible resolution, 'true color' information at somewhat lower resolution, and good stereo ranging of the full scene around the rover. After this early 'Mission Success' panorama, we plan to acquire full 360 degree Pancam panoramas rather infrequently because they take considerable time and generate a large volume of data.
Partial panoramas (i.e., image mosaics less than 360 degree in size) will be the most common use of Pancam. These can be monochromatic or in many colors, and they typically will be targeted on the basis of images from the lower-resolution Navcams. Some images will be acquired with full multispectral coverage, using all of the instrument's geology filters. This will be done when testing of a specific hypothesis requires determination of spectrophotometric properties across the full spectral range of the camera. The spatial coverage of full multispectral imaging will be restricted significantly by time and data volume limitations, so such images will need to be targeted carefully on the basis of previous Navcam or other Pancam imaging.
Imaging of the martian sky will be conducted on a regular basis to monitor atmospheric conditions. The Sun will be imaged directly through both solar filters to determine wavelength dependent optical depth, and the sky will be imaged through the geology filters over a range of angular distance from the Sun to determine aerosol scattering properties.
Pancam will be used to conduct several kinds of coordinated observations with other instruments. For example, both full multispectral Pancam imaging and Mini-TES rastering will commonly be conducted on candidate targets for in situ investigation in order to obtain morphologic, textural, and compositional information before making the decision to drive the rover.
The Pancam will also be used in tandem with the Microscopic Imager (MI), because engineering considerations made it impossible to package a filter wheel with the MI. There are plans to acquire at least three-color Pancam images of every MI target. Software currently in development will allow us to register the lower-resolution Pancam color information with the higher-resolution textural information that the MI provides.
Pancam will also be important for supporting the magnetic properties experiment. The Capture and Filter magnets at the base of the PMA will be imaged in color on a regular basis to monitor the gradual buildup of magnetic dust. These images will be used to assess when the depth of dust is great enough to commit to making Moessbauer and APXS measurements of the magnets. The Sweep magnet is mounted directly adjacent to the Pancam calibration target, so it will be imaged with no additional impact on time or data volume every time the target is imaged. The RAT magnets are located within the RAT, and after a RAT operation the Instrument Deployment Device will be used to position the RAT so that the magnets can be imaged in color by Pancam.
Some of the most important operational considerations associated with Pancam are related to the large volume of data that the instrument can generate. By practical necessity, most Pancam data may end up being transmitted to Earth by UHF relay through the Mars Odyssey or Mars Global Surveyor (MGS) spacecraft. The latency associated with these links is substantial: up to 5 hours for Odyssey, and up to 2 days for MGS. Therefore most Pancam images will probably be used for strategic rather than tactical science planning, though judicious management of direct-to-Earth X-band downlink resources may allow an important subset of Pancam images (including 64x64 pixel 'thumbnail' versions of all Pancam images acquired on each sol) to be downlinked more quickly. Very careful selection of compression and downsampling parameters will also be essential to maximizing the science return from Pancam.
All 9 cameras on each MER rover, plus the descent imaging system on each lander, use a common and nearly-identical set of detectors and electronics. Each MER camera, including Pancam, utilizes a 1024 x 2048 frame transfer CCD detector designed by JPL and fabricated by Mitel (now DALSA Semiconductor, Inc). The CCDs are front-side illuminated, buried-channel devices configured to use one half of the pixels (1024 x 1024) as the active imaging area, and the other half as a storage/readout area masked from illumination by an opaque black-painted aluminum light shield. The CCDs do not use UV-enhancing or anti-reflection coatings. There are no antiblooming structures built into the MER CCD pixels, but blooming is modestly controlled using a 'clocked antblooming' technique that consists of transferring charge between two phases in the same pixel during the integration time. There is also a drain structure that runs along side the serial register that is used to rapidly remove charge from the array during fast transfer or windowing. While these methods are not as effective as having true antiblooming structures in each 12 micron square pixel, they do not impact fill factor or collection efficiency, both of which are important for meeting Pancam measurement objectives.
When powered, the CCD is constantly running in a 'frame flush' mode where charge is drained from the array every 5.1 msec. An exposure is initiated at the start of a new frame flush cycle. Photons are then collected in the imaging area during the specified integration time (from 0 to 65535 exposure counts, where each exposure count equals 5.12 msec) and then once the exposure is complete the accumulated charge is rapidly shifted (shift time = 5.12 msec) into the storage area. This rapid charge transfer obviates the need for a mechanical shutter on the camera, but also limits the minimum exposure time (0 counts) to 5.12 msec and leads to the generation of frame transfer smear signal that must be corrected in calibration (see Section 4.2.4 below).
Once the collected photons are in the storage area, they are clocked out, row by row, into a horizontal serial register for subsequent amplification and digitization. The readout rate is 200 kHz (200,000 pixels/sec), or 5 msec per row, leading to a total readout time of approximately 5 seconds per full frame image. The horizontal register also contains 16 extended or 'reference' pixels at each end that are also read out and digitized by the camera electronics. These pixels provide information on the video offset (bias) level, and can be optionally saved for downlink as a 32 x 1024 'reference pixel' Experiment Data Record (EDR) image file.
The MER camera electronics consist of clock drivers that provide 3-phase timing pulses for transfer of charge through the CCD, as well as a signal chain that amplifies the CCD output and converts it from analog voltages to a 12-bit digitized signal. An Actel Field Programmable Gate Array (FPGA) provides all of the timing, logic, and control functions in the signal chain. The FPGA also inserts a unique camera identification number into the telemetry for each camera to simplify data management and post-processing. A correlated doublesampling Analog-to-Digital Converter (ADC) compares the amplified CCD output voltage against a (commandable) reference voltage from the FPGA to achieve 12-bit (0-4095) digitization of the signal. Gain, read noise, and other performance metrics of the Pancam signal chain are reported below.
The rovers' flight software provides a substantial amount of capability for doing onboard image processing prior to downlink, with the primary goal to increase the compressibility of images and thus to maximize the amount of data that can be sent back to Earth during each downlink session. The image processing services offered by the rover CPU include bad pixel correction, flatfield correction, frame transfer smear correction, image downsampling, image subframing, pixel summing, 12 to 8 bit scaling via lookup tables, and image compression.
However, frame transfer smear correction deserves special notice because of its importance and implications for Pancam imaging. As discussed above, there are analytic or empirical ways to remove frame transfer smear signal from Pancam images. The main advantage of the a posteriori analytic approach of modeling the effect is that no additional image acquisition time or processing time are required on Mars. However, the uncorrected images to be downlinked are likely to be less compressible than corrected images, and substantial additional post-processing time is required in the ground calibration pipeline. The main advantage of the in situ empirical approach of subtracting a zero-exposure image is that the removal of the bias, storage region dark current, and frame transfer ramp components should produce a much more compressible image for downlink than images that have not been corrected. The price for this increased downlink efficiency, though, is a doubling of the time required to acquire images, plus additional overhead for onboard CPU processing. It is anticipated that the tradeoffs will be made so that sometimes onboard frame transfer smear removal is more advantageous, while sometimes post-processing analytic removal may be more advantageous. Each situation will need to be considered on a case by case basis.
Rapid lossless onboard compression of MER camera images can be performed using a routine called LOCO, which is based on the same kind of segmented discrete cosine transform method as the JPEG compressor. High quality lossy compression can be performed using a routine called ICER, which is a wavelet-based progressive compression routine that has been shown in tests by the MER science team to retain excellent image quality even at relatively high compression factors below 1 bit per pixel (compression ratios exceeding 12:1 for MER images).
Each Pancam camera is equipped with a small 8-position filter wheel. Fifteen of the sixteen filter wheel slots contain filters; one slot (L1) was left empty to maximize sensitivity during low-light and ambient Earth temperature (pre-flight) imaging conditions. The filters are glass interference filters, 11 mm in diameter (10 mm clear aperture) and were fabricated by Omega Optical, Inc. Thirteen of the fifteen filters per camera pair are so-called 'geology' filters, designed for imaging of the surface or sky, and the remaining two filters are 'solar' filters, designed for direct imaging of the Sun. The geology filters were designed and fabricated to have peak transmission >85%, transmission ripple within the passband of less than 10%, central wavelength uniformity and central wavelength shift resulting from angle of incidence variations across the FOV of less than 1%, and a wavelength-integrated rejection band response in the 400 to 1100 nm region of less than 1% of that filter's integrated in-band response. The solar filters have the same requirements for their bandpasses, but also are coated with metallic attenuation films to provide an additional factor of 105 reduction in overall transmission. The shortest wavelength (440 nm) and longest wavelength (1000 nm) filters are actually short-pass and long-pass filters, respectively, to provide wider bandpasses to maximize the SNR at these extreme ends of the CCD spectral response profile. The filters are divided between the cameras so that, in general, the shorter wavelength filters less than 750 nm are in the left camera and the longer wavelength filters > 750 nm are in the right camera. Two filters, near 440 and 750 nm, are redundant in the left and right Pancams. This provides stereo imaging capability in two colors, as well as redundancy for generating pseudo-true color images in the right Pancam, in the event of a left Pancam failure.
The science and measurement requirements outlined above (spatial resolution, depth of field, and field of view), the realities of limited payload mass and volume resources, and the harsh martian surface environmental conditions all dictate design constraints on Pancam optics. The resulting design is small (short focal length), lightweight, has a slow focal ratio (greater depth of field), employs discrete spherical or flat elements rather than cemented or aspherical surfaces, and does not allow vignetting of the field. An anti-reflection coated sapphire window protects the filters and filter wheel mechanisms from contamination by airborne dust particles and helps cut down stray and scattered light effects. A short sunshade and set of black internal baffles provide rejection of stray and scattered light. The Pancam lens design is a Cooke triplet. The lens was designed to have a focal length of 43 mm, which yields a field of view (FOV) of 16 degrees x 16 degrees (22.5 degrees on the diagonal) that is approximately equal to the FOV of a 109 mm telephoto lens on a standard 35 mm camera. The Instantaneous Field of View (IFOV) of each pixel was designed to be approximately 280 x 280 microradians, yielding 560 microradians limiting resolution on a pair of adjacent 12 micron pixels, or a Nyquist limit for spatial frequency detection of 41.67 cycles/mm. The lenses were designed to operate at f/20 with a fixed (hyperfocal) distance of 3.0 m and to view objects in focus whose distances range from infinity to 1.5m (the depth of field). The Modulation Transfer Function (MTF) performance of the optics was assessed at the component level at three wavelengths: near the shortest Pancam bandpass around 430 nm, near the peak of the planet's spectral reflectance function at 750 nm, and near the longest Pancam bandpass at 980 nm. At 430 nm, the actual MTF values cluster just below the diffraction-limited value of 55% at the Nyquist cutoff frequency. Similarly, for 750 nm, the actual curves cluster just below the diffraction limit of 25% at Nyquist. And for 980 nm, the curves drop to just below the diffraction limit of 8% at Nyquist.
The Pancam is located atop the Pancam Mast Assembly (PMA)