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.
Some of the most important operational considerations associated with Pancam are related to the large volume of data that the instrument can generate. 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. 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. 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 exposure counts, where each exposure count equals 5. 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. 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 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 '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 bit digitized signal.
The FPGA also inserts a unique camera identification number into the telemetry for each camera to simplify data management and post-processing. 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 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 solar filters have the same requirements for their bandpasses, but also are coated with metallic attenuation films to provide an additional factor of reduction in overall transmission. The shortest wavelength nm and longest wavelength 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.
Two filters, near and 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 Scientists use Pancam to scan the horizon of Mars for landforms that may indicate a past history of water. They also use the instrument to create a map of the area where the rover lands, as well as search for interesting rocks and soils to study.
The Pancam cameras are small enough to fit in the palm of your hand grams or about 9 ounces , but can generate panoramic image mosaics as large as 4, pixels high and 24, pixels around. Pancam detectors are CCDs charge coupled devices.
These devices form the image, just as film does in a film camera. Each "eye" of the Pancam carries a filter wheel that gives Pancam its multispectral imaging capabilities. Images taken at various wavelengths can help scientists learn more about the minerals found in martian rocks and soils. Blue and infrared solar filters allow the camera to image the sun. These data, along with images of the sky at a variety of wavelengths, help to determine the orientation of the rover and provide information about the dust in the atmosphere of Mars.
The Pancam color imaging system has, by far, the best capability of any camera ever sent to the surface of another planet.
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