| DESCRIPTION |
Instrument Overview
===================
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.
Scientific Objectives
=====================
The chief scientific objectives of the Pancam are:
1) to obtain monoscopic and stereoscopic image mosaics to asses the
morphology, topography, and geologic context of each MER landing
site,
2) to obtain multispectral visible to short-wave near-IR images of
selected regions to determine surface color and mineralogic
properties,
3) to obtain multispectral images over a range of viewing geometries
to constrain surface photometric and physical properties, and
4) to obtain images of the Sun and martian sky to constrain aerosol
physical and radiative properties.
Calibration
===========
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 < 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.
Operational Considerations
==========================
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 Mossbauer 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.
Detectors
=========
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 um 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.
Electronics
===========
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).
Filters
=======
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 < 10%, central wavelength uniformity and central
wavelength shift resulting from angle of incidence variations across
the FOV of < 1%, and a wavelength-integrated rejection band response
in the 400 to 1100 nm region of < 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 < 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.
Optics
======
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 urad, yielding 560 urad limiting resolution
on a pair of adjacent 12 um 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.
Location
========
The Pancam is located atop the Pancam Mast Assembly (PMA)
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