The Phoenix Robotic Arm Camera (RAC) is a variable-focus color camera mounted to the robotic arm (RA) of the Phoenix Mars lander. It is designed to acquire both close-up images of the Martian surface and microscopic images (down to a scale of 23 micron/px) of material collected in the RA scoop. The mounting position at the end of the robotic arm allows the RAC to be actively positioned for imaging of targets not easily seen by the Stereo Surface Imager (SSI), such as excavated trench walls and targets under the lander structure. Color information is acquired by illuminating the target with red, green and blue light emitting diodes. Digital terrain models (DTM) can be generated from RAC images acquired from different view points. This can e.g. provide high-resolution stereo information about fine details of the trench walls.

Keller, H.U., W. Goetz, H. Hartwig, S.F. Hviid, R. Kramm, W.J. Markiewicz, C.
                   Shinohara, P.H. Smith, R. Tanner, P. Woida, R. Woida, B.J. Bos, and M.T.
                   Lemmon, The Phoenix Robotic Arm Camera, J. Geophys. Res., 113, E00A17,
                   doi:10.1029/2007JE003044, 2008.

Instrument Overview
  ===================
    The Phoenix Robotic Arm Camera (RAC) is a variable-focus color
    camera mounted to the robotic arm (RA) of the Phoenix Mars
    lander. It is designed to acquire both close-up images of the
    Martian surface and microscopic images (down to a scale of 23
    micron/px) of material collected in the RA scoop. The mounting
    position at the end of the robotic arm allows the RAC to be
    actively positioned for imaging of targets not easily seen by
    the Stereo Surface Imager (SSI), such as excavated trench walls
    and targets under the lander structure. Color information is
    acquired by illuminating the target with red, green and blue
    light emitting diodes. Digital terrain models (DTM) can be
    generated from RAC images acquired from different view points.
    This can e.g. provide high-resolution stereo information about
    fine details of the trench walls.
 
    Information in this instrument description is taken from The
    Phoenix Robotic Arm Camera [KELLERETAL2008]. See this paper
    for more details.
 
 
  Scientific Objectives
  =====================
    The science objectives of the RAC are the following: (1)
    Characterize a soil patch prior to digging (together with the
    Surface Stereo Imager, SSI). This characterization will also
    support the selection of the digging site - an important
    tactical step during mission operations. (2) Characterize
    scoop - soil interactions (e.g. during scraping or digging)
    in terms of physical properties such as cohesion and soil
    strength. (3) Characterize trench walls: (a) Search for fine-
    scale soil/ice layering, (b) monitor possible temporal changes
    due to sublimation of water ice away from a newly excavated
    surface. (4) Characterize soil samples in the scoop prior to
    delivery to the analyzing instruments on the lander deck MECA
    and TEGA. (5) Characterize the surface below the lander, and in
    particular estimate the penetration depth of the foot pads with
    direct implications on the shallow sub-surface environment. (6)
    Provide a fast way to create a Digital Terrain Model (DTM) of
    the landing site. The RAC has a larger field of view (FOV) and
    stereo base than the SSI. In addition the stereo base can have
    any orientation (from horizontal to vertical). In that sense the
    RAC complements SSI data in the generation of the DTM. (7)
    Atmospheric science: (a) monitor the horizon and search for dust
    devils, (b) provide 'one shot' overview images of the brightness
    gradient of the Martian sky. The RAC also fulfils important
    tactical (sol planning) tasks by supporting instrument checkout
    of MECA and TEGA and documenting sample delivery to these
    instruments that cannot be done adequately by the SSI.
 
    More general science goals of the RAC include: (a) Search for
    seasonal and climate records in the sub-surface, (b) search for
    records of diurnal/seasonal, vertical/lateral transport processes
    in the soil (such as evaporative pores or salt cementation), (c)
    constrain long-term weathering processes from shape and texture
    of rocks or rock fragments.
 
 
  Calibration
  ===========
    The bulk of the RAC calibration and instrument characterization
    activities occurred in late 1999. In early 2006 the instrument
    underwent additional testing to verify the 1999 test results and
    prepare for the Phoenix mission. The stability of the parameters
    over more than 7 years is remarkable. During nominal operations
    the raw images must be corrected on board for blemished pixels in
    order to minimize compression artifacts. The active-area frames
    (512 x 256 px) are then JPEG compressed by a factor of ~ 4
    (depending on resources) and downloaded. All remaining calibration
    steps (such as dark current correction (insignificant) and flat
    fielding) will be done on ground.
 
 
  Operational Considerations
  ==========================
    The CCD was qualified for the temperature range 160 - 320 K. However,
    the stepper motors for cover and lens system need to be heated to
    above 210 K before they can be operated. This minimum temperature
    is required by the grease lubricant in the bearings of these stepper
    motors. All the other moving parts in the RAC are lubricated with MoS2
    and need not to be pre-heated, as long as their temperature is above
    140 K.
 
 
  Detectors
  =========
    The size of the RAC was driven by the available charge-coupled
    device (CCD) detector package and Sensor Head Board (SHB) that
    were originally designed for the Descent Imager/Spectral Radiometer
    (DISR) camera on board the Huygens Probe of the Cassini mission.
    The RAC has a double Gauss lens system and a frame-transfer CCD
    detector. The CCD is a front side illuminated frame transfer
    device employing buried channel technology with 2 phase Multi-
    Pinned-Phase (MPP) clocking. The pixel spacing is 23 micron in both
    directions, however, 6 microns in line direction of each pixel are
    covered by an anti-blooming structure to remove excess charge in
    case of overexposure. The CCD has no anti-reflection coating. It
    consists of a (512 active + 16) columns by 256 lines imaging area,
    and a (512 active + 16) columns by 256 lines storage area covered
    by a metal mask. Each line from the serial readout register
    contains 4 null pixels (the 'null strip') providing system noise
    information, 8 dark pixels (the 'dark strip') measuring dark current,
    512 active pixels, and 4 null pixels again. These dark current strips
    are used to scale dark current corrections on board using the line-
    by-line ratios. After exposure the photogenerated electrons are moved
    from the active area to the storage area within 1 ms. The readout of
    the storage area takes 2 s. The CCD output signal is first amplified
    by the sensor head board (SHB), and then transmitted via long (up to
    4 m) electrical lines to the CCD Readout Board (CRB) located inside
    the central electronics box of the lander. The CRB accommodates the
    analog signal chains with correlated double sampling, a sample and
    hold amplifier, 12 bit A/D converter, clock driver, power converter,
    and a digital control unit with a parallel interface to the experiment
    processor.
 
 
  Electronics
  ===========
    After exposure the photogenerated electrons are moved from the active
    area to the storage area within 1 ms. The readout of the storage area
    takes 2 s. The CCD output signal is first amplified by the SHB, and
    then transmitted via long (up to 4 m) electrical lines to the CCD
    Readout Board (CRB) located inside the central electronics box of the
    lander. The volume between front bulkhead and centre accommodates all
    optical and mechanical subsystems while the volume between centre
    bulkhead and rear houses the SHB electronics.
 
    During nominal operations the raw images must be corrected on board
    for blemished pixels in order to minimize compression artifacts. The
    active-area frames (512 x 256 px) are then JPEG compressed by a factor
    of ~ 4 (depending on resources) and downloaded. All remaining
    calibration steps (such as dark current correction (insignificant)
    and flat fielding) will be done on ground. The flight software allows
    for subframing. However, in general mostly full frames will be
    downloaded.
 
 
  Filters
  =======
    The RAC front window is a 2 mm thick, blue-green glass filter (BG40,
    Schott Inc.) that blocks all red and near-infrared light with
    wavelengths longer than 700 nm. The color capability of the RAC
    arises from composing images taken with illumination provided by red,
    green, and blue light emitting diodes (LED). Since the lighting
    system operates in the visible part of the spectrum, the camera's
    color capability is greatly enhanced by cutting off the near-infrared
    part of the incident radiation. A movable transparent sapphire cover
    (referred to as dust cover) driven by a stepper motor protects the
    front window of the camera against atmospheric dust and flying debris
    that may be kicked up during digging operations. The cover will be
    open during normal image acquisition.
 
 
  Optics
  ======
    The optical bench consists of three walls or bulkheads [front, centre,
    and rear] which are braced against each other by two side frames [left
    and right]. The volume between front bulkhead and centre accommodates
    all optical and mechanical subsystems while the volume between centre
    bulkhead and rear houses the SHB electronics, with two connectors
    mounted to the rear bulkhead.
 
  Location
  ========
    For ground-based data, coordinates of the observatory
 
  Operational Modes
  =================
    The RAC employs a movable variable focus objective ranging from macro
    1:1 mode to infinity. The lens in its cell is mounted to a translation
    stage which allows its position to be changed along the optical axis.
    The optical distortion was measured by imaging a target of equally
    spaced holes manufactured out of photo-etched chrome on glass. The
    RAC flight model turned out to have a positive distortion that reaches
    0.28% at the corners of the image, implying that an image point at that
    location is by 0.80 pixels further out than it ideally should be.
 
 
  Subsystems
  ==========
    The RAC consists of the following subsystems: (1) optical
    bench / frame, (2) Sensor Head Board with CCD detector, (3) double
    Gauss lens with lens cell, (4) lens focusing mechanism with stepper
    motor and reference switch, (5) protective dust cover with mechanism,
    stepper motor, and reference switch, (6) upper and lower lamp
    assemblies, (7) two temperature sensors, and (8) a protective shell.
 
 
  Measured Parameters
  ===================
    The mounting position at the end of the robotic arm allows the
    RAC to be actively positioned for imaging of targets not easily
    seen by the SSI, such as excavated trench walls and targets under
    the lander structure. Color information is acquired by illuminating
    the target with red, green and blue light emitting diodes. Digital
    terrain models (DTM) can be generated from RAC images acquired from
    different view points.

Phoenix Robotic Arm Camera

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