Instrument Overview
  ===================
    The MECA Optical Microscope (OM) is part of the Microscopy, Electrochemistry
    and Conductivity Analyzer (MECA) instrument suite. The optical, electronics,
    and illumination systems were designed, integrated, and tested by the Lunar
    and Planetary Laboratory (LPL) at the University of Arizona, Tucson, AZ. It
    was integrated with the MECA platform, including the sample wheel and
    translation stage (SWTS) and Atomic Force Microscope (AFM), at the Jet
    Propulsion Laboratory, Pasadena, CA, where flight qualification was also
    performed. Charge Coupled Device (CCD) detector and readout electronics were
    delivered by the Max Planck Institute for Solar System Research (MPS),
    Katlenburg-Lindau, Germany. The OM is a fixed-focus imaging system utilizing
    a 512 x 256 pixel active imaging area frame transfer CCD detector. It is
    designed to acquire images of particulate Martian material (soil, dust) at a
    spatial resolution of 4 microns/px. The samples to be imaged are moved to
    the optimum focus distance (about 14 mm away from the front end of the lens
    barrel) via a Sample Wheel Translation Stage (SWTS). The imaged area is then
    2 mm * 1 mm. The samples are actively illuminated by red, green, blue or
    ultraviolet Light Emitting Diodes (LEDs). Color images of these samples can
    be generated by combining individual images acquired with red, green and
    blue illumination. The SWTS is shared by the OM and the Atomic Force
    Microscope (AFM). It contains a total of 69 substrates: 10 sets of 6
    substrates plus 9 substrates holding OM and AFM calibration targets. Each
    set of 6 substrates consists of a weak and a strong magnet, 2 (identical)
    microbuckets (empty holes, 2 mm deep, 3 mm in diameter) for bulk sampling, a
    nanobucket (silicon textured substrate with micron-sized pits and pillar
    features for fixing particles and retaining the original size distribution
    of the acquired sample) and a sticky silicone substrate. The SWTS can be
    positioned to extend any one of ten sets of 6 substrates out of the MECA
    enclosure to receive soil samples from the Robot Arm (RA) or to collect
    airfall dust. Excess material is removed by passing the substrates under a
    blade positioned 0.2 mm above the surface, after which the samples are
    rotated for imaging by the OM and AFM. The overall sensitivity of the OM
    extends from 400 to 1020 nm and is chiefly controlled by the sensitivity of
    the CCD (extending from 380 to 1020 nm) and by a Schott GG420 glass filter
    (placed in between CCD and lens assembly) that blocks efficiently all light
    in the spectral range 380-400 nm. Therefore UV LED radiation (central
    wavelength ~ 375 nm and spectral width ~ 12 nm) cannot be directly detected
    by the CCD. Illuminating the target material with ultraviolet light can thus
    reveal fluorescence of that material. OM images belong to a series of images
    (Surface Stereo Imager (SSI), Robotic Arm Camera (RAC), AFM) that inspect
    areas of decreasing size with increasing resolution. OM images will be used
    to place AFM data in context. Refer to the Phoenix MECA/Microscopy paper
    [Hecht et al., 2008] for more details.
 
 
  Scientific Objectives
  =====================
    The chief scientific objectives of the OM are: 1) to characterize fine-scale
    morphology, reflectance and magnetic properties of Martian airborne dust
    that has been accumulated on the substrates of the Sample Wheel Translation
    Stage (SWTS), 2) to characterize fine-scale morphology, reflectance and
    magnetic properties of soil material that has been actively dumped onto the
    substrates of the SWTS, 3) to aid in the interpretation of data gathered by
    other Phoenix instruments, in particular to bridge 'imaging' of these very
    same (or closely related) samples at higher (Atomic Force Microscope, ~ 0.1
    microns/px) and lower resolution (RAC, > 20 microns/px).
 
 
  Calibration
  ===========
    Many of the OM components (such as CCD, optics and LEDs) were tested by the
    vendor or supplier on subsystem level. CCD tests included photon transfer/
    linearity, dark current and flat field. These data were used to select the
    best CCD for the OM. Calibration of the fully assembled microscope included:
    1) Full Aperture Relative Spectral Response, 400-1000 nm in 10 nm steps at
    +25 deg.C and -40 deg.C.
    2) Full Aperture Flat Field Linearity Test (signal versus illumination time)
    with Red, Green, and Blue LEDs using plane, white standard targets at focus,
    temperature range: -90 deg.C to +35 deg.C (in steps of 10 deg.C).
    3) White light flat field images (using an integrating sphere) with
    corresponding dark frames in the same temperature range.
    4) LED emission spectra in the same temperature range.
    5) Images (acquired at +25 deg.C) of standard targets (USAF 1951, Ronchi
    Gratings with 60 and 100 LP/mm) to provide focus, resolution, and MTF
    measurements.
    6) Image Scale and Depth of Field Measurements using (tilted) Ronchi
    Gratings.
 
    Most of the science camera testing and calibration was done in two labs at
    LPL, one for ambient testing and another for thermal/vacuum testing. The
    geometric and other tests that were not significantly affected by
    temperature were performed at room temperature and ambient pressure on
    optical benches with electrostatic discharge protection. During tests in the
    thermal vacuum chamber external targets and sources were imaged through an
    optical grade quartz window. The thermal tests and calibration were
    performed under high vacuum (<10^-6 torr) at a variety of temperatures
    spanning the expected temperature range on the surface of Mars. Flight-
    acceptance thermal cycling was performed before calibration that was
    completed by the end of Nov. 2005.
 
 
  Operational Considerations
  ==========================
    Since the OM shares its readout electronics with the RAC, it cannot be
    operated in parallel with the latter instrument. The performance of the OM
    depends both on intrinsic instrument characteristics and on positioning (and
    associated repeatability) of the SWTS. The SWTS is shared by OM and AFM.
    During normal operations OM images and AFM scans are not acquired
    simultaneously, as the OM images would be out of focus. The SWTS substrates
    are 3 mm in diameter. In order to satisfy the OM requirements and to view an
    area 2 mm * 2 mm of these substrates, two images must be acquired side by
    side. Because the OM has a small depth of field (+/- 25 microns), an image
    of a rough surface (e.g. thick pile of material on the strong magnet) will
    contain both focused and unfocused areas. In this case a stack of images
    must be acquired by moving the SWTS to different distances from the OM: The
    first image is furthest out and subsequent images are taken as the wheel
    steps towards the OM.
 
 
  Detectors
  =========
    The OM uses the same type of CCD as the RAC, while the CCD readout
    electronics is shared by both instruments (as explained in the previous
    section). 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 microns 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 can be
    used to scale dark current corrections using the line-by-line ratios,
    although dark current is basically negligible at operational temperatures.
    The full-well capacity is around 112,000 electrons with a read noise of
    around 16 electrons. The system gain is set to about 27 electrons/DN. The
    exposure time can be varied from 0 to 32 s in steps of 0.5 ms (1 count).
    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 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. Further details on the CCD
    can be found in the Phoenix RAC paper [KELLERETAL2008].
 
 
  Electronics
  ===========
   Refer to previous section.
 
 
  Filters
  =======
    N/A
 
 
  Optics
  ======
    The OM optics employs a fixed focus, f/30 doublet design that provides +/-25
    microns depth-of-field at 4 micron/pixel sampling. The effective focal
    length is 16.5 mm, and the working distance is about 14 mm from the front of
    the lens barrel to the object plane. Given the size of the CCD image area
    (512px * 256px), the FOV is 2 mm * 1 mm at the working distance.
 
 
  Location
  ========
    OM and SWTS are placed together with the Wet Chemistry Laboratory (WCL)
    inside the box shaped (35cm x 25cm x 15cm) MECA enclosure on the Phoenix
    lander deck.
 
 
  Operational Modes
  =================
    The chief parameters for an OM image (or OM image block) are the following:
 
    1) FIELD: Single field (1 mm * 2 mm) or side-by-side images (2 mm * 2 mm).
    2) FOCUS: Single focus or through-focus (in general 4-6 in/out images,
    depending on the available resources during operations).
    3) COLOR: Monochrome mode (only one LED type switched on during image
    acquisition), 3-color mode (red, green and blue LEDs switched on
    separately), 4-color mode (red, green, blue and UV LEDs switched on
    separately), white-illumination mode (red, green and blue LEDs switched on
    simultaneously).
    4) TARGET (five target types available, as explained in the Overview
    section):
    Weak magnet, strong magnet, microbucket, nanobucket, sticky silicone.
 
    Any parameter combinations are possible and will be chosen depending on the
    particular science goal. Standard imaging of a single substrate requires a
    total of 6 images (two side-by-side images, each one acquired in three
    colors (RGB)).
 
 
  Subsystems
  ==========
    The OM has three logical subsystems:
    1) Sensor head (containing CCD detector and optics)
    2) LED assembly
    3) SWTS (containing the substrates and calibration targets to be imaged).
 
    Subsystem (1) is controlled by RAC software/electronics, whereas subsystems
    (2) and (3) are controlled by MECA software/electronics.
 
 
  Measured Parameters
  ===================
    1) Morphology, physical and magnetic properties:
    OM images provide information on morphology, grain size and cohesion of
    Martian soil material. Given the resolution of these images the size
    distribution of the particle size fraction > 10 microns can be determined.
    The magnetic properties of soil particles are inferred from their
    interaction with permanent magnets of different strengths. The OM images
    will also provide useful information on the adhesion, cohesion and magnetic
    properties of airborne dust, although typical (micron-sized) airborne dust
    particles cannot be resolved by these images. This information will stem
    from the particles' interaction with textured, sticky or magnetic surfaces.
    A small subset of airborne dust particles that are within the OM FOV will be
    targeted by the AFM.
 
    2) Color:
    The samples can be illuminated with red, green and blue LEDs during image
    acquisition. OM images contain therefore information on color and
    reflectance properties of these samples.
 
    3) Fluorescence:
    The OM can illuminate the samples with ultraviolet light (wavelength ~ 375
    nm). The CCD cannot detect the scattered UV light, but only reemitted
    fluorescent light (in the visible/near-infrared range). As a result
    potential fluorescent mineral grains can be detected, and their fluorescence
    can be roughly quantified.