MECA Optical Microscope (OM)
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.
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).
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:
Full Aperture Relative Spectral Response, 400-1000 nm in 10 nm steps at +25 deg.C and -40 deg.C.
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).
White light flat field images (using an integrating sphere) with corresponding dark frames in the same temperature range.
LED emission spectra in the same temperature range.
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.
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.
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.
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].
Refer to previous section.
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.
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.
The chief parameters for an OM image (or OM image block) are the following:
FIELD: Single field (1 mm * 2 mm) or side-by-side images (2 mm * 2 mm).
FOCUS: Single focus or through-focus (in general 4-6 in/out images, depending on the available resources during operations).
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).
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)).
The OM has three logical subsystems:
Sensor head (containing CCD detector and optics)
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.
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.
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.
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.