Microscopic Imager (MI)
The Microscopic Imager (MI) is a fixed-focus camera mounted on the end of an extendable instrument arm, the Instrument Deployment Device (IDD). The MI was designed to acquire images at a spatial resolution of 30 microns/pixel over a broad spectral range (400 - 700 nm). The MI uses the same electronics design as the other MER cameras but has optics that yield a field of view of 31 x 31 mm across a 1024 x 1024 pixel charge-coupled device (CCD) image. The MI acquires images using only solar or skylight illumination of the target surface. A contact sensor is used to place the MI slightly closer to the target surface than its best focus distance (about 69 mm), allowing concave surfaces to be imaged in good focus. Coarse focusing (~ 2 mm precision) is achieved by moving the IDD away from a rock target after the contact sensor has been activated. The MI optics are protected from the Martian environment by a retractable dust cover. The dust cover includes a Kapton window that is tinted orange to restrict the spectral bandpass to 500 - 700 nm, allowing color information to be obtained by taking images with the dust cover open and closed. MI data will be used to place other MER instrument data in context and to aid in petrologic and geologic interpretations of rocks and soils on Mars.
Information in this instrument description is taken from the Athena Microscopic Imager Investigation paper [HEREKENHOFFETAL2003]. See this paper for more details.
The chief scientific objectives of the MI are:
- to image fine-scale morphology and reflectance of natural rock and surfaces,
- to image fine-scale texture and reflectance of abraded rock surfaces,
- to aid in the interpretation of data gathered by other Athena instruments by imaging areas examined by them at high resolution, and
- to monitor the accumulation of dust on the capture and filter magnets.
Many of the MI components were tested before they were built into the cameras, primarily to verify performance. Many component-level tests are important to overall camera calibration, including spectral transmission of the optics, filters, and dust cover windows, calibration of temperature sensors, and performance of the CCDs. The spectral transmission of the optical barrel assemblies was tested by the optics vendor, Kaiser Electro-Optics. The spectral transmission of the MI filters was measured at JPL, and the dust cover window spectral transmission was measured at the NASA Johnson Space Center. The temperature sensors were calibrated at the vendor, Rosemount Aerospace. The CCDs used in the MER cameras were thoroughly tested at JPL; the results of these tests (including photon transfer/linearity, dark current, flat field, residual bulk image, and spectral quantum efficiency) were used to select the best CCDs for the flight cameras. Residual bulk image is most prevalent at low temperatures and long (near-IR) wavelengths and is therefore not expected to be significant for the MI. The details of the CCD tests are given in MER project document 420-1-485 (JPL D-20247).
The MER science cameras were assembled, tested, and calibrated in a clean laboratory environment at JPL. The laboratory configuration and equipment were customized for MER testing and calibration. Most of the science camera testing and calibration was done in two labs, 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 pressure on optical benches with electrostatic discharge protection. Three science cameras (2 Pancams, 1 MI) were tested together in the thermal/vacuum chamber, all three viewing external targets and sources 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 camera calibration, and some calibration data were acquired during the acceptance tests. At very low temperature (163 K), the optimum video offset for each camera was determined by measuring the dark current in zero-exposure images and avoiding clipping the signal to zero DN. Most of the MI calibration was done at the extremes of the operating temperature range (218 K and 278 K) and at one intermediate temperature (263 K). All tests were successfully performed during the period July-September, 2002; 18.4 Gbytes of MI calibration data were generated and copied to the USGS for reduction and analysis.
The MI has several performance requirements:
- Instantaneous Field of View (IFOV) of 30 +/-1.5 micrometers/pixel on-axis
- Field of View (FOV) of 1024 x 1024 square pixels
- Spectral bandpass of 400-680 nanometers
- Effective depth of field of >+/-3 millimeters
- Optics MTF >0.35 at 30 lp/mm over spectral bandpass at best focus
- Radiometric calibration absolute accuracy of less than 20% and relative (pixel-to-pixel) accuracy of less than 5%
- Signal to Noise Ratio (SNR) >100 for exposures of >20% full well over the spectral bandpass within the calibrated operating temperature range
- Temperature sensor, accurate to +/-2 K, on the CCD package that can be read out and associated with the image data in telemetry
- Working f/# = 15 +/-0.75
- Operating temperature range within calibrated specifications = 218 +/-2 K to 278 +/-2 K
Other circumstances that would affect the performance of the MI are involved in the positioning of the IDD. The IDD positioning requirements are:
- Position instruments to an angular accuracy of 5 degrees in free space within the dexterous workspace of the Instrument Positioning System (IPS)
- Position instruments to a positional accuracy of 5 mm in free space within the dexterous workspace of the IPS
- Repeatably position instruments to +/-4 mm in position and +/-3 degrees in orientation
- Positioning each in situ payload element to within 10 mm of a science target that has not been previously contacted by another in situ instrument
- Orient each in situ payload element to within 10 degrees of normal to a science target's local surface that has not been previously contacted by another in situ instrument
- After placing the MI in position for imaging, the motion of the IDD shall damp down to an amplitude of less than 30 microns within 15 seconds
Detectors and Electronics
To reduce complexity and cost, all MER cameras share the same electronics design. Some aspects of the MER camera design were inherited from the cameras built for the Athena Precursor Experiment. The MER cameras include a Mitel front-side illuminated, frame-transfer charge-coupled device (CCD) with 1024 x 2048 pixels. Half of the array is covered by aluminum and is used for image storage during readout. Immediately following image integration of 0 to 335.5 seconds, the image is transferred into the storage area in 5.12 msec. Readout of a full image then requires 5.2 seconds, after which another integration may begin. The serial register has 16 extra reference pixels on each end that are read out along with each line of data. The reference pixels are not exposed to light and therefore measure the bias level as each line of data is read out. The value of the last reference pixel is always replaced with the camera serial number. Within the operating temperature range of 218 K to 278 K, the MI has a full well depth in excess of 140,000 electrons and read noise of about 30 electrons. The gain of the MER science cameras (~50 e-/DN) was designed to optimize the 12-bit digitization over the expected full well of the CCDs. The video offset can be set by command to bias the dynamic range of the CCD analog output relative to the range of the analog-to-digital converter. After conversion, 12-bit digital image data are sent to the rover computer. The non-operating (survival) temperature range of the cameras is 163 K to 328 K. The temperature of the MI CCD and electronics will not be controlled during flight, so variations in performance with Athena Microscopic Imager temperature were carefully measured. Temperature sensors on the MI CCDs and electronics will return data for each image obtained, allowing temperature calibration to be applied.
Simple image processing tasks can be performed onboard the rovers to correct for transfer smear, bad pixels, and flat field variations. These processing options can be applied in sequence or one at a time. The correction for frame transfer smear, or shutter effect, can be applied if the exposure time is less than a given threshold. This conditional shutter correction will be very useful in conjunction with autoexposure, when the exposure time will not be known in advance. If the shutter correction is applied, a zero-second exposure is acquired immediately after the image to be corrected and subtracted from the original image.
The MI has a Schott BG-40 (light blue) filter that yields a spectral response similar to that of the human eye. This restriction of the MI bandpass also increases the exposure time needed to image typical scenes on Mars and therefore reduces transfer smear.
The MI optics employ a fixed focus, f/15 Cooke triplet design that provides +/-3 mm depth-of-field at 30 micron/pixel sampling. The field of view is therefore 31.5 x 31.5 mm at the working distance. The focal length is 20.2 mm, and the working distance is 69 mm from the front of the lens barrel to the object plane. The first element in the optics assembly is a durable sapphire window that is less likely to be damaged by windblown debris or inadvertent contact with objects on Mars. It is included to protect the rest of the MI optics. The object to image distance of 100 mm was selected with instrument accommodation as the primary constraint. This design places the MI best focus position at approximately the same distance from the IDD turret axis as the target position for the other IDD instruments. Because the MI has a relatively small depth of field (+/- 3 mm), a single MI image of a rough surface will contain both focused and unfocused areas.
The MI is mounted on the IDD turret, between the RAT and the APXS.