Mastcam Multispectral Imaging on the Mars Science Laboratory Rover: Wavelength Coverage and Imaging Strategies at the Gale Crater Field Site

Introduction: The Mars Science Laboratory (MSL) rover "Curiosity" will land in Gale crater on 6 August 2012 UT to begin a mission to investigate the habitability of an ancient aqueous sedimentary environment on Mars. MSL carries a comprehensive payload of remote sensing and in situ instruments designed to characterize the geology, elemental composition, mineralogy, and geochemistry of materials encountered along a traverse from the landing site crater floor plains and up into at least several hundred meters of section within a 5 km tall mound of layered sedimentary rocks [1-4]. An important payload element is the Mastcam imaging system [5], a pair of CCD cameras mounted on the rover's mast about 2 m above the surface and capable of obtaining high resolution visible to short-wave near-IR images of the scene. The left Mastcam (M-34) has a 34-mm focal length and a 18.4°x15° field of view (FOV) over the 1600x1200 pixels of its Kodak KAI-2020 interline transfer CCD. The right Mastcam (M-100) has a 100-mm focal length and uses the same kind of HD-format CCD to image a FOV of 6.3°x5.1°. Mastcam Multispectral Filters. Each Mastcam obtains images through a Bayer pattern of RGB filters and telecentric microlenses bonded onto the CCD (Figure 1; [6]) for nominal "human color" imaging, and an 8-position filter wheel in front of each camera's optics that provides the ability to obtain additional narrowband images through visible, near-IR, and solar neutral denstity filters. Three of the ten wavelengths of the narrowband "science filters" were selected to provide potentially-diagnostic information on iron-bearing minerals (e.g., Fe oxides like hematite, Fe silicates like pyroxene) and are similar to wavelengths used for the multispectral CCD imagers on the Mars Pathfinder lander [7], the Mars Exploration Rovers (MER) [8], and the Phoenix lander [9]. The effective band centers and band widths of the Mastcam broadband RGB and narrowband science filters were characterized using a monochromator during pre-flight calibration of the cameras, for comparison with design and component-level vendor specifications [5]. Table 1 provides a summary of the as-built system-level (CCD + optics + filter) effective wavelengths (λeff) and half-width at half-maximum bandwidths (HWHM) of the Mastcam filters; normalized bandpass profile curves are shown in Figure 2. Filters in wheel positions 1 through 6, plus the three RGB Bayer filters imaged through filter wheel position 0 (a broadband IR cutoff filter) provide the ability to image in 13 distinct wavelength bands for regions imaged with both Mastcams (but only over an imaging area of about 1/3 of the M-34's field of view because of the different focal lengths of the two cameras). Filter position 7 is a combination of a narrowband filter plus a 10 attenuating neutral density filter for direct "solar filter" imaging of the Sun in two wavelengths designed to distinguish between dust versus water ice cloud sources of opacity [10].