The last of the four camera types is another science camera.  It is the Microscopic Imager, one of which is located on the end of each rover's arm (Instrument Deployment Device).  The camera can be maneuvered to get a detailed look at rocks and other nearby objects of interest.  Also on the arm are a Mössbauer Spectrometer, Alpha-Particle-X-Ray Spectrometer, and Rock Abrasion Tool.  The four instruments are mounted on a rotating turret so one or another can be brought into place as needed.  The front pair of HazCams is particularly useful in verifying how the arm is positioned.

The term Microscopic Imager may be a misnomer; it is not a compound microscope.  The object being viewed is larger than its image, and thus it is what photographers would call a macro close-up lens.  It serves the same function as a geologist's hand-magnifier loupe.

The lens design is a Cooke Triplet and is shown in the layout in Figure 31.4(a).  Focal length is 20.14 mm.  Optical magnification is 2.5-to-1 (or 1-to-0.4).  Thus a 30.7 mm square object area is imaged onto the 12.29 mm square CCD, and a 30 µm object is imaged onto each 12 µm pixel (giving 60 µm limiting object resolution on a pair of adjacent pixels).  At its finite working distance, the lens gathers an f/37.5 cone of light and forms an image having an f/15 cone of light.  Stereo imaging is possible by taking one picture, shifting the camera sideways a bit, and taking a second picture.  A sapphire window protects the lens in case it accidentally bumps against anything during its Martian adventures.  The in-focus working clearance between object and window is 63 mm, and the total length between object and image is 100 mm.

The system is monochrome with an absorption filter (Schott BG40) giving a waveband (width and shape) similar to the human photopic visual response.  Nevertheless, some color information can be obtained.  To protect the Microscopic Imager when not in use, there is a cover that can be closed.  To avoid the chance of a single-point failure by a stuck cover, the cover is transparent.  The lens can image right through the cover if need be.  The cover is also tinted orange.  Combining images made with the cover open and closed allows a quasi-color image to be synthesized (somewhat like the two-color Technicolor system of the 1920s described in Section A2.7).

To view a full-size image of an artist's rendition of the Mars Rover, click on the image to the left. Opens in new window.  Photo Courtesy of NASA/JPL-Caltech.

Figure 31.4(b) is the polychromatic spot diagram.  Again the scale bar length is 24 µm.  The circles indicate the diameter of the Airy diffraction disk for a wavelength of 0.54 µm.  On-axis, the main aberration is secondary longitudinal color.  Off-axis, there is added secondary lateral color and astigmatism.  Nevertheless, all the images are diffraction limited at best focus.  Depth of field is estimated to be at least 3 mm on each side of best focus.  In practice, the usable field depth may be considerably greater.  When in use on Mars, multiple images of rough surfaces are sometimes taken, with the camera distance incrementally stepped, to ensure that all parts are recorded in good focus.  Distortion is less than 0.01%.  Image illumination at the edge of the field is 89% of the central value.

(Actual pictures taken on Mars with the Microscopic Imager can be viewed on the last page of the article - Page 10)