In the lower carriage at both ends of each rover, there are stereo pairs of HazCams.  One pair faces forward, and one pair faces rearward.  These four engineering cameras are another part of the onboard autonomous navigation system.  Their primary purpose is to reveal dangerous objects to be avoided in the path of a rover as it drives in either direction.  But like the NavCams, the HazCams have been giving much science data too.

The HazCam optics are full-frame fisheye lenses.  Their field coverage is 124 degrees from side to side and 180 degrees across the diagonal of the square CCD format, that is, the diagonal field of view is a full hemisphere.  For such an extremely wide field, conventional distortion correction is impossible.  Instead, equal meridional angle increments in the object scene are mapped (ideally) as equal linear increments on the image plane.  When looking at the whole scene, a fisheye lens unavoidably produces a great amount of barrel distortion.  This distortion is actually just the consequence of projecting a hemisphere onto a plane.  Of course, the mapping function can be calibrated.  Of more practical importance, small parts of the scene, even those near the field edge, are imaged quite accurately without needing a lot of subsequent image processing.  But the horizon in most pictures appears strongly curved.

The HazCam lens design is shown in the layout in Figure 31.3(a).  Focal length is 5.58 mm in the center of the field.  They operate at f/15, view objects whose distances range from infinity to 200 mm, and are fixed-focused at 400 mm.  Stereo separation is 100 mm.  They are monochrome systems; as with the NavCams, a pair of absorption filters (Schott OG590 and KG5) working in series gives a reddish waveband extending from roughly 0.60 µm to 0.80 µm.  Absorption filters are used; quasi-reflective interference filters might produce too much stray light.

Figure 31.3(b) is the polychromatic spot diagram.  The scale bar length is 36 µm, the size of a 3 x 3 matrix of pixels.  The circles (which are radially stretched into ellipses off-axis) indicate the size of the Airy diffraction disk for a wavelength of 0.70 µm.  As with the NavCams, primary longitudinal color has not been corrected, but it is small enough to not matter.  At the edge of the field there is a complex mix of several aberrations, but lateral color is virtually zero.  Amazingly, the lens is diffraction limited all across its field.  Departures from perfect fisheye mapping are less than 3%.  Image illumination at the edge of the field is 36% of the central value (off-axis pupil growth, shift, and tilt, plus the large barrel distortion, prevent it from going to zero).

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