It is necessary that the absolute three-axes orientation of a rover be known to enable its high-gain radio antenna dish to be accurately pointed at Earth.  The north heading must also be known while driving on the Martian surface.  With no global magnetic field on Mars to activate a compass, this information is derived from the apparent angular direction of the Sun relative to the rover's body (with help from a vertical-direction sensor and knowing the rover's location on the Martian surface and the time of day).  Shortly before a radio transmission, the Sun's position is observed and used to recalibrate the fiber-optics laser gyro in the inertial navigation system.

Originally, the direction of the Sun was to be measured by a single special camera on each rover called the SunCam.  It was to be mounted next to the antenna dish.  It was to have the same type of lens as the NavCam, except an extremely strong neutral-density filter would be placed in front.  However, as so often happens when building spacecraft, the rovers were exceeding their weight budget.  Although the SunCams had already been built, to reduce weight they were deleted.  Their function was taken over by the PanCams.  On each filter wheel, one of the 8 positions now has a strong neutral-density filter for viewing the Sun.  These solar observations are also useful in determining changes in atmospheric transparency, such as from dust clouds.

However, the SunCams did not go to waste; they later became the Descent Cameras.  The neutral-density filters were removed and one camera was remounted on the bottom of each landing module facing down.  Shortly before bounce-down on Mars, the Descent Camera took three images of the ground in quick succession and immediately compared them on-board.  If a significant sideways motion of the spacecraft was detected that might tear the airbags on landing, the maneuvering rockets could take it out.  This capability was indeed needed for the first rover, Spirit, although not for the second, Opportunity.

The optical prescriptions for the four Mars rover lenses are in the Appendix, along with the prescriptions for all the other lens examples in this book.