The Scan Line Corrector (SLC) is an electro-optical mechanism composed of two parallel nickel-plated beryllium mirrors set at an angle on a shaft. The shaft, driven by a torquer which has primary and redundant tachometers, rotates about an axis normal to the axis of the scan mirror in a sawtooth fashion. The SLC is positioned behind the primary optics and compensates for the along track motion of the spacecraft that occurs during an active SMA cross track scan. A rectilinear scan pattern is produced using the SLC instead of the zig-zag pattern that would be produced without it (Figure 13.12).