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 3.3).
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| Figure 3.3 ETM+ Scan Line Correction |
A feedback control system is employed in the SLC drive electronics to achieve a constant angular rate of 576.6 mr/sec. The primary and redundant SLC drivers are implemented on the MEM A1 and A2 circuit cards respectively. The SLC driver produces a linear sawtooth signal with a fast flyback. The SLC driver receives a 208 kHz clock and a buffered line start signal from the Timing Control Subsystem. Table 3.3 lists the SLC characteristics.
| Table 3.3 Scan Line Corrector Design Parameters | |
|---|---|
| Scan frequency | 13.99 Hz |
| Scan period | 71.462 ms |
| Scan rate in object space | 9.610 mrad/sec |
| SLC rotation rate | 576.6 mrad/sec |
| SLC linear scan angle | 35.02 mrad |
| Linear scan angle in objece space | 583.7 microrad |
| Mirror separation | 1.600 in |
| Linear image displacement amplitude | .056 in |
| Linear image displacement rate | 9.22 in/sec |
| Mirror material | Nickel plated beryllium |
| Mirror coating | Enhance silver |
