The hardpoint attachment geometry causes these six readings to occur in an irregular coordinate system. A matrix, M HP , which depends on hardpoint geometry is used to transform the load cell readings into net forces on the mirror and net moments around the mirror center of gravity. E3 Check that the forces and moments from accelerations and servo errors are within the acceptable hardpoint load vector. The hardpoints measure the six dimensional error signal for the outer loop. Axial load is used to control application of the axial support forces.
Lateral load is used to control the lateral support forces and the axial correction forces.
The distribution of the axial and lateral support forces was optimized to preserve the shape of the mirror surface as the gravity load changed. The axial or rotational moment is used to control a linear gradient across the lateral actuators. In addition to tuning the final balance of the mirror, these linear gradients should compensate the small forces from wind and telescope acceleration. The previous servo loop algorithm distributed the axial and lateral forces on the mirror without ever explicitly calculating the elevation angle of the mirror cell or reading an encoder. Apparently round-off and timing errors in those very small corrections to the individual forces were accumulating to move the force distributions away from the nominal values.