components in the focus mechanism, to minimize CTE mismatch and for optimum mechanical properties  for the pivot and drive flexures which are integral to the lever arm. 

Figure 13. Lever Arm 

Flexures   There are 4 flexures in the focus mechanism design. The pivot flexure, drive flexure, parallel double  bladed flexure and wishbone flexure. All four ar e fabricated from Titanium 6AL-4V and have been  analyzed and designed to have positive stress margins for launch loads and actuation cycles. A fatigue  analysis showed that the flexures will survive 86 lifetimes of actuations.    The parallel double bladed flexures cause the primary interface shelf to rotate a very small amount during  actuation of the focus me chanism. The movement of the interface shelf is not perfectly vertical. At the  extents of travel, the maximum rotation of the interface shelf is 0.21° and the maximum out of plane  displacement is 7.4 μm. This non-axial motion is mitigated with the compliant flexures in the struts that  attach the focus mechanisms to the primary mirror.   Lubrication

The lubricated elements associated with the Focus Mechanism assembly are the motor-gearhead assembly, transfer gears, ball screw, and the ball screw support bearings. All elements were lubricated at  Ball Aerospace, or in the case of the motor gears, under specific instructions from Ball. The selection of  lubrication is highly dependent on operating temperature. The minimum operational temperature requirement for the Focus Mechanism originally was -75C. This made lubrication selection difficult  because at this temperature, the wet lube becomes more visco us, increasing drag a nd friction in the  system, which results in a lower torque margin. Dry lube is an alternative at extremely low temperatures, but restrictions in the operating environment for dry lube increase the cost and test set-up complexity. For  this reason Braycote 601EF wet lube was chosen for all of the lubricated elements in the Focus  Mechanism. 

A torque test was performed on the ball scre w and support bearings usin g the chosen lube at a  temperature of -75C. The resulting rotating torques were dete rmined to be acceptable and used in initial  torque margin calculations.   Initially the Focus Mechanism design called for I 2R heating of the motor windings to warm the lube in the  motor components, which decreases viscosity, if it was required at cold temperatures. Thermal analysis  on the motor driver board later revealed  that the driver chips on this b oard would heat up too quickly and  reach dangerous levels. As a re sult of this, the design changed from I2R heating to a foil heater wrapped  around the motor body. Subsequent drag torque testing at the motor vendor revealed that heating the  gearmotor with a foil heater at -75C resulted in ample torque margin. Ultimately the Kepler thermal model was refined and the minimum operational temperature requirement was increased from -75C to -55C,  which resulted in even higher torque margins in the Focus Mechanism.   Redundancy

There are two LVDT sensors mounted on either side of the primary mirror interface shelf. Only one is  required for position feedback. Figure 14 shows one of the LVDT sensors on the focus mechanism.  368