Focus Mechanism Description  Concept Stepper Motor SM Carrier An initial challenge was to select a concept that  would efficiently constrain the mirror in all but one  direction, yet provide for precise focus control. The  concept in Figure 4 achieves this. The secondary mirror (SM) is held on a carrier and tube  suspended on two diaphragm flexures, which are  stiff in all but the focus direction. Focus is precisely controlled by a stepper motor acting though a lead  screw, lever arm, and flexure system, as shown.  Focus step size and position are simply  determined by counting motor steps. Advantages  of this concept are summarized as follows:  Lead Screw  Lever Arm Pivot &  Drive  Flexures  SM /g120 Simplicity – Minimizes cost /g120 Effectiveness – Meets requirements  /g120 Robustness – Carries high launch loads /g120 No free play in flexures – Repeatable /g120 Low actuation force – Low screw loads /g120 Symmetry of major supports – S table over  temperature Focus Mechanism Engineering Model (FM-EM) The FM-EM shown in Figure 5 was built and  tested to mitigate risk early. Key challenges then  were to find motor and lead screw components and a material suitable for flexures, for operation to 5 K.  A 2-phase stepper motor (Figures 8) was selected with a gearhead for further reduction to achieve the  desired output step size. Motor detent torque and the overall mechanical reduction also prevent  backdriving during launch. A ball screw (Figure 9) was selected to provide for a lo w friction, precision lead  screw. Both these components were tested for operation near 5 K, as described in their testing sections.  Titanium was selected for all the flexures and major structure. Beyond its other desirable properties, it  nearly matches the coefficient of thermal expansion (CTE) of the SM and metering tower (and flexures in  the SM mount and main housing legs compensate for the small mismatch). The FM-EM used 6Al-4V, due  to schedule. For flight, extra low interstitial titanium 6Al-4V (ELI) was chosen since it is generally tougher.   There was concern titanium may become too brittle at 5 K and not flex well over life. This was resolved by  determining its plane strain fracture toughness (K IC) was sufficient, as shown in the lesson learned below  and with fatigue analysis. Cryo-life testing of the FM-EM to 4X life further alleviated this concern.  Lesson learned #1:  Titanium is useful as a flexure material at cryogenic temperatures. 6Al-4V (ELI)  predicted K IC at 4 K is 54.9 MPa*      (50 ksi*     ) per the NASA/FLAGRO manual, JSC-22267A. This was  more than sufficient to meet the focus mechanism require ments for flexure operations below 5 K.  Additionally, 6Al-4V flexures were life tested in the FM-EM at 35 K to 4X life.  Other development included: For modularity of the SM mount and carrier, the carrier mounting screws are  accessible from the cover side. A SM and carrier mass simulator (shown in the photograph) was made for FM-EM testing. The drive flexure passes through the lever arm before attaching to it, to reduce its  deflection. Ball screw nut flexures provide compliance to prevent binding at the screw, yet are stiff in the  drive axis and prevent excessive rotational windup of the nut. And non-jamming hard stops on the screw provide for range limits. Housing  Carrier  TubeDiaphragm  FlexureCover  Focus  Direction  Figure 4.  Focus Mechanism Concept  in m 405