Figure 1. Location of the Secondary Mirror Assembly on the SOFIA observatory. The SMA is mounted  between three spider arms positioning the secondary mirror below the focus of the primary mirror, close  to the cavity opening exposed to harsh environmental conditions.  The Secondary Mirror Assembly  The Secondary Mirror Assembly (SMA) is a three-stage system that enables mirror movements in seven  degrees of freedom. The design can be separated into three subsystems, the light weight silicon carbide  mirror, the Tilt Chopping Mechanism (TCM), and the Focus Center Mechanism (FCM). The FCM is a  hexapod platform, attached to the spider arms of the telescope structure and enables mirror movements in  two rotational degrees of freedom (DoF) (tip-tilt) and three translational DoF to setup the M2 collimation  and focus position. The TCM is mounted on top of the FCM and enables high-frequency tip-tilt M2 motions  for infrared chopping and pointing corrections. When chopping (1125 arcsec max), the mirror is alternating  the field of view by following a square wave pattern up to 20 Hz at a settling time of about 10 ms. Stationed  in the Mojave Desert and flying in stratospheric altitudes, the typical mission temperature range is between  -60°C and 30°C. Figure 2 shows a schematic 120-deg section cut through the TCM presenting the main moving components  and all flexure elements of the mechanism. The system is driven by three moving magnet linear actuators.  The lever arm assembly amplifies the actuator force and reduces the stroke to allow high precision  positioning. Three pivot rods are transferring the actuator load to the mirror holder. A set of force sensors  is measuring the induced load. The mirror cell is suspended by a combination of a central pivot rod and a  membrane that allows the holder to rotate in the tip and tilt domain. The rod provides stiffness and prevents  motion in the focal direction. The membrane prevents side movements of the mirror holder and locks the  rotational degree of freedom around the focal axis. The silicon carbide mirror is mounted to the aluminum  mirror cell by three bipods to prevent stress due to thermal expansion.  A set of three eddy current sensors is used to measure the mirror position indirectly by tracking the motion  of the mirror cell. To eliminate any load induction to the telescope structure when chopping, the lever arm  assembly is tilting a ring-shaped reaction mass, the ‘compensation ring’, in the mirror's opposite direction.  This ring is mounted to the three lever arms by a set of flexures. All moving components of the TCM are  suspended in their center of mass to prevent any excitation due to external loads.  The main structural mode of the SMA is the chop motion of the TCM at about 32 Hz. The first dominant  unwanted resonance that results in mirror jitter is at 300 Hz. So far, this resonance is addressed by the  controller, reducing the unwanted mirror motion at the expense of mirror steering performance (refer to [3],  [4] and [5] for further information). Constraining this resonance on the hardware level would not only lead to a wider actuation bandwidth and therefore a faster transition between the two chop positions but also reduce the image jitter introduced by external disturbances acting on the active mechanism itself. From NASA/CP—2018-219887 166