16 The flex pivots  selected were typical brazed type rather than the higher strength electron- beam welded  type. Elect ron-beam welded type flex  pivot  fatigue life is recommended to be derated according to a  technical note by Donegan, Richard J., “Weld versus Braze,” (n.d.) at the Riverhaw k Co., Inc.  website [2].  

It has become  a common  practice of the GSFC  Electro- Mechanical Systems  branch  to incorporate  significant damping into actuators  of precision pointing and scanning mechanisms as a means to reduce  the effect of both external and internal disturbances, reducing controller burden, thus improving success in   meeting performance requirements . In linear actuators, t his is accomplished by a pair of connected copper  sleeves that surround both the OD and ID of the bobbin. Often, it would be desired to have even higher  damping than we incorporate, but it can become impractical due to actuator size limitations . Scan rates   must be low enough to incorporate high damping such that actuator power to overcome damping is not  significant. For the SMM, the actuator is a stretched version of the BSM actuator in order to meet SMM  stroke needs , but damping is roughly  doubled due to a much lower on- orbit operational temperature range  of -120°C to -89°C, which reduces resistance  thus increasing damping.  At the low temperature extreme,  the system is nearly critically damped.  

The SMM Mechanism Control Electronics (MCE)  incorporates a Field- Programmable Gate Array ( FPGA) .  The Jovian orbit puts the spacecraft rather far from the sun,  reducing solar array effectiveness . This resulted  in the  imposition of  a power requirement for the MCE  of less than 4  watts , significantly lower than the BSM  MCE  which required 13 watts . Fortunately, the sensor system is a relatively low power device requiring  0.4 watt. The BSM MCE  utilized high resolution ADC s with low -noise op amps for the feedback signal.  Identical  components were unnecessarily used in greater quantities for telemetry, along with multiplexers,  all of which are rather power -hungry  components. For the SMM MCE , significantly reducing the number of  telemetry signals , implementing standard noise op amps,  and lower resolution ADCs with built in  multiplexing,  reduced power considerably . For the controller feedback signals, the heritage components  were kept  to maintain optimal closed- loop performance . Power savings also resulted from the reduction in  components since only  single -axis control  was needed. Substituting a low quiescent power amplifier for  driving the actuator pair  saved  an additional 0.5 watt  over the previously used power op- amp.  

Scan Mirror Optical  Challenges  

The primary challenge of the Scan Mirror design depicted in Figure 3 was to satisfy the flatness requirement  of the optical surface, 45 nanometers RMS, whi le balancing size and inertia. In order to minimize thermal  distortion effects, the primary material for the structure of the Telescope Assembly, including all mirrors,  was selected to be aluminum  6061- T651. All components which contribute to the system alignment were  thermal cycled during the fabrication process to provide dimensional stability in the oper ating temperature  environment. The optical surfaces were diamond turned and then silver coated  for optimal reflectance in  the specified wavelength range.  The Scan Mirror geometry was selected to minimize flatness impacts from  gravity release, diamond turning “fling ,” assembly, and on- orbit thermal gradient effects.  Pocketing and  tapering of the mirror were  used to reduce mass while maintaining stiffness.  All of these sensitivities were  predicted and the design guided by finite element and other analysis tools . The Scan Mirror also had to  possess features for mechanical interface, alignment references and fiducials, as well as conductive targets  within the back of the mirror for the inductive  displacement sensors.  

The Scan Mirror is supported by three a luminum  blade -style flexures to provide a secure and reliable load  path that is tolerant to mounting process imperf ections and thermal gradients. Alloy 7075- T651 was  selected to provide the strength necessary to survive launch loads without the coefficient of linear thermal  expansion ( CTE) change that would be necessary with a more traditional t itanium or other alternative  material flexure.  The flexures feature clearances at pin locations to absorb manufacturing tolerances.  The  flexures are bolted to the mirror and moving housing with accompanying plates that possess cavities which,  during assembly,  are injected with epoxy to register the flexure position to the pins. Figure 4 depicts a  flexure as well as an accompanying plate configured for this process, known as “liquid pinning.”