/g120 Likewise, another secondary measurement technique was provided for tilt of the mirror simulator  in two axes (tip / tilt) using two quad-cell autocollimators, on a stable base. One measured mirror simulator tip/tilt motion and the other measured t he reference plate tip/tilt motion. The difference  between these measurements represented the desired tip / tilt motion, relative to the plate.  /g120 The dewar has two stages, one for liquid nitrogen and one for liquid he lium. It was modified to fit  windows for the secondary measurement techniques.    /g120 Thermocouples were used to monitor temperature at various locations.  Success Criteria Meet the key performance requirements, as shown in Table 1, over ambient and cryogenic temperatures,  before and after vibration testing, and over 1X life for the FM-EM, and as a goal for up to 4X life.  Results The FM-EM met the above success criteria. Table 1 shows the FM-EM performance before and after  launch vibration and over 1X life. Performance did not change appreciably for up to 4X life, meeting the  goal to provide significant life margin.  Table 1 shows maximum values for step size and repeatability. The mean and standard deviation values  for these over 1X life are provided in Table 5.   Table 5.  Mean and Standard Deviation for FM-EM Step Size and Repeatability  Parameter Mean Standard Deviation  Step size 0.42/g80m 0.17 /g80m Repeatability (bi-directional) 0.0005/g80m 0.29 /g80m The FLT-FM performance was similar, based on abbreviated testing. A difficulty occurred during FLT-FM  testing when it was erroneously vibration tested to 170 G in the lateral direction, due to a faulty control of  the vibration table. This was far beyond the required 70 G.  The effects of the 170 G over-test were carefully evaluated. Fortunately, due to the robustness and  conservatism in the design, there were relatively few issue s. However, the ball scre w was loaded beyond  its rating. So we loaded a spare flight ball screw to the same over-test level and well beyond. Based on  inspections of the overloaded spare ball screw, we concluded the flight ball screw was still acceptable for  use. This 170 G over-test also reduced the fatigue life of the flexures from 10X to 6X. But this was  deemed acceptable, with customer approval.   Notable Items Contributing to Successful Testing : /g120 The FM-EM was tested at 30 K to 49 K since the dewar had only one, liquid nitrogen shroud and due  to parasitic radiation from the optical windows. This was accepted since the motor and ball screw  components were tested near 5 K. Also, most material shrinkage occurs by these temperatures and  the predicted toughness (K IC) for titanium at 5 K was more than acceptable.   /g120 Including secondary measurement techniques proved to be a prudent decision. As anticipated might  happen, the VIT calibration changed below 80 K. Using interferometer and autocollimator data, it was found the slope of the calibration changed but the VITs retained good linearity. After accounting for  this, the data between all the measure ment methods agreed well. This also allowed for interpretation  of VIT data measuring the lateral motions, which did not have the secondary measurements.  /g120 Vibration testing without force limiting showed high magnification factors at resonant frequencies that  would have resulted in over-testing the mechanisms capability, if vibrated to the required input levels on a standard shaker. As recommended by JPL, using force limiting [2] resolved this problem and it  was successfully tested to its full levels. (Force limiting is now more routinely used at BATC)  412