32 Once the OTE is deployed, the primary and secondar y mirror support structures and mechanisms must  remain stable to prevent degradation of telescope  image quality and to eliminate the need for timeconsuming adjustment of the adaptiv e optics. The thermal stability r equirements of the primary mirror  wing latches and SMSS (shown in Table 3) are provided in the form of allowable distortions during the  worst case operational temperature swings. The s ources of these distortions include non-zero CTE  materials, variations in CTE within a material, manu facturing tolerances and thermal gradients across the  structures and mechanisms.  

Table 3.  Thermal Stability Requirements 

In addition to errors caused by changes in the therma l environment, there are allocations for errors due to  nano-lurching caused by stresses internal to the stru ctures and mechanisms and/or on-orbit loading. The  JWST image quality requirements allow for 20 nm of wavefront error (WFE) for these micro-dynamic  events. Because of these requirem ents, the systems must be designed with micro-dynamics in mind and  will be tested under operational load conditions to demonstrate stability within their allocation.  

Disturbances from the spacecraft (e.g. reaction wheels ) are attenuated by a 1-Hz isolator located at the  base of the OTE deployment tower. Deployed natural frequency requirements have also been set for the  OTE to further prevent the spacec raft disturbances from causing unac ceptable motions of the primary  mirror. These are referred to as line of sight (LOS) and WFE jitter requirements. The required natural frequencies for jitter are determined through integrated modeling using NASTRAN finite element math  models in conjunction with Code V optical analysis  models. Based on these models, the PM wings and  SMSS must have deployed natural frequencies of 15 Hz and 7 Hz respectively.  

The Design 

Design Practices for Deployable Optics

When we began our mechani sms design, NASA Langley Research Center, with the help of JPL and the  University of Colorado had recent ly published a study identifying guidelines for good design practices for  micro dynamically stable deployable optics. Much of the design practi ces, however, had not been proven  out in working designs. The approach NGST took for the PM and SMSS latch designs was to apply the published good design practices [1] that fit with our design approach, along with standard practices typically applied to mechanism design at NGST. T hen an in-house system was developed to test the  performance of the resulting hinge and latch designs.   Among the good design practices applied was to have kinematic (or quasi-kinematic) interfaces between  optical components. One of the primary benefits we sa w with a kinematic interface was the ability to  preclude the development of unwant ed interface loads (i.e., loads due to manufacturing and assembly  tolerances and thermal loading due to cool-down) , hence reducing the likelihood of friction-induced  slippage. In addition, we believed using this type of interface in a deployment latch would result in good  deployment repeatability. This is clearly dem onstrated in the PM wing latch design.   Non-conforming contact geometries (i.e., point or line contacts) were used at the latching interfaces on  the PM wing latches and for the mid-hinge of the SMSS. It is believed that this type of interface helps  insure that the interface stress distribution is ac curately known and prevents sensitivity to localized  imperfections over large mating surfaces. However, in our applications we found that trying to use non-conforming contact geometries in a truly kinematic configuration resulted in unacceptably low stiffness. PM Latch Distortion Parameter req't units Z translation (parallel to hinge line (HL)) 0.05 µm dec Y translation (perpendicular to HL) 0.1 µm dec Piston of Mirror Segment 0.005 µm piston Rotation about HL (Z) 0.01 µrad Rotation perpendicular to HL (about Y) 0.0025 µrad Gamma rotation(about X) 1 µradSM Motion  Parameter req't units Focus 1.34 m Translation 5 m Tip/Tilt 0.5 arcsec