379 LOS path . Dynamic FEM s11, due to computationally intensive eigenvalu e calculations, were judiciously  reduced (limited to ~ 900 dynami c Degrees -of-Freedom (DOFs) and 4 12 nodes -gridpoints ) as compared to  much larger DOF s (>10 times)  in structures -stress static models . This helped offset  conflicting needs of  having adequat e analysis fidelity yet  computer runs ( via the most capable  computers available)  that could  complete  within  practical timeframes . These were t ypical ly a cluster of  all-night runs (~15 hour run- time if  not encountering convergence problems ) taking seve ral day s to compile 157 modes (0- to-75 Hz ). Many   cross- check s/iterations  occurred on the reduced dynamic FEM  to prove stiffness  properties  replicated more  refined stress FEMs plus mechanical assembly prototype test result s. High criticality was placed on meet ing  regular  re-analysis deadlines, so a significant amount of pre- computation effort s were expended on  sparsematrix sequencing methods and trial-and-error  connectivity -optimization schemes  to compress run-times   lower  within an assortment of additional compet ing constraints,  e.g., most capable computer available  (Sperry Univac 1100/92; 36 -bit, 133MFLOPS ) vying with top-priority classified programs’ runs.  

During a typical  LOS j itter evaluation ( re-assessed often due to evolving mass properties data , structural  refinements, and updates of  stiffness values),  that involved  checking all mechanical disturbers  (e.g., tape  recorders, high- gain antenna gimbals, RWAs ), an unexpected RWA jitter impact was unexpectedly   discovered coming from a very subtle model input coding er ror. Upon reviewing a typical  RWA overnight  run’s graphical jitter results, a high -amplitude ( i.e., ~5 times  normal;  significantly exceeding specification)  spike of jitter app eared at a previously unseen higher frequency ( ~30 Hz) that had no analog in any prior  results. This highly  anomalous result prompted an intensive and focused effort of input data review and rechecks/cross -checks. Using some model insights ( e.g., what compound dynamic modes were showing high  generalized responses near  that frequency) and localization ( e.g., what input -output response combinations  are proximate to responsive LOS optical elements) plus multiple trial-and-error eliminations, appearance  (and disappearance) of the large jitter value was related to a single  RWA mass property input line of code.  Although continuation of data values on a line was allowed by the model ing-analysis program  (with no error  indications) , data extensions were truncated/ignored at 80 characters due to prior program limits of punched  card input s not noted in computer code documents. S ubsequently , this got  correct ed to allow unlimited   input strings via continuation symbols.  However , this investigation created a totally  new serendipitous  insight  beyond just a trivial code error . This  tiny error caused a specific RWA mass property (local “rocking”  rotor inertia) to be reduced,  but valid within the code’s execution/error detection abilities . Thereby , on the  given base stiffness  (a validated value),  driving RW A local IV disturbance moment s to more effectively  stimulate a very -high-frequency complex mode (~90 Hz)  previously  not believed to be a jitter issue.  Amplified by the local RWA  mode, IV inputs were injecting much larger  generalized modal moments directly  into the rigid SSM  core/ base structure; in turn, efficiently driving a previously unrelated angular coupled  modal displacement at a higher and complex -system -mode frequency involving  a smaller relay Optical  Telescope Assembly optical element, namely secondary mirror motion. This creat ed the very first analytical  awareness that, after significant mitigation of  the 4 main RWA -scissors -mode/fine -balancing concerns   discussed above, a “new” system issue suddenly developed into a formidable jitter threat from RWA highfrequency inputs (not amenable to simple unbalance correcti ons) stimulating local IV -input modes that could  then couple with higher frequency resonant modes locally dithering optical components. Such highfrequency modal response, combining with test -derived low structural dampi ng (~0.005) at small forcing  ampli tudes, needed much less (~ 25 times ) input  forces/moments t han previous IV/unbalance disturbance   values . HST’s secondary mirror’s suspended mass is ~5% of the primary mirror’s mass ; minute angular  movements through a very high effective optical magnification displaced it sufficiently to impact LOS jitter.    This near -overnight revelation radically transformed previous jitter -problem paradigm s. E ngineering  pursuits immediately  shifted to ferret out high- frequency disturber  characteristics,  along with much more  attention to accurately  model refinement s of input disturbers ’ mounts  and regions near optics , especially  proximate support structure stiffness elements. All this while st ill adhering to severely limited FEM DOF  detailing/frequency -range constraints . This also led directly to assur ing highly optimized ( i.e., high local  stiffness  still within tight  mass limitations/budgets) RWA support -mount  structural element representations   to drive local response modes up in frequency (target >60 Hz , based on the next higher local compliance  being RWA mounting- tab-to-outer ring stiffness  that was difficult to redesign), without  creating new side