Hubble Space Telescope Pointing Performance Due to Micro-Dynamic Disturbances from the NICMOS Cryogenic Cooler

The Vibration Emittance Test (VET) was performed tomeasure the emitted disturbances of the Near InfraredCamera and Multi-Object Spectrometer (NICMOS)Cryogenic Cooler (NCC) in preparation for NCCinstallation onboard the Hubble Space Telescope (HST)during Servicing Mission 3B (SM3B). Details of theVET ground-test are described, including facilitycharacteristics, sensor complement and configuration,NCC suspension, and background noise measurements.Kinematic equations used to compute NCC mass centerdisplacements and accelerations from rawmeasurements are presented, and dynamic equations ofmotion for the NCC VET system are developed andverified using modal test data. A MIMO linearfrequency-domain analysis method is used to computeNCC-induced loads and HST boresight jitter from VETmeasurements. These results are verified by a non-linear time-domain analysis approach using a high-fidelity structural dynamics and pointing controlsimulation for HST. NCC emitted acceleration levelsnot exceeding 35 micro-g rms were measured in theVET, and analysis methods herein predict 3.1 milli-arcseeonds rms jitter for HST on-orbit. Because theNCC is predicted to become the predominantdisturbance source for HST, VET results indicate thatHST will continue to meet the 7 miili-arcsecondpointing stability mission requirement in the post-SM3B era.The purpose of this document is to describe details ofthe Vibration Emittance Test, define the analysismethod used to compute NCC-induced disturbanceloads, and predict Hubble Space Telescope on-orbitjitter due to the NCC. Four objectives of this work arelisted in order of descending importance: 1) verify thatHST will comply with the 7 milli-arcsecond pointingstability requirement after the installation of theNICMOS Cooling System (NCS) which includes theNCC, 2) quantify NCC-induced disturbances asmeasured in the VET, 3) characterize the spectralcontent of NCC emitted disturbances, and 4) assessstructural dynamic coupling between NCC emitteddisturbances and other HST structural modes.Beyond application for HST, this work illustrates threeitems significant for structural dynamic analysis onother spacecraft programs. Methods are presented toquantify and characterize micro-dynamic disturbances(greater than 9 micro-g rms) in ground test over a broadfrequency range (0.001 Hz - 50 Hz) for application toprecision orbiting platforms. The emitted disturbancesfrom one cryogenic cooling machine are characterizedspectrally, revealing spectral content at integermultiples of the fundamental disturbance frequency.Finally, the NCC demonstrates new cryogenic coolingtechnology applicable to other infrared science spacemissions with stringent disturbance requirements.This material is declareda work of the U.S. Governmentandis notsubjectto copyrightprotectionin the UnitedStates.1American Institute of Aeronautics and Astronautics