Hubble Sp ace Telescope Fine Guidance Se nsor Po st-Flight Bearing Inspection 

J. Pellicciotti*, S. Loewenth al**, W. J ones J r.+ and M. Jumpe r++ 

Abstract 

Aero space mech anism engin eering su ccess sto ries o ften, if not always, consi st of overcomi ng  developm ental, test an d flight ano malies. Ma ny time s it i s these ano malie s that stimulat e technolo gy  growth and more reliabl e future syste ms. Ho weve r, one must le arn fro m these to achieve an ultimately   successful mi ssion. 

It is not often that a spa cecraft en ginee r is abl e to insp ect ha rdware that ha s flown in o rbit for seve ral  years. However, in Feb ruary 1997, the Fine Gui dance Sensor-1 (FGS-1 ) was removed fro m the Hubbl e  Space T elescope (HST) and returned to NASA Goddard Sp ace Flight Center (GSF C) during the second  Servicin g Mission (SM2 ). At the time of removal, FG S-1 had nearly  7 years of service and th e beari ngs in  the Star Selector Servos  (SSS) had accumul ated approximately 25 million Coarse Track (CT) cy cles. The  main rea son for its replacement was due to a bearing torque anomaly leadi ng to stalli ng of the B Star  Selec tor Serv o (SSS-B) when revers ing direc tion during a vehic le offset maneuver, referred to herein as a  Revers al Bump (RB). The returned HS T FGS SSS bearin gs were disassembled fo r pos t-servic e condition  asse ssment to bette r understan d the actual cau se of the to rque spike s, identify potential  process/design  improvem ents, and p rovide informatio n for rem edial o n-orbit ope ration modifications. 

The meth ods and te chnology utilized fo r this in spection are not u nique to thi s system a nd can be ad apted   to most inve stigations at v arying stage s of the me chanism life fro m developm ent, through te sting, to po st  flight evaluati on. The systemat ic methods used for the HST Fi ne Gui dance Sensor (FGS) SSS and  specific findin gs are the subject s presented in thi s paper. T he le ssons lea rned include the  importan ce of  cleanline ss and handlin g for precision instrument bearings and the pote ntial ef fects f rom contamination.   The paper describes in detail, the analytical techni ques used for the SSS and t heir import ance in this  investigatio n. Inspe ction a nalytical data a nd photo graphs are incl uded throu ghout the paper. 

Introduc tion & FGS De scription 

Hubble's three FGS’s — it s targeting camera s — provide feedb ack u sed to ma neuver the te lescop e and  perfo rm cele stial mea surements. T wo of the sen sors point the te lescop e at a n astronomi cal target an d  then hol d tha t target in a scientific in strume nt's fiel d of view. Th e third sensor is available  to perfo rm  scientific o bservation s. The sensors aim the tele scope by lo cking onto " guide sta rs" an d mea sure the  positio n of th e telesco pe relative to th e obje ct bei ng viewe d. Adjustme nts b ased on th ese constant,  minute mea surem ents keep Hub ble poi nted pr ecisely in the right dire ction.   

Each FGS en closure houses a very precise optical interferomete r. The pointin g control system uses the  Fine Guid ance Senso rs to point the telescop e at a ta rget with a n accura cy of  0.01 arcsec. The se nsors   detect when the telescope drifts wh ich gives Hubble the ability to rema in pointed at that target with no  more th an 0. 007 a rcsec of deviation ov er lon g periods of time. T his level of stability and preci sion is like  being a ble to hold a lase r beam focuse d on a dime 320 km (200 miles) a way (about the distan ce from  Washington, D.C. to Ne w York ) for 24 h ours. 

The Star S elector Servo s “A” a nd “B” a ssemblie s move indep endently of each o ther to affe ct the tilt of the   wave front through the instrument. Each SSS is  comprised of a brus hless  DC mo tor and 21-bit enc oder                                                         * NASA Goddard Spac e Flight Center, Greenbelt, MD  ** Lockhee d Martin Spa ce Systems Co mpany, Sunn yvale, CA  + SEST Inc ., Middleburg Heights , OH  ++ BEI Precisi on Systems & Space Divi sion, Mau melle, AR 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  343