the 30° steps of the stepp er moto r, and  the planeta ry/spur g ear reduction s, each moto r step resulted in  ~0.6 a rcsec of beam deviation. 

Torque m argin was measured by dete rmining th e minimum volta ge at which the me chanism wo uld still  step consistently durin g the therm al vacuum  testin g of the me chanism  at bot h tempe rature extreme s. It  was dete rmined that the BAM/BAME could be op erated with voltage s as lo w as 6V un der worst-ca se  begin ning of life conditio ns. Give n the no minal operatin g volt age of +30V , this m argin was deemed  sufficie nt for this rel atively low duty cycle appli cation. Durin g the test program  over 80,0 00 steps we re  logge d on the  actuato rs without inci dent.  

Although the re wa s no redunda ncy req uirement pla ced on the BA M, a failsafe feature was inco rporated  into the de sign that wo uld allow fo r continued operation of the BA M given the f ailure of one of the drive   assembli es. Backdrive  pins protrude  from the i nner surfaces of th e rotatin g shaft s of each drive assembly.  They overl apped in the axial dire ction such th at if one drive a ssembly contin ued to rotate it  would engage  the pin of the second d rive assembly th us ba cking driving the se cond drive a ssembly. It would therefo re  be po ssible t o rotate b oth wedges to an y desired p osition. In additi on, if for any reason th e orientation of  the two prism s relative to each other was in dou bt, the back drivi ng pins could  be used to re-e stabli sh a  known relatio nship betwee n the prisms.  

Prism Mo unts and Prelo ading  The pri sm assembli es where taken a s a “qualified ” assembly. The f used silica prisms are b onded into an   invar rin g. These a ssembli es were p reloaded a nd lo cked in rotatio n by a custo m desig ned ti tanium wave   spring. Th e spring  provide d axial p reload of the prism cell to the sh aft while also enga ging ang ular slot s in  both the cell and the capture ring to ensure that t he prism could not rotate relative to the shaft. 

Bearin gs and Lubri cation   The b all bearings (MPB # 4050 ) were 4 40C stainl ess angul ar contact ball bearings mounted in a b ack to   back configu ration with a spa cer betwe en the outer-r aces a nd were p reloaded with a cu stom desi gned  titanium wave sprin g applying 13 3 N (~30  lb) to th e inn er race. They were cleaned post delivery and  lubricated at GSFC using Braycote  815z oil and 601 grease. Th e output pi nion and spur ge ar were also  lubricated with a light coa ting of the same lubri cant. This lubricant was selected prim arily for its low  outga ssing chara cteristics and  availabil ity. The a ctuator was also lub ricated with the  sam e lubricant b y  the vendo r. An anti-wetting coating  was applie d to critical surfaces to in hibit creep  of the  lubri cant to  unwanted surface s. Great  care wa s taken to en sure that no lub ricant cam e in conta ct with the pri sm  assembli es or any surface s they interface with. The re wa s concern t hat the high-energy laser radiatio n  may bre akdown the l ubricant and eithe r reduce optical throu ghput or cause fail ure of the co atings appli ed  to the p risms which could prove to b e a catastro phic failure of the la ser tran smit path. The tran sparency of   Braycote to t he wavel engths of light in question was investigated but it was d etermined that the effects of   the high energy radiation on the lubri cant were unknown. The sam e lubric ant was al so utilized in the l aser  assembli es, which had even more stri ngent co ntami nation re quirements.  

GSE Sens ors  Two sen sors were incorpo rated into the  BAM for gro und testin g purposes. The  BAME had no available  telemetry, so  the se nsors we re operated an d mo nitored usin g Grou nd Support Electronics (GSE). A   magneto -resistive se nsor was pla ced in proximity to  the output  pinion of e ach of the a ctuators. The   pinion s were mad e of a soft ma gneti c materi al. As t he te eth of  the pi nion passed  by the  sensor, th e  variations in th e gap were sensed and recorded. The resolution of the se nsors pr ovide d an easily vi sible  respon se for each an d every ste p take n by the mot ors. This val uable data was u sed to v erify that the   motor n ever missed steps duri ng characteri zatio n, thermal va cuum, and EMI /EMC testing  as well as  provide d a m eans of dete rmining to rque margin. Althoug h there i s no reason that this sensor could not  have flown, the plan was to remove it prior to flight. 

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