hugh.radkins@LIGO.ORG - posted 17:38, Monday 12 December 2011 - last comment - 17:36, Tuesday 13 December 2011(1891)
BSC8 ISI Optical Table In-Chamber Survey
We did a Differential Elevation Survey of the BSC8 Optical Table this afternoon. Attached is my survey note page(just the right side.) We observed the mon TP202 (see T1100187) which was surveyed wrt BSC2 way back. Then looked in the chamber where Jim held the rod up against the Optical Table. We took several shots. I then kicked a leg of the Level and shot again and turned back to the starting monument. My survey closed to within 0.1mm--this means I at least didn't read any numbers wrong. All the elevation on the Optical Table were within 0.5mm. This is good in that we leveled the Support Tubes from the outside quite awhile ago and have since pulled the iLIGO CAS stack and swapped in HEPI and the Crossbeams are currently sitting on mechanical stops. I wouldn't have been too surprised if the table had been more out of level. I believe this puts the table very close to our requirement of within .5mrad level. This of course can (will?) be repeated once we float the optical--we certainly can get it more level. The average of the 5 shots on the table is 1662.6mm above BSC2(0,0,0). That is in local elevation. We subtract 0.1mm to put that in the Global frame. I don't know what the Optical Table elevation should be at this point, which is good, that way you know I'm not cheating! I'll look into this or ask someone and update you all soon!
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Comments related to this report
According to E1100690-v8_WBSC8 Initial Alignment Procedure: "The local BSC chamber coordinate system origin is nominally located 65.421 in [1661.7 mm] below the BSC-ISI optics table surface." This would be the end of it except the chambers aren't necessarily exactly positioned on the global coordinate system. The Global vertical zero is 0.1mm above the local gravity zero at BSC8-See T1100187 & T980044. Our survey yesterday puts the Optical Table at 1662.6mm above the local level where BSC2 center is 0,0,0. We subtract the 0.1mm from our survey result to put the elevation of the Optical Table into the global frame. So the Optical Table is 1662.6 - 0.1 - 1661.7 = 0.8mm higher than nominal. Not too bad since we set the support tubes 15 to 30mils high shooting the bottom side of the support tubes out side the chamber. I was expecting some depression when the Cartridge landed but we didn't measure that as the dial indicators were no installed. Any way, the level of the Optical Table is not to spec according to the requirement in E1100690 although I didn't think the requirement was as low as .1mrads; currently we have 0.53mrads. However the elevation is within spec, if, if the ITM was not too far off in spec when it was measured at the test stand. You know, the errors may combine to be fine or they may conspire to take the ITM out of position vertically. I don't know that yet. Regardless, the level will have to be tweaked anyway so we can fine tune the elevation as well. If anyone doesn't understand my surveying notes or other gyrations, please feel free to ask.