jason.oberling@LIGO.ORG - posted 17:13, Tuesday 01 May 2012 (2739)
BSC06 ETMy Pitch Alignment and XY Position Check
ETMy Pitch Alignment
IAS: J. Oberling
SUS: T. Sadecki
We aligned the ETMy pitch this morning. We started at 300 µrad down and after a series of adjustments left the pitch at 658 µrad down. Our target for mechanical adjustment is 639 ± 160 µrad, so this is well within our spec.
ISI XY Position Check
IAS: J. Oberling
This afternoon I checked the lateral (X) and axial (Y) position of the ISI table. Since we are using the PLX retroreflector (those of you that have been in chamber have probably noticed the large structure sitting in the spool piece; this is the PLX) and setting the total station up behind BSC06 we can not directly measure the position of the ETMy. What we decided to to instead was to measure the position of the ISI and use the position numbers from the test stand alignment to then calculate the ETMy position.
The ISI position is measured in two steps: Y position and X position. We had set a monument on the North side of BSC06. This monument represents the desired Y position of the ISI, corrected for the +4mm of the ETMy axial positioning measured on the test stand. In other words, for the ETMy to be at its correct axial position, the Y centerline of the ISI (line is perpendicular to the optical axis of the Y arm cavity) should line up with this monument. To measure this, we set a total station on the monument, bucked in on the North offset line and turned it 90 degrees. We then used a bracket designed to hang a plumb bob from the ISI centerline, since the centerline falls in between screw holes on the ISI table; the bracket has two holes for attaching to the ISI table and smaller hole in between the two larger for hanging the plumb bob. We can sight the wire used to hang the plumb bob with the total station and measure the angle; it is then a matter of simple trig to calculate the position error of the centerline.
Unfortunately, the bracket was designed with 2" between the screw holes instead of 1 (the ISI table is on a 1" hole pattern). This makes it impossible to use the bracket as-is for its intended purpose. To get around this I used a bar supplied by Travis to clamp the bracket in position. This worked, but since the bracket is being aligned visually an approximate ±1mm error is introduced into the position of the plumb bob hole (the centerlines of the ISI are scribed, so there is a visual reference for placement of the bracket, but they are wide, ~2mm). I was then able to sight the wire and measure the angle to it. As the plumb bob was swinging slightly I had to sight the edges of the swing and find the midpoint. There is some error here in sighting the wire as it swings. I was able to see angular changes of ~5 arcseconds, which leads to a position error of approx. ±0.08mm. This puts the error estimate for this measurement at approx. ±1.08mm. With that in mind here are the numbers for the ISI position, reported as distance from its nominal position:
- ISI X position: 2.7mm South
- ISI Y position: 1.2mm West
Correcting these numbers with the ETMy test stand positions gives the current ETMy position errors (error spec is ±1.0mm for X, ±3.0mm for Y):
-
ETMy X position: 1.7mm South
- ETMy was 0.97mm North on the test stand
-
ETMy Y position: 1.2mm West
- The +4mm test stand error was already corrected for with the setting of the monument, so no change
With the estimated error, this puts the ETMy lateral position either within spec or wildly out of spec. The axial position is in spec regardless.
