J. Oberling, R. Short

After the recent PRC alignment work (moving PR3 to better center the beam on PR2) the OpLev beam had fallen off of its QPD, so today I attempted to re-center the QPD.  Using only the translation stages, I could not re-center the QPD since the horizontal translation stage hit its max travel limit well before the beam was back on the QPD (SUM counts went from ~700 to ~850, well short of the usual ~22k).  I enlisted Ryan S. to help and we set out to adjust the OpLev beam alignment from the transmitter.  Turns out that the beam gets clipped before we can get it centered on the QPD.  We first tried to move the translation stage away from its limit but could only get the beam on 2 of the QPD segments (segments 1 and 3) before the SUM counts started dropping; at no point could we get the beam onto QPD segments 2 and 4 in this configuration.  We then moved the horizontal stage to its limit and tried to align the beam onto the QPD; this worked a little better, but still not great.  The best SUM counts we could get and have a "centered" QPD was ~17k, roughly 75% of the total SUM counts for this OpLev, so still some significant clipping.  We decided to unclip the beam to make the OpLev more trustworthy (we've had questions about this OpLev in the past).  Max SUM counts were ~22.7k, and this gives a Yaw reading of roughly -12.5 µrad; any closer to zero and the beam begins to clip again.  We left the OpLev in this configuration and will investigate the clipping at a later date.

Tagging OpsInfo: Around -12.5 µrad is the current "yaw zero" for the PR3 OpLev until we can figure out and alleviate the clipping.

As for the clipping, my best guess at this point is we're clipping somewhere on/in the ~4' long tube that runs from the exit viewport for the OpLev receiver and the OpLev's QPD.  We'll have to climb on top of HAM3 to check this, which we will do at a later date.

This closes WPs 12336 and 12341.

Edit: Fixed typo in yaw reading in bolded sentence (12.7 to 12.5).