(Cheryl, Chris)

In order to set the Gouy phase of the beam on the IMC WFS, we used the NanoScan to check the beam profile after the L3 lens on IOT2L.  Compared to a previous beamscan at this location (alog 5131), it looks like we still have essentially the same beam.  Both the horizontal axis and the vertical axis were measured -- and they differ slightly about where we should place the WFS.  So we will split the difference and place the WFS in between the two indicated positions.

The attached plot shows the measurements, the fits, and the magnitude of the disagreement, which amounts to +/- 10 degrees of Gouy phase.  The upper panel plots the 1/e^2 radius, and the lower panel plots the Gouy phase accumulated from each profile's waist.  The vertical lines correspond to the ideal WFS positions, as indicated by each measurement.  (We want to put the WFS at +/- 1 Rayleigh length from the waist, for a Gouy phase difference of 90 degrees.  At the intermediate position, one profile gives 81 degrees difference and the other gives 98.)

Also to note: the beam path wasn't quite level when we started -- so we had to temporarily remove both L2 and L3 lenses, and touch the M6 steering mirror in order to fix it.  As a result, the MC REFL PD has to be realigned (we'll take care of this today along with placing and aligning the WFS).