Dan, Jeff, Keita, Koji

As part of the closeout activities for the HAM6 work, we have taken TFs for all the suspensions in the chamber.  The OMC-SUS, OM2 and OM3 all checked out fine -- current TFs are the same as the Phase3B measurements that Stuart took last week.

OM1 has changed, mostly in yaw.  The frequencies of the fundamental modes in longitude and pitch have shifted slightly -- this is not surprising since we have slightly changed the mass (the mirror) in the pendulum, and the change is small enough that it doesn't prevent our acceptance of the chamber for closeout.

In yaw, the fundamental mode has split into two high-Q peaks.  The mechanism for how this could happen, while not changing the peak structure in pitch or longitude, is not clear to us.  Rubbing would seem to be ruled out.  The eddy current dampers are a possibility, and the symmetry of the system (two dampers oriented horizontally, aligned to the middle of the optic in the vertical direction) seems to imply that a misalignment of the dampers would primarily couple to yaw.  One leading, somewhat unsatisfying hypothesis is that one of the dampers is too close to the mirror holder, and this is coupling the transverse (side-to-side) mode into yaw.  (But, why not transverse to longitudinal?  And why is this coupling coherent with an excitation in yaw?)  Another idea is that the flags have become misaligned relative to the BOSEMs in just the right way to couple this new mode into yaw.  But, this suffers from many of the same complaints as the first idea.

Regardless of the mechanism, the motion is yaw is well-damped using the local damping loops.  We think this means the motion will not be a problem during low-noise operations, and we are comfortable closing out HAM6.  We can fix it during a subsequent vent.

The attached figures are:

Fig 1 - comparison of OM1 Y-->Y TF with Stuart's measurement from last week.  Note that the Apr 3 measurement was in vacuum, today's measurement is in air with the HAM door on.

Fig 2 - comparison of Y-->Y TF for all OMs (top), comparison of the three DOF TFs for OM1 (bottom).  The bottom panel is meant to illustrate that we are not coupling a pitch or longitudinal mode into yaw.

Fig 3 - comparison of undamped/damped spectra for all OMs.  Dashed references are undamped, solid lines are damped.  Differences between the traces for each OM are due to uncompensated gain differences in the BOSEMs - this has been fixed since the plot was made.  The motion in OM1 yaw, quite large when undamped, is about the same as the other OMs when the damping loops are engaged.

Figs. 4, 5, 6 - A yaw excitation impresses the low-frequency peak onto the other DOFs; the structure is evident in all four BOSEMs; the low-frequency peak is present in a L-->Y TF.

Let's bolt on the door and pump down the chamber.