On wednesday evening (Dec 19), we performed several measurements in order to investigate how to better tune the OMC mode matching. During the shift we tested 5 different configurations:

- ITF locked in Low Noise 1, with the DARM offset adjusted to have the same power in the carrier as on Dec 7 (see entry 43962) ;

- Single bounce beam (NI mirror aligned, NE WI and WE misaligned) with the TCS DAS still ON ;

- North arm locked with the TCS DAS still ON ;

- Single bounce beam with the TCS DAS switched off ;

- North arm locked with the TCS DAS switched off.

For each ITF configuration we tested several longitudinal positions of SR. For each SR position, we ran a temperature scan of the OMC in order to evaluate the amplitude of the TEM00, the first and second order modes.

The DAS was switched off at 18h57 utc and switched back on at 21h43 utc.

The detailed list of measurements with UTC times is given in the first attached file.

The results of the OMC scans performed with single bounce beam and north arm locked, with or without DAS, are summarized in a table inside the second attachment. The preliminary analysis leads to the following conclusions:

* With the north arm locked, the state of the DAS has no impact on the mode matching.

* With the single bounce, the mode matching tuning is worsened by a factor 2-3 when the DAS is off, but stays relatively good (mismatch lower than 2%).

* There is no clear effect (or at least not big) on the mode matching when moving SR longitudinal position from -50 to +550. We noticed a small worsening of the mode matching when moving SR to -1000, which seems consistent with what was previously observed in low noise 3 with the optical gain (entry 43962).

* The OMC alignment is better when SR is at -1000. This is very likely due to the fact that the OMC alignment was optimized while SR was close to this position.

* The OMC alignment is better with the north arm locked than with the single bounce. This is probably due to the fact that the locked arm helps keeping NI well aligned.

Analysis of the low noise 1 data is still in progress. During these measurements in LN1 we noticed that the B1p_DC power was fluctuating quite a lot, indicating alignment fluctuations in the ITF. This clearly impacted the amplitudes of the modes observed during the OMC scans and makes the interpretation of these scans more difficult. Moreover in this configuration, the amplitudes of the first and second order modes only give at best upper limits on the alignment and mode matching tuning, given the fact that the dark fringe signal is spoiled by side bands and high order modes present in the ITF.