Last night a big worsening of the weather condition  arrived (fig 1) and produced the feared increase of correction on NE / WE mirrors, touching some time 10 V (fig 2). There was no need to wait for the strong wind of today, because the microseismic region, between 0.5 and 1 Hz, was enough to create problems (fig. 3).

Today we had in mind to test a filter for the windy condition, so we proceeded like that. In fig 4 we see that a result was achieved for the low frequency (200 mHz and < 100 mHz): a worsening of the sismic signal (first plot) did not produce an equivalent worsening of the correction. Unfortunately, we were again close to the instability at 1.8 Hz.

The next planned test was to move the marionette correction from ITMs to ETMs and see the effect on the 1,8 Hz instability. We started moving the north arm and the 1.8 Hz became quickly unstable, unlocking the ITF (fig 5, fig 6). Probably the right way to do the test would have been to move both the arms at the same time, but we had no opportunity to try.

We developed a new filter, which is supposed to gain less at low frequency, improve a bit between 0.5 and 1 Hz, and behave at 1.8 Hz about like the one which worked well the past days. We are waiting for a successful lock to know if it works.

In order to achieve LN2 in the current weather condition, a change of transition strategy was required, because normally we need to pass through a locking done by only a pair of mirror coils instead of all the four. If the mirror correction often exceeds 5 V like now, very likely it will saturate when 2 coils are used with the double gain. It happened like that the time we tried the normal transition. The new strategy consists in skipping the transition to LN1: setting directly the LN2 state for ETMs actuation and moving form ITMs to ETMs in this condition, we can continue using the four coils. The strategy worked, but it was not totally tested: the final step of disabling the ITMs high power state through the relays was not tested yet. The usual glitch caused by this action could be too large for LN2. The glitch could be eventually reduced because it is due to a DC value present on the high power actuation, which can be better subtracted digitally.