Today we investigated the DARM offset with the following procedure:

• we locked at first in LOW_NOISE_1, then we made a full scan of OMC1 of 2 FSRs, using the usual DARM offset and then the same one but with opposite sign; unfortunately this last one measurement was spoiled by a non optimal working point of the DIFFp loop;
• we worked then on such working point, and we found out that on the other side of DARM the offsets for both TX and TY of the DIFFp loop had to change sign as well, in order to restore the usual dark fringe behaviour;
• just for curiosity we tried to flip also the BS_TY offset (BS_TX does not have one) but the effects were very bad from the CMRF point of view (Figure 1: blue is Monday, Gold is today with usual offset, Purple is today with flipped offset);
• in order to hand-off DARM correctly to DC Readout, the DC Readout global sensing has to change sign (i.e. the weight of LSC_B1_DC_INPUT inside LSC_DARM_INPUT); the particular weights and gains of B1s2 and B1 do not, as the global sign change takes care of everything;
• we made a scan of a few points of the DARM offset while still on the other side, from -1e-3 to -4e-3 (3e-3 is the standard), adjusting the DARM loop gain in the extremal points to cope with the change of the UGF; precise GPSs can be found by looking at the LSC_DARM_SET channel, approximate times are the following:
  • 21:52 LT: DARM_SET = -3e-3 (dur = 300s);
  • 21:59 LT: DARM_SET = -2e-3 (dur = 300s);
  • 22:13 LT: DARM_SET = -1e-3 (dur = 300s);
  • 22:21 LT: DARM_SET = -4e-3 (dur = 300s);
• at 22:31 LT we engaged the full bandwidth loop for BS, but only on TX, manually disaabling the TY one on the DSP card.

The current lock is in the usual configuration, just with this flipped DARM offset; however, the Automation has already been reverted, and on the next lock the usual configuration will be applied (BS_TX in full bandwidth included).

Notes:

• from now on, to explore either side of DARM it is sufficient to change the several 'darm_offset' values inside ITF_LOCK.ini; the DC Readout path and the DIFFp offsets are automatically changed on the basis of numpy.sign(darm_offset);
• the final FmodErr signal had a weird behaviour, similar to a sign flip, when changing the BS_TY setpoint (Figure 2);
• we set up a quick workaround for the LNFS bug reported recently; we will see in the next unlocks how it is working; the idea is to have a timer after which the reset command is sent again;
• both h(t) and the Ranges have not been computed for the whole shift, as changing side to DARM made them negative. In the forthcoming analysis we will have to use DARM.