Laser frequency noise has been a major limitations during these measurements, making any conclusions on the effect on the BNS range in general, or on the HOM noise (mystery noise) difficult to make. The higher laser frequency noise is due to something that happened on Oct 7 around 19:00 UTC and above 300Hz due to the more noisy LNFS: https://logbook.virgo-gw.eu/virgo/?r=69929.

Figure 1 shows a noise budget in the starting tuned SR configuration. The fitted HOM noise level is comparable to the case of January, when the first stage of SR diaphragm was already installed, for the times when the etalon effect was making the B1p higher. It is unclear in what position the etalon effect was on Oct 14. The estimated frequency noise contribution is comparable to the HOM noise, so the fitted level of HOM noise depends strongly on whether that frequency noise estimation is correct or not.

Figure 2 shows the SR detuned configuration. The frequency noise estimation decreases, because the coupling was lower by a factor ~2 in detuned SR. The estimated HOM noise also decreases by about 20%, which is less of decrease than what was measured in March with the SR diaphragm. It is hard to draw a conclusion from that, as it may be due to many reasons. A tentative conclusion is that the SR detuning is not dramatically better at reducing the HOM noise without the SR diaphragm. At low frequency the sensitivity is much worse, because this data is in LN2 (with many dither lines on), unlike the figure 1 in LN3.