The SR3 tests last week showed broadband improvements in DARM below 40 Hz in one lock last week, and below 200 Hz in another. I made some BNS range integrand plots at a few points during Friday Thursday night/Friday morning's SR3 heater test to see where in the spectrum our range improvement was coming from, and as sanity check that we're not being fooled by the changing calibration. Conclusions:
- within an hour of the SR3 heater being turned on we have significant improvement in DARM below 30 Hz, which gives us ~4 Mpc
- later in the lock, once the heater has stabilised, we get sensitivity improvement from 40-100 Hz
- the 48 Hz wandering peak was (in this lock) eating ~2Mpc
- while the calibration changed during the lock that effect is small compared to the real change in the displacement

Craig explains the physics behind the range increase here.

First attachment is DARM spectra and range integrands for 3 points in the same lock. Blue is before the SR3 heater is turned on, red is while the heater is still thermalising (and roughly around the same time as the PCAL to DARM sweep was run), yellow is later in the lock once the SR3 temperature had stabilised.

The most noticeable difference is the low frequency improvement in the first hour of the heater being turned on. The 48 Hz lump also improves between these three locks but I''m not sure if that is related to the SR3 heater at all. Finally the 72.3 Hz line (injected into the 9MHz RIN excitation point to monitor its coupling to DARM) decreases dramatically.

The second attachment top plot is the cumulative range in the three spectra, and the bottom plot is the difference in cumulative range between them. This shows that the initial range improvement (in the first hour and a half of the lock) is almost entirely attributable to the noise below 30Hz, as well as some possible improvement above 100 Hz. The red trace, comparing the 88 and 95 Mpc times, shows a 2Mpc jump at 48 Hz since the time I got for 95 Mpc also coincided with low 48Hz lump activity.

Craig pointed out that from Dan's PCAL to DARM sweeps, the relative calibration changed between the two spectra. Below 40Hz in our SR3-heater-on spectrum we are underestimating DARM and overestimating the BNS range. Above 40 Hz our actual calibration is better in the 94 Mpc lock, and relative to the pre-SR3 heater time we are over estimating DARM and hence underestimating range. We had a look at the ratio of the PCAL-DARM transfer functions after and before the SR3 heater was turned on (i.e. the red and blue traces in this plot) to check that our range improvement is not due to the changing calibration. The ratio of PCAL-DARM sweeps before and after SR3 (yellow stars on the 3rd attachment), is much smaller than the ratio of DARM displacements before and after SR3 (red line). Assuming PCAL is the same at these two times, our improved displacement is real. Dan has already calculated that the difference in calibration at high frequency causes us to underestimate our range.

The final attachment shows the DARM BLRMS during this lock. The jumpy behaviour in the RLP 4 BLRMS (60-100Hz) is because it is dominated by the injected lines around 70 Hz.