J. Kissel, L. Dartez, F. Llamas

As Louis and I were wracking our brains for all the changes to the IFO that might have impacted the IFO's calibration in 2024 in order to update / maintain T2300297, we realized that Francisco's work -- changing the PCAL "inner beam" spot position of PCALX to explore and validate models of systematic errors the PCAL system -- would potentially impact the measurements that are constantly running on PCALX -- the so-called "roaming lines" which is a really long duration, repeating sweep of the sensing function between 1 and 5 kHz. Because this long-duration sweep relies on nominal low noise data, the speed at which the sweep frequency advances is dependent on the IFO's duty cycle, and is thus irregular; sometimes complete in 6 days, other times taking 10 days or more.

To facilitate this research, I plot the trend of the PCALX Oscillator Frequency that's used to define the sweep frequency over time and overlay Francisco, et. al's PCALX spot moves.

And just so you can join us in our worry -- while Francisco's work (see latest in LHO:78964) is showing 0.03%-level changes in the comparison between PCALX and PCALY at 283.91 and 284.01 Hz respectively (via plots like this), the roaming lines measurement is only PCALX and the sweep is from 1-5 kHz, where it's been known for a long time that PCAL spot position changes of ~5 [mm] at RX, or ~2.5 [mm] scale at the Test Mass, can cause 1-to-10%-level errors in reported displacement values at 3, 4, 5 kHz (largest at highest frequencies) -- see, for example, figures 2.30 through 2.35 in T2300381.

Stay tuned!

This aLOG does not suggest we've done the leg work to quantify that we've seen the a problem, but the start of doing such a study.