KAGRA Logbook

With Fabian, Liu-kun.

Today we measured the height of the BF keystone as a function of payload mass:

We did an audit of the trim mass. See logbook pages in Google Photos BS Test Hang album for details.
We used this as the "baseline" (BL) configuration for the rest of the measurements.
We stood rulers next to the keystone on both the high and low sides and remeasured the heights, taking photos with the camera (a Canon PowerShot G7X) leveled with its internal electronic level) to minimize parallax.
We added weights on top of the keystone in steps of 37 g up to 225 g, taking photos of the keystone from both sides each time.
We took the extra weights off and measured the baseline configuration again as a consitency check.
We had taken off so much trim mass over the last week, that the trim mass stacks at each corner of the IM had little more than a single 335 g mass and one or two 75 g masses, so we replaced the trim masses in the +X,+Y and -X,-Y corners with equivalent stacks that had many smaller masses.
We remeasured the keystone position with the supposed equivalent stacks as a consistency check.
We moved two 75 g masses from each of the +X,+Y and -X,-Y corners (total 300 g) to the top of the keystone and remeasured the height again.
We then removed mass in steps of 37 g and measured the height with the camera each time.
The section of the data with the most trim mass has a slope of about 130 g/mm. This could be due to non-linearity in the spring, or it could be stiffness in the cables. Liu-kun measured the period during that phase of the data taking by timing 10 cycles and got a period of about 0.6 Hz, which is consistent.
Overall however, the data was a fairly good fit to a straight line with slope -91 g/mm. This is a bit disappointing because Hirata-san had tuned it to a resonance frequency of 0.4 Hz in T1605099, which for a mass of 80 kg corresponds to 52 g/mm.
From the fitted slop and intercept we calculated that we need to remove a further 91 g to get to the CAD height of 65.5. (There's no point in targeting Hirata-san's measured optimum of 65.0 mm because something has clearly changed in the tuning and there's no optimum there.)

Tomorrow we will work out a combination of trim mass that is lighter by 91 g and symmetrical, and then see how far we get installing the IRM and OSEMs.