Krishna,

The upgrade to capacitive actuation/damping is done for all practical purposes. The original plan was to install one capacitor plate under each arm of BRS-X. However, after opening up the vacuum can, we found the space insufficient to install two. Rather than take the thermal shields apart and machine them, we decided to install just one. Just one has proved sufficient for BRS-Y, so I think this was a reasonable call. The installation went well. The Beckhoff code/parts proved easy to install and worked perfectly, after some hiccups with incorrect directory paths,etc, which Patrick helped me out with. The damping works perfectly well. I will tune the Q value over the next few days.

I also reduced the 'd value', which is the distance offset between the center of mass of the beam and the pivot/suspension axis. Measurements I made on Monday had suggested that d was ~ 26+/- 2 micrometers (first attachment). The measurement was made by using a piezo stack actuator under the BRS-X platform and driving it with sinusoidal drives at different frequencies. The absolute value of the the relative tilt-transfer function is then calculated from the responses at these frequencies.

I added ~1.5 grams to the top part of the beam-balance. The resulting frequency was ~5.6 mHz. Measuring the transfer function turned out to be impossible to do in air due to thermal drifts. So I had to close up and pump down and restore thermal insulation. Luckily, my first attempt for mass adjustment turned out to be reasonably good. Latest measurements indicate d = -1 +/- 1 micron. For BRS-Y we got d = 0 +/- 1 micron, but I think this is good enough.

BRS-X was pumped down with a Turbo pump to ~2e-7 torr. I switched on the ion pump this morning and I have valved off the Turbo. The ion pump current is at ~0.5 mA and going down steadily.

Over the next few days, I will monitor the DC position of BRS-X and adjust the tilt-subtraction filter banks based on the new response functions.