F. Matichard, H. Radkins, J. Warner, K. Venkateswara
Due to the problems described in 15690, Z sensor correction to the corner station BSC HEPIs was causing excess tilts at low frequencies. Based on directions from Fabrice, Hugh and I ran tilt decoupling measurements described in 15726 and fixed this to some extent. The pdf attachment (BS_HEPI_Tilt_Decoupling.pdf) shows the effect of the tilt decoupling at the Beamsplitter. The measurements show the X, Y CPS signals with no sensor correction, with sensor correction and finally with sensor correciton and tilt decoupling. There is still room for improvement and a more careful and longer measurement might reduce this further.
For the moment this seems to be good enough to keep MICH locked with perhaps some improvement in the MICH_OUT as seen in the image attached (MICH_OUT.png). The benefit of the Z sensor correction can be seen in the second image (BS_Zsensor_Correction.png). The improvement is not as significant as some of the other chambers. We should investigate more to see what limits the subtraction.
Edit: I made a mistake in the labeling of the lines. The blue and green labels are switched and so are the blue/cyan and pink labels. In summary, the sensor correction increased x and y motion by a factor of ~10 and ~40 respectively and tilt decoupling reduced it by a factor of ~3 and ~8. A more careful/longer approach may reduce it to the original levels.
I attach a screen shot of
(1) The tilt-decoupling, IPS Align elements that have been installed for the three BSC chambers
(2) The current, raw IPS position position values for these chambers
(3) The current alignment offsets stored on HEPI
Remember that we're concerned that if the IPS raw position values, (2), reach the edge of their linear range (around +/-25000 = +/-2.5e4 [ct]), then the tilt decoupling numbers could change. However, with the current set points / alignment offsets stored on HEPI, (3), the raw IPS values, (2), are at worst less than +/-14000 (most less than +/-6000), well within the linear range, so we expect the tilt decoupling values (1) to be static and valid.
And just for ease of parsing later, if they get lost in a reboot or something, the alignment decoupling values are
ITMX BS ITMY
RX to Z (none) -0.0172 -0.0049
RY to Z 0.0015 0.0038 (none)
