[Aritomi, Matteo]
Recently we had a problem that QPD2 DC centering loop cannot be locked.
Today we found that when we move the green beam horizontally, QPD2 DC signal somehow moves vertically and vice versa. This means horizontal and vertical correction signals for QPD2 galvo should be swapped. After we swapped the horizontal and vertical correction signals, QPD2 DC centering loop locked stably even when the green is reflected from filter cavity. Maybe the cabling of QPD2 is wrong.
This issue is originated by the fact that the servo galvo and the DGS are following two different convention for the WFS segment order:
The galvo servo from initial TAMA is following the convention in pic 1 (take from this document), while the DGS is following the convention on pic 2 (KAGRA convention).
The cables from the WFS DC segments have been connected into the galvo servo followig the convention from KAGRA/DGS and this resulted in a swap between X and Y error and correction signals.
We decided to keep the current configuration to be coherent with the DGS beam position monitor. Note that for WFS1 the cable for the correction signal for X and Y were originally inverted by mistake.