There was some suspicion that the REFL beam is somehow clipped on LSC REFL_A diode. Yesterday I made a long scan of RM1 and RM2, separately and at the same time.
LSC-REFL_A_LF is good, ASC-REFL_B too.
I had to move RM1 and/or RM2 by a large amount to make the LSC-REFL_A_LF drop.
ASC-REFL_B is also good, though the safe range is smaller than LSC (not surprising considering the short focal length lens in front of LSC diode).
ASC_REFL_A is unhealthy.
Something is wrong with SEG4.
It seems as if SEG4 has a lower trans impedance, in that when I steer the beam to SEG4 (all power falls on that segment) we get 18000 counts, but when I do the same thing to SEG1/2/3 they rail at 32k counts.
In the attached, the beam was in SEG3 at first, and I walked the beam to SEG4, then SEG1, then SEG2, and then back to SEG3 by turning RM1.
The problem seems to be in the WFS head, not in the WFS interface board. Connected the WFS output cable from the REFL_A head to REFL_B input and the problem is still there.
For the moment I adjusted the digital gain of SEG4 such that there's no coupling from PIT and YAW motion to the SUM (new H1:ASC-REFL_A_DC_SEG4_GAIN=1.9), but this needs to be investigated further.
The second attachment shows the REFLA SUM spectrum of before (green) and after (red) of the gain adjustment when I was modulating PIT of RM1 at 3.3Hz and YAW at 5Hz. For comparison, Blue/Brown are the REFLB SUM data which show no alignment coupling.
Could be the electronics in the head or the cable (one end of the differential signal is dead?). I hope that the diode is not damaged.
Though not impossible, I don't think it's clipping because SEG4 never went larger than 19000 counts during RM1/RM2 full scan.
Followup: WFSA SEG4 DC seems to have connection issue at the chamber (Kiwamu, Keita)
I checked the analog signal coming to the WFS interface chassis using a DB15 breakout board, and sure enough, there was no signal coming from one of the SEG4 differential drivers (don't remember if it was pin4 or pin12).
To check if the problem is in the cable between the rack and the chamber, we swapped the cable for WFSA and WFSB at the chamber.
Right after they were swapped, for a few minutes both WFSA and WFSB looked good and we were confused, but eventually WFSA SEG4 (which showed up in WFSB SEG4 channel because of the cable swap) got back to crappy half-signal state.
We swapped the cables back, WFSA SEG4 got back to good state for some minutes, and again after a while it went back to the bad state.
It seems to me that a crappy feedthrough is doing its own thing. The problem feedthrough is D6-1C1 on HAM1.
(However, it's not totally impossible that the feedthrough is OK but the seg4 in-vac circuit works only for a few minutes after it is powered on because of slowly developing oscillation or thermal problem or whatever.)
We know from the cable swap excersize that it's not the WFSA in-air cable.
I wiggled PRM at 4.13 Hz in PIT (attached, left) and 6Hz in YAW (right).
The DC level during the measurement was: about 80 for LSC-REFL_A_LF, about 40000 for WFS DC SUM.
The OSEM witness is supposed to be calibrated in micro-radians.
Using the above information, the PRM angle to the RIN coupling for these sensors are:
About 1 RIN/urad for LSC-REFL_A_LF, about 10 for WFSs.
