Kiwamu, Rana, Sheila
We wonder if the spots are far off center and might explain the low recycling gain that we see. The camera images are not detailed enough to tell us about positions better than a couple cm.
The concept of the A2L measurement is that we drive the optic angle and measure the response in the cavity length sensor (REFL 45, in this case). We assume that the LSC photodiode has a very small angle sensitivity (which is true as long as the cavity is non-degenerate and has a high finesse and the beam on the photodiode is a few times smaller than the diode active area and that the photodiode has a small non-uniformity). We also assume that the mechanical coupling from torque to longitudinal motion is smaller than the centering precision we want. I don't know if all of these assumptions are true, but we proceed as if they are.
To make the measurement robust, we want to avoid the GDS testpoint dropouts, and so we use the front end digital lockins. Unfortunately, the LSC does not have capability to drive mirror angles and the SUS/ASC do not have capability to demodulate the LSC signals. So we make a temporary workaround in the SUS DRIVEALIGN matrix by routing the L drive to P or Y**. Then the LSC drive can drive apply torque on PR2-M3 and demodulate REFL45_I. We then null the demodulated signal by also sending some of the torque to the L2L path. By taking into account the size of the optic (R = 8 cm from T0900435-v9), we can use the ratio of L2L/L2A gains to determine the beam position on the optic (we also take into account the non-unity gains in the EULER2OSEM matrix: L=0.25, P/Y=5.3).
For PR2, we used a drive frequency of 9.57 Hz and amplitude 555 counts. Since we do not have WFS feedback yet, there were large beam spot motions. The 9.57 Hz digital lockin we use to demodulate the 45 MHz demod outputs have I and Q outputs. The I-phase corrresponds to the the steady beam spot position and the Q-phase gives us information of how much the beam is moving (?).
** This hacky technique won't work for the optics being used for LSC feedback, so we need to come up with a better hack or make some model rewiring. Now that we have larger range coil drivers, we can swap the control from PRM to PR2 to allow us to measure the spots all over the DRMI. And, of course, we can use the front end dither system to automatically center the spots as well.
(some details)
As for the vertical measurement, we needed a coefficient of -2.5 in the L2L drive-align matrix element. Taking the Euler matrix into account, I computed the imbalance alpha (see the previous alog 13765 for more info about the definition of those numbers) to be 0.12. Note that the L2P path had a coefficient of -1 while the L2Y had 1 as usual. This indicates an offcentering of 5.3 mm downdward on the optic.
As for the horizontal measurement, we needed a coefficient of -1.5 in the L2L drive-align matrix element. This corresponds to an offcentering of 3.1 mm. Since the UL and LL coils needed to be pushed harder than that of Rs, the spot must be off toward the right side.
Note that we have not readjusted the coil balance on PR2 after the recent coil drive modification.