Kiwamu, Alexa,
This morning, we moved IM4, PR2 and PR3 in order to reduce the amount of the clipping that Keita found the other day (alog 14567).
I moved PR2 and PR3 approximately by +150 and +15 urad horizontally which are the angles that we tried the day before yesterday. As a result, this increased the recycling gain for the 45 MHz sidebands from 21 (alog 14532) to 25.
We may want to go further in these angles for more improvements, but first I need to go through some careful analysis to see if all the observed numbers makes sense.
(some details)
Before starting the project, we realigned the X arm using the green light. The idea is to use the X arm as a reference for recovering good alignment for the infrared light around the corner station. Also we checked the location of the ALS X green light in the GigE camera which can serve as a reference when we touch a picomotor. According to the centroid fitting, the ALS X beam was at (X.Y) = (406.5 pix, 239.7 pix).
After moving PR2 and PR3 to 784 and -162 urad respectivly in the YAW bias sliders, as had been observed, we lost the POP beam at ISCT1. So we moved the picomorot in HAM3 by (dX, dY) = (900, 0) counts using the ALS X green light as a guidance. After picomotoring, we confirmed that there was no clipping according to the camera images. Note that it seemed that this operation somehow introduced vertical displacement slightly in the POP and ALS paths. We did not correct the vertical displacement as it was not considerably large. We locked the infrared laser to the X arm in order to automatically fine-tune the angle of IM4 and PR2 using the REFL WFSs. At this point, we went to ISCT1 and realigned the POP path. In addition to the YAW displacement that we had to correct on ISCT1, the bottom periscope mirror of the POP beam was clipping the beam a little bit. So we steered the top periscope mirror to correct it. Note that the spot position on the top periscope mirror seemed fine and therefore we did not have to move the location of it. We did not correct the ALS path, but it looks like at least the X green light can hit the TRX diode.
We did an OMC scan to measure the power recycling gain. I repeated the same process as we did on this past Monday (alog 14532). The highest DCSUM I obtained for the 45 MHz sidebands was about 7.5 mA when the PRMI was locked. Note that the PRMI alignment was optimized by maximizing POPAIR_RF18 manually. A single shot measurement for the 45 MHz sidebands gave us DCSUM of 0.27 mA. Using the equation in alog 14532, we estimated the recycling gain to be 25.
After moving IM4, PR2 and PR3, the PRC build-up observed by POP18 increased roughly from 170 to 200 uW. Good.
(How much did we shift the beam on PR2 ?)
Actually, this is unclear to me at the moment. I need to look into this issue more carefully. Basically, we seem to have a contradiction between two expected shifts, one is a value derived from the IM4 angle and the other from the PR2 angle.
If we believe that the IM4 bias is accurate, we must have moved its angle by more than +500 urad. This means that the spot positin on PR2 must have moved by ~ 2 x 500 urad x 15 meter = 15 mm toward the positive y-direction, which sounds too large. On the other hand, if we use the PR2 angle instead, my ABCD analysis suggests an expected shift of 0.5 mm on PR2 toward the positive y-direction. So they don't match up at all. Something is wrong. Since both expected shifts are based on the suspension biases, we probably should check the accuracy of them.