Naoki, Vicky, Dhruva, Camilla, Sheila
To measure the green beam spot position on FC2, we used the method described in LLO28266. We dithered FC2 M3 P/Y and measured the dither lines in green feedback signal to testmasses (H1:SQZ-FC_SERVO_SLOW_OUT_DQ). The dither frequency is 7.1Hz for P and 16.1Hz for Y. We removed those lines by changing the drivealign P2L and Y2L gain of FC2 M3 (H1:SUS-FC2_M3_DRIVEALIGN_P(Y)2L_gain). The beam spot position from the center of FC2 can be calculated from these gain based on the formula in LLO28266. The attached figure shows the spectra of H1:SQZ-FC_SERVO_SLOW_OUT_DQ with/without drivealign gain (Green: without drivealign, red: with drivealign). With g_P2L = 1.6, g_Y2L = -13, the dither lines dissappeared.
Vicky wrote a script to calculate the formula. The script is in /opt/rtcds/userapps/release/sqz/h1/scripts/filterCavity/measure_fc2_spot.py. With g_P2L = 1.6, g_Y2L = -13, the calculated beam spot position from the center is 2.9 mm in pitch and -23.8 mm in yaw.
FC2 camera (looking in the +Y direction) can see 532 scatter off FC2 surface when locked
The photo of the FCGS beam on the HAM8 viewport is attached to alog 66133 was taken with the same pointing as Naoki's above 2.9 mm in pitch and -23.8 mm in yaw on FC2 (the beam is closer to -x) measurement. The photo and measurement should be used together.
I've attached a photo of the FC2 camera with the illuminator on.
The FC dither template is stored in userapps/release/sqz/h1/Templates/dtt/filter_cavity/FC2_dither.xml.