(Alexa, Sheila, Stefan, Evan)
Using the picomotors, we have centered the beam onto both POP A and POP B QPD. For reference... with a whitening gain of 45dB, H1:ASC-POP_A_SUM_OUTMON = 11450 cnts, H1:ASC-POP_B_SUM_OUTMON = 10436 cnts.
We have also calculated how many counts we expect on each quadrant: We have 8.8W into MC. There is 15% loss due to input optics losses (IMC and faraday), 3% transmission of PRM and 230ppm from PR2. This gives about 6.4uW for each quadrant of the QPD. With a responsivity of .8 A/W, a transimpedance of 1kOhm and a gain of 45dB, there is 1V on each quadrant. This amounts to about 3000 counts on each quadrant, which is consistent with what we see.
Once these were centered we wanted to get a (rough) calibration of the PIT and Yaw in terms of beam radii. To do this we moved the IM4 alingment slider until the QPD sum dropped to half of its centered value, then moved IM4 back until the sum reached it maximum again. This gives us an approximation of the beam radius in units of counts on IM4 slider. (for PIT, QPD A hwhm =1600 IM4 PIT, QPD B hwhm=1720 counts IM4 PIT).
Then we returned IM4 to its original alingment, (so the beam was again centered on the QPDs) and measured the response of the QPD PIT and YAW to moving IM4. We saw:
For the calibration of PIT and YAW in beam radii we get:
This is only rough.
We also saw that the QPDs are about 1.8 times the size of the beam
Attached are spectrum of the POP QPDs, and RMS, not calibrated. You can see that the beam motion is a small fraction (less than 1%) of the beam radius, so input beam motion (at PR2) is not the cause of our difficulty aligning the IR into the arm.