Julianna Lewis, TonyS, RickS

This morning we moved the upper (inner) Pcal beam at X-end down by 5 mm at the entrance aperture of the Pcal Rx sensor to test the impact on the calibration of the Pcal system at X-end.

We expect that the impact on the calibration of the Xend Pcal will be given by the dot product of the 

The work proceedes as follows:

• Disabled all excitations for the Xend Pcal system. Rx PD output ~0.443 W. (GPS time 1375545485)
• Blocked the lower (outer) beam in the transmitter module. Rx output ~0.2206 W.  (GPS time 137554600)
• Assessed the position of the upper Pcal beam at the entrance aperture of the Rx integrating sphere.  It appeared to be well centered (see first attached photo).
• Installed a target at the entrance aperture of the Rx sensor and adjusted the roll angle so that the rows of holes in the target were aligned with the vertical and horizontal directions (see third and second attached photos.).
• Increased the Pcal AOM offset in the OFS servo from 3.33 to 5.0. (GPS time 1375547403)
• Adjusted the beam position at the Rx sensor aperture (and on the ETM) using the last steering mirror in the Tx module to optimize the sensor output voltage (and thus center the beam at the center of the Rx sensor aperture). (GPS time 1375547610)
  • The Rx sensor output level increased to about 0.301 W from about 0.295 W by tweaking the aligment. (see fourth attached photo)
• We initially moved the Pcal beam up by 5 mm, but became concerned that we might be getting close to the edge of the coating on the output vacuum window because the transmitted light level was significantly lower (few percent).
• We moved the beam down to the lower hole in the target and aligned it to optimize the transmitted level at about 0.300 W (GPS time 137549300) (see fifth attached photo).
• Removed the target from the Rx sensor.  Rx sensor level 0.33 W. With OFS offset at 5.0.  (GPS time 1375549520)
• Reduced the OFS offset to 3.33 from 5.0.  Rx sensor level 0.221 W.  (GPS time 1375549950)
• Removed block from lower (outer) beam in the Tx module.  Rx sensor level 0.4438 W. (GPS time 1375551330) (See last attached photo)

We expect that this upper beam movement will change the unintended rotation of the test mass and change the calibration of the Rx output by about 0.2 % This assumes that we have moved at roughly 45 deg. with respect to the roughly 22 mm interferometer beam offset and that the offset on the surface of the ETM is roughly half of that seen at the Rx module: - 2.5 mm / 2 x 22 mm x 0.707 x 0.94 hop / mm^2 = -18 hop (hundreths of one percent).  So we expect to see that X/Y comparison factor will change from about 1.000 to 0.9982.