[Jax, Aidan, Elli]

This morning we calibrated the lever arm of the HWS. We added a beam splitter (CAL-BS) into the optical path to hit the HWS CCD with two beams (REFL and TRANS) at ~ 10mrad to each other. We removed the Hartmann plate from the sensor and recorded the resulting interference pattern on the CCD. The spatial frequency of the interference pattern allowed us to determine the exact angle between the two beams, 9.86mrad, with a precision of approximately 1:500. Next we replaced the Hartmann plate and measured the Hartmann sensor spot pattern of each beam separately by blocking first one and then the other beam. We centroided the spots and determined the mean centroid displacement, 8.507 pixels. Knowing the measured angle, the pixel size (12E-6 m) and the spot displacement, we determined that the lever arm distance (between the Hartmann plate and the CCD) is 10.35 +/- 0.03mm.

The data is attached. Also attached is an optical layout showing the additional beam splitter producing the reflected and transmitted beams.

We made certain to remove the beam splitter and check the final alignment of the HWS system was restored to nominal before closing up the ETM ALS table.

The reference lever arm that has been used before now is 10.0mm.