I finalized the position of the LEN QPD and it should be minimally sensitive to tilt.

Tilt QPD calibration data is available at 7985, and other useful parameters are available at 7952. See attacted srmlenoplevcal20190319.png for LEN QPD calibration data.

I worked on diagonalization matrix. For the tilt sensing QPD, a rotational transformation of ~-0.4° is needed while ~1.3° is needed for that of the length sensing QPD.  It is estimated that the beam spot is 2.1 mm off-centered horizontally towards the +T direction.

The coupling between all degrees of freedom is less than 1%. I also observed some L2Y coupling and it is possiblly due to some non-zero transverse component of the beam. It can be cancelled later with the diagonalization matrix but I didn't bother for now because the coupling ratio is very low.

Please see srmtml.png, srmtmp.png and srmtmy.png for the transfer function measurements.

I have also implemented damping control for the TM, and IM to TM DC control.

I also measured IM to TM transfer functions which will be used to generate L2P cancellation filters later. See srmiml.png and srmimp.png for the transfer functions.