Calculations using DTT with 0.001 Hz BW FFTs and 10 averages indicate that chi_XY, the comparison of the X-arm and Y-arm calibrations, is about 1.0013 during the recent 9 hour lock stretch from 08:30 - 17:30 UTC.

This agrees pretty well with the Pcal X/Y comparison calculation in the front end: H1:CAL-CS_TDEP_PCAL_X_OVER_Y_REL_MAG (see attached plot).

Since we are already applying correction factors for a previously-measured chi_XY of 1.0020 (Xend: 1/0.9991; Yend: 1.0011), the X/Y comparison should be 1.0.  

These X/Y correction coefficients will likely need to be updated after some more investigation.  We will also need to investigate and take into account differences in the response function between 410.1 and 410.2 Hz.