[Sudarshan Karki, Aaron Viets]

Sudarshan asked me how close to the 331.9 Hz line it is safe to inject another line using the X end photon calibrator without affecting the kappa_c and f_cc calculations. The short answer is very close, as long as the line injected is a very pure sinusoid.

I ran a test on about an hour of LHO data starting GPS second 1181534652, in which I added a 332.0 Hz sinusoid to DARM_ERR of similar amplitude to the 331.9 Hz line (see spectrum plot). I then collected data for kappa_c and f_cc from both C01 and the test I ran to compare. They were essentially identical:

                             mean(kappa_c)      std(kappa_c)      mean(f_cc)         std(f_cc)

C01                          1.0305                       0.0030            342.7737            1.6903

DCS_TEST               1.0305                       0.0030            342.7737            1.6903

Not convinced I did this correctly, I did a simpler test of demodulating a 332.0 Hz sinusoid at 331.9 Hz and applying the same smoothing that is done in the gstlal calibration pipeline. The sinusoid was attenuated by a factor of 1e-17, consistent with the above results.