Here are plots of the long term drift of the arm offset frequency (as measured my H1:ASC-Y_TR_A_NSUM_OUT), and the 3 relevant VCO frequencies. All curves are calibrated in Hz IR.
Between 1600sec and 2000sec we manually moved the ALS-C_COMM_VCO_FREQUENCY to the next fringe and back.
The alignment was tweaked up between 2600sec and 2800sec.
Traces, top:
- Blue: IR transmitted light, calibrated in Hz using zpk([81.9545+i*81.9529;81.9545-i*81.9529],[850;850],0.656,"n")
- Red: ALS-C_COMM_VCO
- Bright green: Green light transmitted (arb. units)
Traces, bottom (note that the scale is 1000x bigger):
- Green: ALS-Y_VCO
- Gold: IMC-VCO
Conclusions
- For 3h the IR frequency offset was well predicted by the ALS-C_COMM_VCO - I don't what happened at about 11000sec.
- To epics channel precision the IMC-VCO cancels out ALS-Y_VCO exactly (actually, I'd expect the difference to be equal to ALS-C_COMM_VCO, but they are too noisy to tell)
- The alignment tweak at 1600sec to 2000sec changed the offset by about 50Hz.
- A typical RMS over 10min is about 12Hz, dominated by VCO motion.