This evening we took things a little bit farther than last night (elog 9791) and transitioned CARM to fully IR signals, no ALS at all for CARM error signals! We were unsuccessful at doing the same for DARM.
As we discussed at 40m Meeting this afternoon, the big key was to remove the PRCL ASC from the situation. I don't know specifically yet if it's QPD saturation, or what, that was causing PRM to be pushed in pitch last night, but removing the ASC loops and reengaging the PRM optical lever worked like a dream.
Since we can now, using ALS-only, get arbitrarily close to the PRMI+2 arm full resonance point, we decided to transition CARM over to the 1/sqrt(transmission) signals. We have now done this transition 5 or 10 times. It feels very procedural and robust now, which is awesome!
To make this transition easier, we made a proto-CESAR for the CARM signals in the LSC. There's nothing automatic about it, it's just (for now) a different matrix.
ALS lock conventions:
We have (finally listening to the suggestion that Koji has been making for years now....) set a convention for which side of the PSL the X and Y beatnotes should be, so that we don't have to guess-and-check the gain signs anymore.
For the X beatnote, when you increase the value on the slow slider, the beatnote should increase in frequency. For the Y beatnote, when you increase the value on the slow slider, the beatnote should decrease in frequency.
The input matrix (the aux input part) should then have +1 from ALSX->carm, and +1 from ALSY->carm. It should also have -1 from ALSX->darm and +1 from ALSY->darm.
The output matrix should be carm -> +1's for both ETMs. darm should be -1 to ETMX and +1 to ETMY.
With these conventions, both carm and darm should have negative signs for their gains.
Since we don't have (although should whip up) Watch scripts for the CARM and DARM servo filters, we were using the Xarm filterbank for carm, and the Yarm filterbank for darm again.
Transitioning CARM to 1/sqrt(trans) signals:
As with last night, we were able to easily acquire PRMI lock with a CARM offset of 3 counts. We then moved down to 2 counts, and saw transmission values of 0.1-0.2. We set the offsets in the TR_SQRTINV filter banks so that the difference between the outputs was zero, and the mean of the outputs was 2 (the same as the CARM offset we had).
We looked at the relative gain and sign between the ALS and 1/sqrt() signals, and found that we needed a minus sign, and half the gain. So, we stepped the 1/sqrt() matrix elements from 0 to -0.5 in steps of 0.1, and at the same time were stepping the ALS matrix elements to CARM from +1 to 0, in steps of 0.2. This was, excitingly, very easy!
The first time we did this successfully, was a few seconds before 1081143556 gps.
Here is a set of spectra from the first time we locked on the 1/sqrt(trans) signals.
Failure to transition CARM to RF signals, or reduce CARM offset to zero:
While locked on the 1/sqrt(trans) signals, we looked at several RF signals as options for CARM. The most promising seems to be REFL55, normalized by (TRX+TRY). The next most promising looks like REFL11 normalized by POPDC. Note that these are entirely empirical, and we aren't yet at the resonant point, so these may not be truly the best. Anyhow, we need to reconfigure the LSC input of the normalized error signals, so that they can go into the CESAR matrices. This was more than we were prepared to do during the nighttime. However, it seems like we should be about ready to do the transition, once we have the software in place. Right now, we either normalize both ALS and the RF signal, or we normalize neither. We want to be able to apply normalization to only the RF signal.
Just sitting on the tail of the CARM resonance, there were some random times when we seem to have swung through total resonance, and spoiled our 1/sqrt(trans) signals, which aren't valid at resonance, and so we lost lock. This implies that auto-transitioning, as CESAR should do, will be helpful.
Attempt at transitioning DARM to AS55:
Next up, we tried to transition DARM to AS55, after we had CARM on the 1/sqrt signals. This was unsuccessful. Part of the reason is that it's unclear what the relative gain should be between the ALS darm signals and AS55, since the transfer function is not flat. Also, we didn't have much coherence between the ALS signals and AS55Q at low frequencies, below about 100 Hz, which is concerning. Anyhow, more to investigate and think on here.
Transitioning CARM to 1/sqrt signals, with a DARM offset:
As a last test, Q put in a DARM offset in the ALS control, rather than a CARM offset, and then was still able to transition CARM control to the 1/sqrt signals. As we expect, when we're sitting on opposite sides of the arm resonances, the 1/sqrt signals have opposite signs, to make a CARM signal.
Conclusions / path(s) forward:
We need to redo the LSC RF signal normalization, so that the normalized signals can be inputs to CESAR.
We need to make sure we set the AS55 phase in a sane way.
We need to think about the non-flat transfer function (the shape was 1/f^n, where n was some number other than 0) between the ALS darm signal and AS55. The shape was the same for AS55 I&Q, and didn't change when we changed the AS55 phase, so it's not just a phasing problem.
What DC signals can we use for auto-transitioning between error signals for the big CARM CESAR?