Nutsinee, Elli
To take a measurement with the end-station HWS, the green beam cannot be resonant in the arm cavity. To take a measurement while the IFO is locked in red, this means we want to add a misalignment offset to one of the ALS_X/Y_PZT1/2 drivers to misalign the green beam in the cavity. Today we are working out what offset is required to do this.
First we looked at the Y arm. We looked at what offset we need to add to H1:ALS-Y_PZT2_YAW_MISALIGN_BIAS in order to offset the beam enough that green is both not resonant in the arm but the beam yet not so badly aligned that we see evidence of clipping on the HWS camera. We put -3250 counts in H1:ALS-Y_PZT2_YAW_MISALIGN_BIAS , requested 'misalign' state for H1:ALS-Y_PZT2_MISALIGN_SWITCH and 'sum' state for H1:ALS-Y_PZT2_MISALIGN_TYPE. We turned off the input to the filter at H1:ALS-Y_PZT2_YAW_OUTEN. This gave us a good image of the ETM on the EX HWS. Changing the MISALIGN_BIAS by +/- 100 counts made the image on the HWS camera noticabally worse.
When we came in this morning the interferometer was unlocked and trying to lock. We set ISC_LOCK to down and ALS_YARM and ALS_XARM to unlocked. We have left the guardian in this state. I have left the ALS-Y_PZT2 driver in it's nominal 'aligned' state (and the BIAS values have not changed). When we statred the flashes in the y-arm were about 1.2 counts, indicating good alignment. After 30mins the flashes were down to 0.8 counts, indicating the alignment had gotten worse. This suggests the hysteresis in misaligning and realigning these PZTs may noticablly effect the green alignment.
We are planning to do the same thing on the x-arm, although we have not yet gotten to a good alignment with bright flashes in H1:ALS-X_TR_A_LF_OUT this morning. But we have noticed the end-X HWS camera isn't taking images properly, so we are going to diagnose that now. We will leave the X-arm alignment as we found it this morning.
Elli, Nutsinee
After Patrick restarted the frozen Beckhoff computer at EX (alog 17861), we were able to power on EX HWS camera and align the ALS beam to the X arm. We had flashes of 0.9 counts in ALS-TRX. Looking at the HWS camera output, we could see that adding an offset of -3300 counts to H1:ALS-X_PZT2_YAW_MISALIGN_BIAS changes H1:ALS-X_PZT2_YAW_MONITOR to 8700 counts. (H1:ALS-X_PZT2_YAW_OUTEN is set to 'OFF', 'Misalign' is requested in H1:ALS-X_PZT2_MISALIGN_SWITCH and the type is set to 'sum' in H1:ALS-X_PZT2_MISALIGN_TYPE.)
There is with this alignment offset we can see some flashes from ITMx at the edge of the HWS image, as well as a little fringing from clipping on the HWS path. If we increase the offset, the clipping gets worde and if we reduce the offset the flashes from ITMx move towards the center of the image. An offset of -3300 seemed to us a good compromise between these undesirable effects. See attached HWS camera images at different offsets.
When we were finished the flashes in ALS-TRX were at 0.85 counts (they dropped about 0.05 counts in 20 minutes), so maybe moving the ALS PZTs doesn't add much of a hysteretic misalignment after all... but we will keep an eye on this. The ALS X PZTz are now running in their nominal aligned state.