Night 6 of the October run was dedicated to commissioning the LBTI phase loop and acquiring HOSTS nulling data for testing. We had clouds move in during the afternoon. We waited for them to thin and opened a couple hours late. Seeing was good (0.7-0.9 arcsec) but the transparency at 10 microns was unusable.
We further tested the phase loop. Several bugs were found in the new implementation which improved our stability compared to the previous nights. Seeing degraded during the night (1.2 arcsec) and we demonstrated continued good performance of the loop under these conditions.
During a rare clear patch Denis started to test his automated nulling setpoint script. While it appeared to work, testing was interrupted by clouds. It did allow enough testing for us to refine the logic behind the script from our pre-run plans.
We tested and had success with the fringe envelope outer tracker. We were able to demonstrate the ability of the loop to stay on the "white light" fringe at ~1 Hz, while the pathlength loop stabilized the phase at 1000 Hz.
Finally, we again tested the accelerometer feed-forward. This night the dominant frequency on the secondary was at 13.5 Hz. We set our filter and were able to reduce the amplitude by 2-3x.
We ended the night by taking some Fizeau images using LMIRcam to demonstrate our phase stability.
Night 7 was the beginning of tropical storm Simon moving into Arizona. We did not open.
We did, however, align our artificial source and start to use it for control loop refinement and internal system checks. The sources have some noise at 35-40 Hz, but we can also see the beamcombiner mirrors contribute some noise at 90-200 Hz. The total contribution from the beamcombiner appears to be approximately 150 nm RMS.
The interesting point came when we tested which components are creating the peaks in the power spectrum. By perturbing each mirror we were able to determine that the ellipse mirrors are the culprit for the noise at ~90 Hz and maybe the higher peaks. All the other mirrors looked solid.
While this noise does not dominate our current performance, we will want to improve the ellipse mounts in order to reduce this source of noise eventually.