[johannes, lydia, gautam]

• The Y arm has been locked to IR in air using POY11 as an error signal 
• We had been seeing flashes in the arm since yesterday, but were unable to lock
• Today we re-did the alignment procedure much the same way as yesterday
  • It is useful to put in the slide-on irides onto the suspension tower for this sort of alignment
  • We were a bit more systematic in aligning back-reflections to overlap each other today
  • It is useful to stick the IR card just in front of the iris, and align the tip tilts by looking at the scatter on the camera. At least for Yaw, this works pretty well, probably a more reliable reference than contorting oneself inside the vacuum chamber to see if we are well aligned or not.
• Two fixes that made locking possible today:
  • The POY error signal had a large DC offset. I zeroed the offset and adjusted the demod phase to make the error signal 0 when the IMC was unlocked
  • I replaced the 50-50 beam splitter that was dividing the transmitted light between the QPD and Thorlabs PDs with a 2" Y1 CVI mirror - this meant that the flashes we had with the arm roughly aligned went from < 0.1 to a healthier 0.25, which allowed easier locking
• The POY whitening gain was unchanged from when we locked the Y arm in air just after venting and before taking the doors off
• The mode is barely visible on ITMY face, although I guess this is to be expected given we are at low power
• Lydia then tuned the arm alignment more finely such that the transmission is now ~0.65 (See Attachemnt 2 for slider values)
• From values from normal (pre-vent) IFO operation, I would have expected us to get a transmission of about 1 assuming 100mW going into the IFO from the IMC - and so with the BS switched out for an HR mirror, a transmission of ~2. What we get is about 1/3 of this value. Perhaps the IMC isn't so well aligned, but it is hard to imagine we have only 30mW going into the IFO. Or perhaps the input pointing is sub optimal (I did not run ASS, perhaps I should have)

GV EDIT Sep 5: These numbers do make sense if the ND filter that was on the Transmon QPD had ND = 0.6 (there are two at the end, one labelled ND 0.6 and the other labelled ND10 though the latter label looks like some custom label so I don't really trust that value), even though only one was on, unfortunately I don't remember which. So, for 10% of input power with a factor of 8 increase because the ND filter is removed and also that the 50% BS has been replaced with a HR mirror, we expect a transmission level of ~0.6 (compared to the normalized value under normal IFO operation) which is close to what we see...

• The UL coil problems continue to plague us but we were able to lock the arm regardless

In any case, I think we can work on putting in the X arm now and work on recovering that. 

To do for the Y-arm (now that the F.C. is off, we should try and do this in as few chamber openings as possible):

• Fix problematic ITMY UL coil
• Rotation of all 5 ITMY OSEM coils for B-R peak reduction in sensor outputs
• Adjustment of axial position of all OSEM coils on ITMY and ETMY to better center the PD outputs to half their saturation value, given that the pitch and yaw biases to the optics have changed since this was last done
• Insertion of new baffles - try and center the IR and green beams as best as possible on these so that they serve as an alignment reference in the future

Then we need to do all of this for the X arm as well. The PRM LR coil is still giving no output - I will try moving the bias sliders around to see if this is a stuck magnet situation, but perhaps it is not. Since Eric's 3-satellite-box-monte did not yield any positive results, we have to consider the possibility that the LED or PD themselves are damaged. If so, I don't see any workaround without opening up the BS-PRM chamber, but if we can avoid this, we should. Perhaps when ITMX is open we can use the camera with the IR filter removed to see if all the OSEM LEDs are functional through the beam tube.

We are also piping POY11 error to the DAFI model and can hear it in the control room.

Rana suggested reviving the MC autolocker - I've made some changes to the low power MC autolocker scripts and they've been working the few times I tried today evening, but let's see how it does over the weekend. I've also changed the Y axis of the StripTool on the wall to better reflect the low-power range..