R. Savage, J. Oberling, J. Driggers, S. Dwyer, P. King (via phone), R. McCarthy, F. Clara

This morning we completed the initial in-air alignment of the ISS 2nd loop PD array.  Recall from yesterday afternoon, we did not make much progress in the alignment.  This was due to the signals out of the PD array dropping for apparently no reason.  After some intial investigation it was decided to re-convene this morning, this time with Rick helping out (as he has worked with the ISS PD arrays extensively).

Some notes from this morning's work:

• The ISS 2nd loop transimpedence amplifier chassis was OFF, thereby preventing us from seeing any signals from the 8 PDs in the array.
  • After talking with Peter, it is unclear if this chassis was ever turned back ON during the vent.
  • This does not explain the issues we were having seeing the beam yesterday afternoon, as the QPD in the ISS 2nd loop array enclosure is read out via a different chassis (per P. King).
  • This chassis, which was a prototype installed before O2, was swapped with a tested and approved production version.  The swap was performed by R. McCarthy and F. Clara.
• We discovered that to really see the beam entering the array, we needed to re-install the temporary HWP after IM1 that Sheila removed yesterday afternoon.
  • We talked to both Ed and Cheryl and were told that this was necessary due to a temporary HWP placed at the base of the PSL periscope.  This HWP was placed in order the purposefully send the wrong polarization into the IMC, I believe to make the IMC flashes easier to see (I may be mis-remembering what I heard, if this is incorrect please correct me in the comments).  This temporary HWP in the PSL enclosure will be removed.
  • This does explain the issues we were having in seeing the beam yesterday afternoon.  As soon as the temp HWP was re-installed in front of IM1 the beam was once again obviously visible on an IR card and the ISS 2nd loop signals were greatly elevated.
• To increase the rate of IMC flashing, we injected a small sine wave (via awggui; f=0.1 Hz, amplitude=1, gain=30) into the L path of MC2.  This made seeing the flashes on an IR card and via the PD array signals much easier.

This taken care of, the beam was then centered on the PD array enclosure input aperture using the first steering mirror (located on the -Y side of the table), and then the signals maximized (to the best we could achieve with a flashing IMC) using the second steering mirror (located on the +Y side of the table).  This was iterated several times until we had a strong signal on all of the PDs and the array enclosure QPD, and the beam was centered on the entrance aperture of the array enclosure.  This is more than sufficient for the in-air alignment; the PD signals will be maximized when the IMC is once again locked (after the corner station is pumped down).